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An apparatus for vapor-liquid equilibrium measurements under pressure. Whittle, Donald James 1958

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AN APPARATUS FOR VAPOR-LIQUID EQUILIBRIUM MEASUREMENTS UNDER PRESSURE by DONALD JAMES WHITTLE B.A.Sc, U n i v e r s i t y of B r i t i s h Columbia, 1956 Ay THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of CHEMICAL ENGINEERING We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1958 ABSTRACT Equipment and methods used to measure v a p o r - l i q u i d equilibriums at pressures above one atmosphere are reviewed, and the methods of t r e a t i n g the r e s u l t s obtained from such equipment are also discussed. An appara-tus s u i t a b l e f o r the study of v a p o r - l i q u i d e q u i l i b r i u m at pressures up to 3000 pounds per square inch and temperatures up to 550°F. has been designed. P r o v i s i o n i s made i n the apparatus f o r measuring the volume of each of the two phases and f o r removing samples of the i n d i v i d u a l phases at constant temperature and pressure. Recommendations f o r the c a l i b r a -t i o n .. and use of the apparatus and f o r the p u r i f i c a t i o n of the solvents to be studied are given. In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree'at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of The U n i v e r s i t y of B r i t i s h Columbia Vancouver Canada. TABLE OF CONTENTS TITLE PAGE Introduction 1 H i s t o r i c a l Review 8 Methods and Equipment 8 Treatment of Vapor-Liquid E q u i l i b r i u m Data 21 Material s 48 Benzene 48 Mercury 54 n-Propanol 55 Apparatus 62 Procedure f o r Making Measurements 72 Bibliography 83 TABLES 1. Table of Symbols 46 2. Physical Data f o r Benzene from the L i t e r a t u r e 58 3. Physical Data f o r Propanol from the L i t e r a t u r e 61 LIST OF ILLUSTRATIONS AT END OF TEXT Figure 1 - Freezing Point Apparatus Figure 2 - Mercury Transfer Flask and Sample C o l l e c t i o n Flask F i t u r e 3 - Transfer Apparatus Figure 4 - Tubing Diagram Assembly Drawing 1 - Assembly of Equi l i b r i u m C e l l Assembly Drawing 2 - Assembly of Measuring Head D e t a i l Drawing 1 - Equi l i b r i u m C e l l D e t a i l Drawing 2 - Mercury Storage Bomb Det a i l Drawing 3 - Cap and L i d f o r Bomb D e t a i l Drawing 4 D e t a i l Drawing 5 D e t a i l Drawing 6 D e t a i l Drawing 7 TABLE OF CONTENTS (cont'd.) Packing Support Rings Gland Nut and Guard Ring Measuring Head D e t a i l s D e t a i l s of Magnetic S t i r r e r ACKNOWLEDGEMENT The author wishes to express h i s appreciation f o r the constructive c r i t i c i s m and the encouragement given by Dr. L.W. Shemilt, under whose supervision t h i s research was c a r r i e d out. Acknowledgement i s also made of the Standard O i l Company of B r i t i s h Columbia Limited f o r t h e i r f i n a n c i a l help during the winter months of 1956-1957 and of the National Research Council f o r t h e i r assistance during the f o l l o w i n g year. INTRODUCTION When a l i q u i d composed of two or more chemically pure substances i s heated, the composition of the vapor given o f f w i l l normally be d i f f e r e n t from that of the l i q u i d remaining. This change i n composition with change i n phase forms the basis of such separation processes as d i s t i l l a t i o n and absorption, and therefore a qu a n t i t a t i v e knowledge of the change i s essen-t i a l f o r the a n a l y t i c a l treatment of these processes. Although i n a d i s -t i l l a t i o n column the vapor evolved i s not generally i n phase e q u i l i b r i u m with the l i q u i d i t leaves, corrections can be made f o r t h i s f a c t and equ i l i b r i u m values are used as a basis f o r c a l c u l a t i n g the composition d i f f e r e n c e s . Since v a p o r - l i q u i d e q u i l i b r i u m values are used, i t i s impor-tant that extensive tables of these be a v a i l a b l e , and the determination of such values has become an important f i e l d of study. As w e l l as being of p r a c t i c a l importance, the determination of vapor-l i q u i d e q u i l i b r i u m values i s of t h e o r e t i c a l i n t e r e s t . Many of the studies made i n recent years on the theory of solutions have been made from a 78 "molecular" viewpoint . With t h i s method of treatment, expressions are found f o r bulk properties i n terms of molecular properties and intermole-cular forces. In order to determine the v a l i d i t y of such expressions and thus of the molecular theory on which they are based, the calc u l a t e d values must be compared with experimentally determined ones, and one of the basic sources of data f o r such comparisons i s from v a p o r - l i q u i d equi-l i b r i u m measurements . A considerable amount of v a p o r - l i q u i d e q u i l i b r i u m data i s a v a i l a b l e 83 i n the l i t e r a t u r e , as exemplified i n the compilation of J u Chin Chu ; but much of i t i s f o r systems composed of s i m i l a r or re l a t e d compounds, f o r 12 5 167 example, that of Banks and Musgrave , Ainer et a l , and Woodson et a l . Mixtures of t h i s type often form nearly i d e a l solutions, and many of the problems associated with an understanding of the more general non-ideal s o l u t i o n s do not a r i s e . Although some experimental values are a v a i l a b l e 90 99 159 f o r non-ideal systems ' ' , most of these are f o r systems which are at or near atmospheric pressure where again the mixtures often approach i d e a l s o l u t i o n s . For these reasons i t i s obviously of i n t e r e s t to study a system i n which the components would form a non-ideal s o l u t i o n and to make measurements over most of the range of temperature and pressure where a l i q u i d and vapor phase can co - e x i s t . Some valuable work has of course been done at pressures above one atmosphere and with non-ideal systems. Comings**^, S m i t h * 4 5 , and Newitt^^^ have compiled l i s t s of workers who have made measurements at elevated 37 pressure,and Comings has also discussed the theory, apparatus, and t y p i -c a l r e s u l t s of these workers. Among the we l l known workers i n t h i s f i e l d 127 128 129 are Sage and Lacey, ' ' who have studied a large number of systems which are important i n the f i e l d of petroleum at elevated pressures. „. 8 5 , 8 6 , 8 8 . , 4 8 , 9 2 , 1 6 3 . , + + i . i Kay 1 and Katz have also studied s i m i l a r systems at elevated 123 pressures. Prigogene has attempted to predict the behaviour of mixtures at conditions up to the c r i t i c a l ones from molecular structure and i n t e r -molecular forces. Several sets of measurements have been made at t h i s u n i v e r s i t y on the va p o r - l i q u i d e q u i l i b r i u m of the normal alcohols with benzene and with t o l -u e n e 2 9 ' 5 2 ' 5 4 ' 7 5 ' 8 1 ' 1 5 7 ' 1 6 2 . Since a mixture of polar and non-polar mole-cules of t h i s type forms a non-ideal s o l u t i o n i t was decided to extend the measurements f o r one of these, the benzene-normal propyl alcohol system, to - 3 -the c r i t i c a l region. The p a r t i c u l a r system benzene-n-propanol was chosen because the c r i t i c a l values of the two components were not too high f o r convenient measurement and because there i s a reasonable diffe r e n c e bet-ween the two c r i t i c a l temperatures and c r i t i c a l pressures. A second f a c t o r which influenced the choice of t h i s system was that some measurements have been made on benzene-methanol mixtures at elevated pressure****. Since r e l a t e d systems often lend themselves to group c o n t r i -bution p r e d i c t i o n s , that i s pr e d i c t i o n s i n which each element or group of the system contributes a set value, i t was thought i t might be worthwhile to obtain information on another s o l u t i o n of t h i s type. Many types of equipment have been used to experimentally determine 133 v a p o r - l i q u i d e q u i l i b r i u m . Robinson and G i l l i l a n d have c l a s s i f i e d these under the headings of (a) C i r c u l a t i o n Method (b) Continuous D i s t i l l a t i o n Method (c) Dynamic D i s t i l l a t i o n Method (d) Dynamic Flow Method (e) Bomb Method ( f ) Dew and B o i l i n g Point Method. The c i r c u l a t i o n method consists of pl a c i n g the mixture to be studied i n an evacuated v e s s e l , c o l l e c t i n g the vapor from above the l i q u i d and c i r -c u l a t i n g i t back through the l i q u i d u n t i l the composition of both phases becomes constant. Although t h i s method i s b a s i c a l l y very simple, several precautions have to be taken to obtain accurate r e s u l t s . The system con-t a i n i n g the mixture must be leak-free or the amount of material i n i t w i l l p rogressively change and cause corresponding change i n the eq u i l i b r i u m - 4 -p r o p e r t i e s . Not only does the t o t a l quantity of material i n the system have to be kept constant, but the t o t a l q u a n t i t i e s of each phase must also not change. In order that the volume of each phase does not change,it i s necessary that the apparatus be kept at a constant temperature and that the displacement of the pump used to c i r c u l a t e the vapor remain e f f e c t i v e l y con-stant. An inherent error e x i s t s i n t h i s type of measuring equipment caused by the f a c t that the pressure at the bottom of the l i q u i d phase where the vapor i s reintroduced,is d i f f e r e n t from that at the top of the phase, where the vapor leaves,and thus the e q u i l i b r i u m composition i s d i f f e r e n t at the two l e v e l s . However, at most t o t a l pressures the change i n composition with t h i s small change i n pressure can be neglected. The continuous d i s t i l l a t i o n method i s a le s s accurate but simpler method of measurement. The vapor i s c o l l e c t e d from above the l i q u i d , con-densed, and returned to the s t i l l as a l i q u i d . This method has been quite generally used but has the disadvantage that there i s some doubt as to whether or not the vapor formed by the b o i l i n g l i q u i d i s i n eq u i l i b r i u m with the l i q u i d . Another d i f f i c u l t y a r i s e s because the vapor returned as a condensate i s not of the same composition as the s t i l l l i q u i d . I f any of t h i s condensate i s vaporized before i t i s completely mixed, the vapor produced w i l l not be i n eq u i l i b r i u m with the l i q u i d phase. In order to eliminate some of the d i f f i c u l t i e s a r i s i n g from t h i s method of c i r c u l a t i o n , the condensate i s often vaporized before returning i t to the s t i l l . In t h i s case the r e s u l t i s equivalent to r e c i r c u l a t i n g the vapor but the equipment i s sometimes easier to operate. Care must be taken that the condensate i s completely vaporized and that i t i s super heated only enough to make up f o r heat losses,or the s t i l l w i l l not operate under steady state conditions. - 5 -The dynamic d i s t i l l a t i o n method i s a very simple one f o r obtaining approximate v a p o r - l i q u i d e q u i l i b r i u m values. I t i s based on the d i s t i l l a -t i o n of small q u a n t i t i e s of vapor from a large quantity of l i q u i d . The l i q u i d mixture i s placed i n a s t i l l , a small quantity vaporized, and the composition of both phases measured. The procedure i s then repeated u n t i l several samples have been obtained. The average composition of each of the phases i s p l o t t e d on a graph versus the amount vaporized,and the curves obtained are extrapolated back to zero vaporization to obtain the e q u i l i -brium compositions. The values obtained by t h i s method, of course, w i l l only be the true values i f the b o i l i n g l i q u i d produces an e q u i l i b r i u m vapor. Another approximate method of determination i s the dynamic flow method i n vhich a vapor i s bubbled through a s e r i e s of vessels containing l i q u i d of constant composition. As the vapor passes through each v e s s e l , i t s com-p o s i t i o n changes u n t i l by the time i t reaches the l a s t one i t i s assumed that the vapor i s i n e q u i l i b r i u m with the l i q u i d . The composition of the vapor i s then measured and that of the l i q u i d i s known since i t i s that of the o r i g i n a l mixture. A serious weakness i n t h i s method of measurement i s that a pressure drop must occur i n each vessel,and thus the composition of the e q u i l i b r i u m vapor i s s l i g h t l y d i f f e r e n t i n each one. The bomb or constant-volume method i s an accurate one that requires only f a i r l y simple equipment. A l i q u i d sample i s placed i n an evacuated bomb,and the mixture then e i t h e r s t i r r e d or shaken u n t i l the two phases come to e q u i l i b r i u m . Once the phases are at e q u i l i b r i u m , samples of each phase are taken by displacement with an equal volume of some i n e r t material such as mercury. Although accurate r e s u l t s are possible by t h i s method, -6-care must be taken i n order to obtain them. A very common error that a r i s e s i s that some of the l i q u i d phase i s splashed or else condenses i n the vapor sampling l i n e . Since the volume of the vapor sample when con-densed i s u s u a l l y very small, a small amount of l i q u i d i n the sample l i n e would represent a large percentage of the t o t a l sample and could cause a very serious e r r o r . Because the equipment required f o r t h i s method i s quite simple, i t i s often used f o r high pressure measurements! The dew and b o i l i n g point method i s another method commonly used to measure v a p o r - l i q u i d e q u i l i b r i u m at high pressures. A sample of known composition i s placed i n a c e l l of v a r i a b l e volume which i s surrounded by a constant temperature bath. The volume of the c e l l i s then varied u n t i l the sample f i r s t s t a r t s to vaporize. The point at which the vaporization f i r s t occurs i s found by p l o t t i n g the pressure volume isotherms or, i f the c e l l i s of glass, by observation. The volume at which the vapor f i r s t s t a r t s to condense i s found i n a s i m i l a r manner. Since the composition of the mixture i s determined before i t i s placed i n the c e l l , no a n a l y s i s of the phases i s necessary. Two f a c t o r s were considered when choosing the design of the equipment b u i l t f o r t h i s i n v e s t i g a t i o n . Since i t was hoped to obtain e q u i l i b r i u m values of t h e o r e t i c a l i n t e r e s t , i t was important that the apparatus should give as accurate r e s u l t s as p o s s i b l e . As w e l l , however, i t was desirable that the basic design be kept as simple as p o s s i b l e , since the apparatus had to be b u i l t to withstand elevated temperatures and pressures. On the basis of these two considerations i t was decided that, of the various types of equipment used to measure v a p o r - l i q u i d equilibrium,a m o d i f i c a t i o n of 134 the constant volume apparatus used by Sage and Lacey was most s u i t a b l e . -7-' The d e t a i l s of the design and construction of the apparatus, along with the proposed operating procedure and d e s c r i p t i o n of the methods of p u r i f y i n g the solvents to be studied are presented here. Included as w e l l i s a b r i e f d e s c r i p t i o n of some of the equipment used by other workers to measure v a p o r - l i q u i d e q u i l i b r i u m at elevated pressures and temperature, and, since i t i s important from the standpoint of the structure of l i q u i d and vapor solutions,the method of t r e a t i n g and u t i l i z i n g data obtained under these conditions. -8-HISTORICAL REVIEW Methods and Equipment Of the various types of apparatus mentioned above, those that have been used f o r measurement at pressures above one atmosphere w i l l be d i s -cussed more f u l l y . The majority of measurements obtained at elevated u v -+u i 4.- • ,11,92,134,135 pressure have been with n o n - c i r c u l a t i n g equipment, ' 7 ' such us i s used i n the constant-volume method or dew and bubble point method,but some important r e s u l t s have been obtained with equipment where e i t h e r the l i q u i d + u u • • i * ,71,119,120,134 . , . , . „ , .T or the vapor phase i s c i r c u l a t e d ) ' ' ' and a d e s c r i p t i o n of both types w i l l be given. The constant-volume method has been used extensively f o r the deter-mination of v a p o r - l i q u i d e q u i l i b r i a at elevated pressures. This method of measurement i s advantageous i n r e q u i r i n g simple equipment, making accurate r e s u l t s t h e o r e t i c a l l y p o s s i b l e , and allowing any number of components to be studied. One apparatus of t h i s type that has been used s u c c e s s f u l l y 134 i s that of Sage and Lacey . Their equipment consisted p r i m a r i l y of an o o e q u i l i b r i u m c e l l with a working temperature of from 0 to 460 F. at press-ures up to 10,000 pounds per square inch. The temperature i n the e q u i l i b r i u m c e l l was c o n t r o l l e d by immersing i t i n a w e l l agitated o i l bath. Mercury could be added to or removed from the c e l l through a high pressure l i n e connecting i t to a storage vessel, and the pressure on the c e l l was c o n t r o l l e d by regulating the pressure applied to t h i s v e s s e l . A v e r t i c a l rod extended into the c e l l through the bottom and c a r r i e d a mercury l e v e l i n d i c a t o r at i t s upper end. The i n d i c a t o r consisted of an e l e c t r i c contact p o i n t i n g downward, and i t and the e l e c t r i c lead wire which extended from i t through the rod were i n s u l -ated from the rod and c e l l . The contact and the c e l l were connected -9-through an i n d i c a t i n g c i r c u i t so that a si g n a l was given when the contact point touched the mercury surface. The height of the va p o r - l i q u i d i n t e r f a c e i n s i d e the c e l l was measured by means of a hot wire anemometer which was also supported by the rod. The anemometer consisted of a short length of platinum wire stretched across two insulated pins. A small current was passed through the wire, and since the rate of d i s s i p a t i o n of heat from the wire was d i f f e r e n t i n each phase, the temperature and thus resistance was also d i f f e r e n t i n each one. The l o c a t i o n of the i n t e r f a c e could therefore be found by determin-ing the l e v e l where the resistance of the wire suddenly changed. The lower end of the rod extended into another c e l l which was f i l l e d with mercury and connected to the top one so that no change occurred i n the free volume of the top bomb when the p o s i t i o n of the rod was changed. The rod was raised or lowered by r o t a t i n g a worm which engaged a gear attached to a nut threaded on to the rod. The gear and thread were con-structed so th a t , a f t e r i t was c a l i b r a t e d , a counter on the worm shaft in d i c a t e d the p o s i t i o n of the rod. Two valves were b u i l t i nto the c e l l , one at the top and the other half-way down the maximum working volume of the c e l l . The top valve was con-nected to a vacuum pump and to an apparatus f o r adding samples to or with-drawing samples from the bomb. Samples could also be added or removed through the lower valve. Excellent mixing of the material w i t h i n the c e l l was achieved with a s p i r a l a g i t a t o r which was designed so that the free cross section w i t h i n the c e l l was the same at every p o s i t i o n where l e v e l measurements could be made. This a g i t a t o r was driven by an electromagnet which revolved around the outside of the c e l l . -10-The composition of each phase at e q u i l i b r i u m was found by withdraw-ing samples through the two valves b u i l t i n to the c e l l . As the samples were removed, mercury was added so that i s o b a r i c conditions and thus equi-l i b r i u m were maintained. Other workers have used d i f f e r e n t forms of t h i s apparatus to measure 92 phase e q u i l i b r i a . Kobayashi and Katz have used an e q u i l i b r i u m c e l l at pressures up to 2800 pounds per square inch and temperatures to 300°F. i n which the i n t e r f a c e between the two phases was determined through a glass window. The c e l l was immersed i n an a i r bath f o r temperature control, and the c e l l contents were s t i r r e d with an e l e c t r i c s t i r r e r mounted e n t i r e l y w i t h i n the c e l l . C e l l pressure was varied by changing the amount of mat-e r i a l present. Samples of the e q u i l i b r i u m phases were obtained through four posts set at d i f f e r e n t l e v e l s . The l i q u i d sampling posts and l i n e s connected to them were f i l l e d with mercury to prevent accumulation of material i n them. When samples of e i t h e r phase were taken, mercury was i n j e c t e d into the c e l l at the same rate as the sample was removed, thus preventing any change i n the e q u i l i b r i u m conditions. 2 Aktrs, A t t w e l l , and Robinson have made eq u i l i b r i u m measurements i n a bomb-type e q u i l i b r i u m c e l l at pressures up to 5000 pounds per square inch and temperatures up to 300°F. A g i t a t i o n of the mixture i n the c e l l was accomplished by rocking the e n t i r e e q u i l i b r i u m c e l l i n a constant tempera-ture bath. Samples of the vapor were obtained by cracking a needle valve on the top of the c e l l . A constant pressure was maintained during the sampling by the i n j e c t i o n of mercury from a mercury pump. Sampling was continued u n t i l three samples of the gas had been obtained, a f t e r which the remaining vapor was forced out of the c e l l . The point where the l i q u i d i n t e r f a c e reached the e x i t valve was detected by a sudden jump i n c e l l -11-pressure. When i t was c e r t a i n that only l i q u i d remained i n the c e l l , the pressure was increased by about a thousand pounds per square inch and l i q u i d samples then withdrawn. 39 Copeland, Silverman, and Benson have designed an apparatus which has been used f o r the sampling of one phase of a v a p o r - l i q u i d e q u i l i b r i u m system at pressures up to 300 atmospheres and temperatures up to 400°C. The e q u i l i b r i u m c e l l consisted of two chambers, a sampling chamber and a valve chamber, which could be i s o l a t e d from one another with a spring valve. Normally the valve was i n the open p o s i t i o n and was held there by a small shear p i n . When i t was planned to close the valve, an a l i e n screw on the outside of the valve was tightened. The screw operated through a diaphragm on the valve stem and when tightened, broke the shear p i n and closed the valve. When a sample was to be taken, the equi l i b r i u m c e l l , which could be rotated, was placed i n such a p o s i t i o n that only one phase was present i n the valve chamber and the valve closed. The eq u i l i b r i u m c e l l was then c h i l l e d , the valve chamber opened, and the sample removed with a small p i p e t t e . 51 Drago and S i s l e r used a s t a i n l e s s s t e e l e q u i l i b r i u m apparatus at pressures up to 100 atmospheres i n which a l l inner surfaces were coated with t e f l o n enamel and i n which the eq u i l i b r i u m c e l l contained a glass l i n e r . In t h i s apparatus no pro v i s i o n was made f o r e q u i l i b r i u m displacement of samples. When a sample was required, a small amount of material was bled o f f through a dip tube extending into the eq u i l i b r i u m c e l l . The equipment described above i s representative of the types that have been used to measure v a p o r - l i q u i d e q u i l i b r i u m by the constant volume method. 56 Many workers, i n c l u d i n g Evans and Har r i s , Ottenwelter, H e l l e r , and Wein-r i c h 1 2 1 , De an and Took43 } a n a Benedict Solomon and Rubin 16 have used -12-s l i g h t l y d i f f e r e n t designs, but i n each case the general form i s the same as i n those already described. Many determinations of v a p o r - l i q u i d e q u i l i b r i u m at elevated pressures also have been made by the dew and bubble point method. The p r i n c i p l e involved i n t h i s method of measurement i s quite d i f f e r e n t from that i n the constant volume method:, where actual samples of the e q u i l i b r i u m phases are obtained. In a binary, two phase mixture, f i x i n g two degrees of f r e e -dom f i x e s the e n t i r e system. I t f o l l o w s , therefore, that f o r any binary system at any p a r t i c u l a r temperature and pressure the. compositions of the vapor at the dew point and the l i q u i d at the bubble point are f i x e d and are equal r e s p e c t i v e l y to the vapor and l i q u i d compositions of any two phase mixtures formed from the same components at the same temperature and pressure. For t h i s reason, the determination of v a p o r - l i q u i d e q u i l i b r i u m i n a binary mixture can be reduced to the measurement of dew and bubble points of mixtures of known concentration. This method i s r e s t r i c t e d to binary mixtures, of course, because f o r a mixture of more than two compon-ents, f i x i n g the temperature and pressure does not f i x the e q u i l i b r i u m l i q u i d and vapor compositions. Probably the most generally used apparatus f o r dew and bubble point 126 determinations i s that of Ramsay and Young as modified by Bahlke and Kay**. Their equipment consisted of a long s t e e l compressor f i l l e d with mercury i n which the volume could be varied with a plunger f i t t e d i n to the block through a pressure-tight j o i n t . The block was also f i t t e d with a v e r t i c a l branch through which mercury could be forced. A t h i c k - w a l l e d glass c a p i l l a r y was f i t t e d i n to t h i s branch and sealed o f f with a s t u f f i n g box. The sample to be tested was placed i n the tube and confined there by mercury from the block. To ensure adequate mixing of the sample, the -13-tube was f i t t e d with a so f t i r o n s t i r r e r which was activ a t e d by an ex-t e r n a l electromagnet. The experimental tube was also surrounded by a glass jacket through which organic vapors were passed. These vapors were produced from a ser i e s of organic l i q u i d s whose b o i l i n g points lay w i t h i n the tempera-ture range desired. By b o i l i n g the l i q u i d under reduced pressure, a range of temperature s u f f i c i e n t to overlap the b o i l i n g point of the next l i q u i d i n the se r i e s could be obtained. The pressure applied on the sample was determined from a measure of the pressure on the compressor block and a s t a t i c head c o r r e c t i o n f o r the height of mercury i n the tube. When a known weight of sample had been placed i n the tube and the tube i n s t a l l e d i n the block, vapor at the desired temperature was bubbled through the jacket and the pressure slowly increased. Measurements were made of temperature, pressure, and phase volume at the dew point, bubble point, and several intermediate p o i n t s . Since the tube was constructed of g l a s s , the dew point, bubble point, and phase boundaries could a l l be determined by d i r e c t observation. When the measurements had been com-pleted f o r one sample, the tube was r e f i l l e d with a sample of a d i f f e r -ent composition and the measurements repeated. From the measurements made on dew and bubble points,the v a p o r - l i q u i d e q u i l i b r i u m was determined. The d e n s i t i e s of the unsaturated l i q u i d phase, the co- e x i s t i n g l i q u i d and vapor phases, and the super heated vapor phase could also be calc u l a t e d from the volume measurements and a know-ledge of o r i g i n a l weight of m a t e r i a l . 84 Kay l a t e r modified the apparatus so that the pressure was applied from a high pressure gas c y l i n d e r instead of the plunger, and Kay and Rambosek 8 8 modified the pressure regulator on the jacket surrounding the -14-experimental tube, but i n both cases the e s s e n t i a l operation of the appara-tus remained the same. 135 Sage and Lacey have used a s l i g h t l y d i f f e r e n t technique to deter-mine dew and bubble p o i n t s . I f pressure-volume data are determined f o r a system over the e n t i r e two phase region and p l o t t e d as pressure versus volume isotherms, d i s c o n t i n u i t i e s w i l l occur i n the curves at the dew and bubble points except where the data i s measured near the c r i t i c a l condi-t i o n s . The apparatus used by these two workers at pressures up to 10,000 pounds per square inch and at temperatures up to 600°F. to determine the pressure-volume data consisted, i n essence, of a U-tube. closed at each end and p a r t l y f i l l e d with mercury. The sample was confined i n one arm, while a i r under pressure was admitted to the other i n order to change the volume occupied by the sample. The temperature of the arm containing the sample was c o n t r o l l e d by surrounding i t with a constant temperature bath. E q u i l i b r i u m w i t h i n and between the phases was obtained by means of a s t i r r e r driven by a magnet r o t a t i n g on the outside of the bomb. Since the t o t a l quantity of mercury i n the two c e l l s was constant, a measure of the height of mercury i n the pressure c e l l gave the height and thus free volume i n the e q u i l i b r i u m c e l l . This mercury l e v e l was determined with a movable e l e c t r i c contact which extended down from the top of the c e l l and gave a s i g n a l when i t touched the mercury surface. The measuring procedure consisted of adding a known weight of sample to the c e l l , s e t t i n g the constant temperature bath at the desired value, and then varying the pressure u n t i l pressure volume measurements had been obtained f o r the e n t i r e two phase region. The temperature was then i n -creased and the procedure repeated. When one sample had been completely 15-investigated i t was removed, replaced with another, and the measurements repeated. Once the f u l l range of temperature, pressure, and composition had been investigated, the pressure volume data were p l o t t e d , the dew and bubble points found, and the v a p o r - l i q u i d e q u i l i b r i u m determined. 18 Bloomer and Parent used a d i f f e r e n t method f o r varying the system pressure. Their apparatus, used at pressures up to 750 pounds per square inch, consisted of a graduated glass c e l l immersed i n a constant tempera-ture bath. S t i r r i n g was accomplished by magnetically r a i s i n g and lowering a s t e e l b a l l i n s i d e the c e l l . The contents of the c e l l were brought through the gas phase region to the dew point and then through the two phase region to the bubble point by the a d d i t i o n of measured increments of the material being studied. The dew and bubble points were determined by d i r e c t observation and checked from a pressure versus volume p l o t . 89 Katz and Kurata have used an e q u i l i b r i u m c e l l c o n s i s t i n g of a glass-windowed s t e e l tube at pressures up to 3100 pounds per square inch. A g i t a t i o n to insure intimate mixing was obtained by rocking the e n t i r e c e l l . Pressure was applied to the c e l l by the a d d i t i o n or removal of mer- -cury. The p o s i t i o n s of the mercury-sample i n t e r f a c e and v a p o r - l i q u i d i n t e r f a c e were determined from a scale placed beside the window. 38 53 Many other workers, i n c l u d i n g Cook , Eaken, E l l i n g t o n , and Garni, 34 and Clegg and Rowlinson have used the bubble and dew points method to determine v a p o r - l i q u i d e q u i l i b r i a i n binary mixtures. Although the appa-ratus that these workers used d i f f e r e d s l i g h t l y i n d e t a i l from the ones described above, the general features were the same. Several workers have adapted atmosphere e q u i l i b r i u m s t i l l s f o r use at elevated pressures. Although the equipment required f o r t h i s method of measurement i s f a i r l y complicated, accurate r e s u l t s can be obtained from a w e l l designed s t i l l . Scheeline and G i l l i l a n d have designed such a l i q u i d c i r c u l a t i o n s t i l l from gauge glass tubing. The top of the tubing was sealed with a packing gland and the bottom was f i t t e d into a e t e e l base. Four tubes entered the s t i l l through the top gland: a s t i l l sampling l i n e , a thermocouple w e l l , a vapor e x i t l i n e , and a l i q u i d return l i n e . In order to prevent r e f l u x on the s t i l l w a l l s , the s t i l l was surrounded by a pyrex jacket through which hot a i r could be blown. A length of t h i c k walled glass tubing, sealed at the upper end, was used as a condensate t r a p . Condensate from the s t i l l condenser entered through a tube extending up nearly to the top of the trap and was returned to the s t i l l through a s i m i l a r shorter l i n e . P r o v i s i o n was made f o r removing samples from the condensate return l i n e . The pressure i n the s t i l l was regulated by c o n t r o l l i n g the heat i n -put with a mercury switch which also operated as a pressure manometer. The bottom of the condensate trap served as one arm of the manometer so that when the pressure i n the s t i l l rose, the mercury l e v e l i n the con-densate trap was depressed,and the l e v e l i n the other arm of the mano-meter also rose. This r i s i n g mercury surface closed an e l e c t r i c a l c i r c u i t which reduced the heat input to the s t i l l . When the s t i l l press-ure dropped, the contact was broken and the heat input to the s t i l l was increased. Measurements were made with t h i s s t i l l at pressures up to 600 pounds per square inch. A s i m i l a r one, modified s l i g h t l y by Griswold, Andris, and K l e i n was used to pressures up to 1000 pounds per square inch at a temperature of 250°C. 119 Othmer , S i l v i s , and S p i e l have b u i l t a l i q u i d type c i r c u l a t i o n s t i l l of s t a i n l e s s s t e e l s u i t a b l e f o r pressures up to 1000 pounds per square inch. Two small sight glasses were b u i l t i nto the s t i l l body to allow observation of the contents, and one into the drop counter so that the b o i l i n g rate could be determined. The temperature i n the s t i l l was measured by means of two thermocouples which were placed i n we l l s i n both phases. The temperature and pressure were c o n t r o l l e d by adjusting the heat input to the b o i l e r and the condenser cooling water flow r a t e . Heat losses from the s t i l l body were prevented by i n s u l a t i n g and wrapping Nichrome wire around the outside. The power supplied to the heating wires was c a r e f u l l y c o n t r o l l e d so that the temperature on the outside of the s t i l l was the same as that on the i n s i d e . Samples of the l i q u i d phase were obtained through a l i n e from the condensate return l i n e . 120 Otsuki and Williams measured v a p o r — l i q u i d e q u i l i b r i u m at atmos-65 pheric pressure i n a s t i l l based on the one designed by G i l l e s p i e . Measurements at pressures up to 500 pounds per square inch were then made i n a copper duplicate of the atmospheric s t i l l . 9 Aroyan and Katz obtained eq u i l i b r i u m between the l i q u i d and vapor phases at pressures up to 8000 pounds per square inch by c i r c u l a t i n g the vapor from the top of a s t i l l back through the l i q u i d phase. Their ap-paratus consisted of an equ i l i b r i u m c e l l placed i n a constant temperature bath from which vapors were pumped with a magnetically operated high pressure pump. Since the pump maintained a constant displacement during the c i r c u l a t i o n , no pressure f l u c t u a t i o n occurred i n the system. The vapor from the pump was passed through c o i l s i n the constant temperature bath to bring i t to the equ i l i b r i u m temperature before returning i t to the c e l l . Two b a f f l e s were placed in s i d e the c e l l to prevent any e n t r a i n --18-ment of l i q u i d i n the vapor. Samples of each phase were taken d i r e c t l y from the e q u i l i b r i u m c e l l when equ i l i b r i u m had been reached. The equi-l i b r i u m pressure was maintained during sample withdrawal by the i n j e c t i o n of mercury into the pressure control cylinfer. Cines, Roach, Hogan, and Roland*** u&eola s i m i l a r apparatus f o r deter-mining v a p o r - l i q u i d e q u i l i b r i u m at pressures up to 650 pounds per square inch. The eq u i l i b r i u m c e l l was placed i n a cryostat which i n turn was surrounded by a vacuum jacket to reduce heat t r a n s f e r from the cryostat to the surroundings. C i r c u l a t i o n of the vapor phase was obtained through the action of a mercury pi s t o n pump and two mercury valves. Fluctuations i n pressure from the pumping were l e s s than one pound per square i n c h . The vapor was passed from the pump through two p a r a l l e l l i n e s . With t h i s arrangement i t was possible to obtain samples from one l i n e and allow vapor c i r c u l a t i o n through the other. The vapor was returned to the l i q u i d phase through a tube i n which four small holes had been d r i l l e d . The vapor passing through these holes gave excellent mixing of the l i q u i d phase. Samples of the l i q u i d phase were obtained d i r e c t l y from the s t i l l . Most v a p o r - l i q u i d e q u i l i b r i u m measurements at elevated pressures have been made using one of the four types of equipment discussed above. How-ever, other types have been used by some workers. Ashley and Brown*** investigated v a p o r - l i q u i d e q u i l i b r i u m at pressures up to 220 pounds per square inch with a c e l l i n which i t was possible to c i r c u l a t e e i t h e r the l i q u i d or vapor phase or both. Samplers were provided i n the c i r c u l a t i o n l i n e s so that a portion of the equ i l i b r i u m f l u i d could be i s o l a t e d and analyzed without d i s t u r b i n g the e q u i l i b r i u m i n the c e l l . A magnetic pump, with a displacement of approximately two per cent of the volume of the - 1 9 -system,was used to c i r c u l a t e the f l u i d s . A l i q u i d - l e v e l i n d i c a t o r , con-s i s t i n g of an inverted bucket type f l o a t attached to a l i g h t i r o n stem, was used. The stem formed the core of a transformer, and with a f i x e d primary voltage, i t was possible to determine the f l o a t p o s i t i o n from the secondary voltage. The sample to be studied was charged to the e q u i l i b r i u m c e l l and, with at lea s t an inch of l i q u i d i n the bottom of the c e l l , the two phases were c i r c u l a t e d u n t i l e q u i l i b r i u m was reached. Both phases were returned to the c e l l at the bottom, thus g i v i n g intimate mixing. An an a l y s i s of each phase was then obtained by i s o l a t i n g the samplers and l e t t i n g a por-t i o n of each flow through an analysis t r a i n . Akers, Burns, and F a i r c h i l d have used a s i m i l a r apparatus at press-ures up to 1500 pounds per square inch, except that each phase was analyzed continuously. The gas mixture to be studied was passed through a compres-sor and cooling c o i l i n to a separator. The vapor and l i q u i d were then removed from the separator through d i f f e r e n t l i n e s , expanded to a pressure s l i g h t l y above atmospheric, and passed through thermal conductivity c e l l s f o r a n a l y s i s . The two streams were then recombined and fed into the com-pressor. The c i r c u l a t i o n was continued u n t i l no change of composition occurred i n e i t h e r c e l l . 33 Clark, Din, and Robb determined v a p o r - l i q u i d e q u i l i b r i u m at press-ures up to 120 pounds per square inch by a batch d i s t i l l a t i o n method. The mixture to be studied was placed i n a small copper c y l i n d e r immersed i n a constant temperature bath. The vapor phase was then s t i r r e d by means of a magnetically operated plunger. When eq u i l i b r i u m was reached, a small sample of the vapor was qui c k l y withdrawn and analyzed. The wi t h -drawal had to be performed r a p i d l y or a change i n composition would occur -20-i n the vapor phase during sampling. The contents of the c e l l were then allowed to return to e q u i l i b r i u m and the procedure repeated. When several samples had been obtained, a graph of quantity removed versus composition was p l o t t e d and the curve extrapolated to zero amount removed to f i n d the composition of the vapor i n e q u i l i b r i u m with the o r i g i n a l l i q u i d . 112 Mertes and Colburn have used a flow type apparatus to determine v a p o r - l i q u i d e q u i l i b r i a at pressures up to 100 pounds per square inch. An e q u i l i b r i u m c e l l , b u i l t from a glass-windowed s t e e l c y l i n d e r , contained the l e a s t v o l a t i l e l i q u i d of the mixture to be studied. A vapor of con-stant composition was then continuously bubbled through the e q u i l i b r i u m c e l l u n t i l the l i q u i d i n the c e l l was i n e q u i l i b r i u m with t h i s vapor. The c e l l was kept i n a constant temperature bath,and the pressure i n the system was regulated by passing the vapor from the c e l l through a condenser and then to a condensate receiver where a constant back pressure of carbon dioxide was maintained. From the above discussion i t can be seen that experimental vapor-l i q u i d e q u i l i b r i u m measurements have been made at pressures up to 10,000 pounds per square inch with.the s t a t i c type of equipment and up to nearly that pressure with the c i r c u l a t i o n type. Both types have advantages and disadvantages when used at elevated pressures. The chief advantages of the c i r c u l a t i o n type l i e i n the f a c t that i t i s r e l a t i v e l y easy to obtain e q u i l i b r i u m between the two phases. Samples of the c o - e x i s t i n g phase being studied are also easy to obtain. However, because the equipment necessary i s r e l a t i v e l y complex, t h i s type has, not been as generally used as the s t a t i c one f o r pressure measurements. Of the s t a t i c methods, the dew and bubble point one i s p a r t i c u l a r l y s u i t a b l e f o r studying binary mixtures at elevated pressures because the -21-equipment required i s very simple. Unfortunately t h i s method can be used f o r measurements i n binary systems only, a f a c t which l i m i t s i t s u s e f u l -ness very much. As w e l l , a large number of readings must be made to define the e q u i l i b r i u m composition of the l i q u i d and vapor phases. The constant volume method i s a more v e r s a t i l e one but s l i g h t l y more complex equipment i s required. A second disadvantage i s that great care must be taken to get a t r u l y representative sample of the l i q u i d and vapor phase i n e q u i l i b r i u m . With both of the s t a t i c methods, of course, i t i s much more d i f f i c u l t to obtain e q u i l i b r i u m between the phases than with the c i r c u l a t i o n methods. Treatment of Vapor-Liquid E q u i l i b r i u m Data Since a knowledge of the e q u i l i b r i u m formed between l i q u i d and vapor sol u t i o n s i s important i n modern i n d u s t r i a l processes, and since e x p e r i -mental values are d i f f i c u l t to obtain, t h i s f i e l d of thermodynamics has 15 27 105 been treated t h e o r e t i c a l l y at considerable length i n recent years ' ' 131,164^ object of t h i s study has been, p r i m a r i l y , to obtain r e l a t i o n -ships from which v a p o r - l i q u i d e q u i l i b r i u m values can be c a l c u l a t e d from a knowledge of the properties of the pure components or from a t h e o r e t i c a l measure of t h e i r i n t e r a c t i o n s . Another, and also important purpose has been to obtain methods of checking the i n t e r n a l consistency and accuracy of experimentally measured values. The thermodynamic basis of both studies i s the same and from t h i s basis by empiricism or by a s t a t i s t i c a l thermodynamic approach the two purposes have been achieved to some degree. From the point of view of t h i s research, the more important of the two i s the checking of experimental values and therefore the theory w i l l be discussed front t h i s approach. The development of the thermodynamic r e l a t i o n s h i p s given below i s essen--22-+ - H + 1 . +u + • i *> i- i 45,74,80,133 . x t i a l l y the same as that i n a number of books 7 on the subject, but has been rearranged to serve as a basis f o r the equations which fol l o w i t . In order to completely define a sin g l e phase i n which the t r a n s f e r of material can occur, as i s the case f o r e i t h e r phase i n a v a p o r - l i q u i d mixture, i t i s necessary to spec i f y the mass and the composition of the phase as w e l l as two other independent var i a b l e s such as temperature and pressure. Thus, any extensive property, such as free energy, w i l l be a functio n of each of these v a r i a b l e s or, i n symbols P - F(T,P, n i n 2 n3...) ( l ) For an i n f i n i t e s i m a l change i n the free energy the equation may be wr i t t e n c/f^iI)dT + (il)dP t(±F)dn, (2) and since - s / 2 £ \ =. V lo>T/p 0 (^ \ o)P /r.n, Ad-equation (2) has the form jr-.-SilT For the sake of convenience, the d i f f e r e n t i a l c o e f f i c i e n t s of free energy with respect to mass are often represented by f*- and c a l l e d the chemical p o t e n t i a l . Therefore equation (3) can be w r i t t e n c/f = - S J T +V</P +• JUL,tin, / JUX<*\ t • (4) dr.- -SJT + VdP iT^u- d%- ( 5 ) -23-where R J F For a closed system composed of two or more open phases i t can e a s i l y be shown that, at e q u i l i b r i u m under conditions of constant temperature and pressure, the chemical p o t e n t i a l of a component i n one phase i s equal to that of the same component i n every other phase. Since by i t s d e f i n i t i o n , the chemical p o t e n t i a l , o r p a r t i a l molal free energy, i s an intensive property, that i s , i t depends only on the r e l a -t i v e proportions or concentration of each component and not on the t o t a l amount, and since the equation i s homogeneous and of the f i r s t degree i n number of moles, equation ( 5 ) can be integrated by applying Euler's 7 4 Theorem to give under conditions of constant temperature and pressure. Zf*. nL (6) Equation (6) can now be d i f f e r e n t i a t e d to give dFrt/u^dn- ( 7 ) I f t h i s equation i s subtracted from equation ( 5 ) the f o l l o w i n g r e s u l t i s obtained. S J T * VJP-TmdK =0 ( 8 ) This r e s u l t , known as the Oibbs-Duhem equation, shows the r e l a t i o n s h i p between simultaneous changes i n temperature, pressure, and chemical poten-t i a l . I t i s often r e s t r i c t e d to conditions of constant temperature and pressure so that r ni C//U:--<D ( 9 ) The equation may be expressed i n terms of mole f r a c t i o n instead of the -24-number of moles by d i v i d i n g both sides by the t o t a l number of moles to give Z A/c d/uL *0 (10) In t h i s research the number of components i s r e s t r i c t e d to two and there-fore equation (10) can be w r i t t e n as /V,</y", f A/xdfl^--0 (11) At constant temperature and pressure the chemical p o t e n t i a l i s a function of composition only, and therefore equation ( l l ) can be w r i t t e n i n terms of p a r t i a l molal q u a n t i t i e s as follows / K / M I ' - A ^ v / J / M = o ( 1 2 ) This form of the Gibbs-Duhem equation may be w r i t t e n i n terms of f u g a c i -t i e s rather than chemica) p o t e n t i a l . By d e f i n i t i o n , the chemical poten-t i a l and f u g a c i t y are r e l a t e d by the equation d/U,-- RTd^nf, (13) and the d e f i n i t i o n i s completed by the r e l a t i o n s h i p j, ~v o A S P-'C ( 1 4 ) S u b s t i t u t i n g equation (13) into equation (12) and d i v i d i n g by ET gives M ijAif, \ •+ ^ ( J _ M | , o (15) Since NL + N = 1 and dN, = «dN , equation (15) can be rearranged as Since the a c t i v i t y c o e f f i c i e n t Y f o r any component i s the r a t i o of the a c t i v i t y and the mole f r a c t i o n f o r that component, i t can e a s i l y be shown -25-that equation (16) can also be w r i t t e n i n the form N, /cJ A Y, ) , A/z (IAlL ( I 7 V dA/, ' V d Nu ) V / which i s perhaps more useful f o r l i q u i d s o l u t i o n s . Equations (16) and (.17), when applied to the l i q u i d phase under con-d i t i o n s of constant temperature and pressure, can be used i n the d i f f e r -e n t i a l form or can be integrated to t e s t experimental v a p o r — l i q u i d equi-l i b r i u m data f o r thermodynamic consistency. When the d i f f e r e n t i a l form of the equation i s used, the values of Ad^kJj and j^?.Xj- or i f x i s used f o r N i n l i q u i d mixtures, and y f o r N i n vapor mixtures, a n t* 4-^?lX<- are found by measuring slopes from a graph on which ^ and Vu are p l o t t e d versus mole f r a c t i o n . I f the data are thermodynamically consistent, the r a t i o of the two slopes at every value of ~X. w i l l be equal to the r a t i o — at the same X . I t should also be noted that i f i t i s not convenient to p l o t the logarithm of a c t i v i t y c o e f f i c i e n t s equation (17) can be rearranged to give j^l. Jjd ^ J K (18) In t h i s case the a c t i v i t y c o e f f i c i e n t i s plo t t e d versus x»and the r a t i o of the slopes must equal -' J-In order to use equations (16), (17), or (18) to t e s t v a p o r - l i q u i d e q u i l i b r i u m data f o r thermodynamic consistency, the f u g a c i t i e s or a c t i -v i t y c o e f f i c i e n t s must be re l a t e d to experimentally measurable q u a n t i t i e s , As stated e a r l i e r , the fugacity i s rel a t e d to the chemical p o t e n t i a l by d e f i n i t i o n as follows d/u, -, Rl 4&-f (13) -26-D i f f e r e n t i a t i n g both sides of equation (13) with respect to pressure at constant temperature and compositions In an analogous proof to the one showing t h a t / — ^ ) 5 V , i t can be proved that - V, wh ere 17 i s the p a r t i a l inolal volume. Substitut-ing t h i s r e l a t i o n into equation (19) gives - % ( 2 0 ) Under conditions of constant temperature and composition dA /, - 2 d P (21) I f i s subtracted from both sides of t h i s equation, then dAf. -d&,(x,P)-- X dP x,.P m I - RT I ' Since conditions of constant composition were s p e c i f i e d d£n?t, = o a n a Integrating (22) under conditions of constant temperature from A 0 r P--P P 2.P Since at P- O , /, - z., P , the equation becomes " <e~ t -foP(iL-±)dp +J*px, ; ( 2 3 ) I f the value of V, i s known as a function of pressure at constant compo-s i t i o n and temperature, t h i s equation can be integrated and the value of found. I f the values of the p a r t i a l molal volumes are not a v a i l a b l e to a s u f f i c i e n t degree of accuracy to allow the use of equation (23), then some other method of evaluating the fugacity must be found. I f the temperature and pressure are not too near the c r i t i c a l values, the need f o r p a r t i a l molal data can be eliminated by assuming that Lewis and Randall's*^^ rul e f o r an i d e a l s o l u t i o n i s true f o r the vapor phase. This rul e states that the f u g a c i t y of a component i n an i d e a l mixture i s equal to the mole f r a c -t i o n of that component i n the mixture m u l t i p l i e d by the fugacity of the pure component at the temperature and pressure of the mixture. Expressing the r e l a t i o n s h i p i n terms of symbols (24) I f the fugacity of a component i n the vapor phase can be cal c u l a t e d using t h i s r u l e , then the desired quantity, the f u g a c i t y of the component i n the l i q u i d phase, i s known, since at equ i l i b r i u m the two are equal. In order to use Lewis and Randall's r u l e , the f u g a c i t y of the pure vapor must be known at the temperature and pressure of the mvxture. This f u g a c i t y i s found by in t e g r a t i n g equation (23) w r i t t e n f o r a pure substance. ''Jo ( }R1~?^P (25) The value of the molar volume f o r a pure vapor i s much easier to f i n d than the p a r t i a l molal volume, since i t can be ca l c u l a t e d from an equation of s t a t e . I f no p a r t i c u l a r equation i s a v a i l a b l e , the i n t e g r a t i o n can be performed using the generalized c o m p r e s s i b i l i t y chart or one of the more recent generalized methods. Once the i n t e g r a t i o n has been performed, the fugacity i n the mixture i s calculated from the product of the pure compon-ent value and the mole f r a c t i o n or . (26) Writing the Gibbs-Duhem equation f o r t h i s case gives or i n terms of a c t i v i t y c o e f f i c i e n t s where the standard state f o r a c t i -v i t y i s chosen as the pure component at the temperature and pressure of the mixture (28) The value of the f u g a c i t y of the pure l i q u i d f o r use i n equation (28) may be found i n two stages. F i r s t the fugacity of the l i q u i d at tempera-ture of the mixture and the vapor pressure i s c a l c u l a t e d from an i n t e g r a -t i o n of equation (25) between the l i m i t s of P = 0 and P = vapor pressure. The value at the pressure of the mixtur'e i s then found by i n t e g r a t i n g equation (21) w r i t t e n f o r a pure component between the vapor pressure and the s o l u t i o n pressure. The value of the l i q u i d volume as a function of pressure can be found from a generalized chart i f experimental values are not a v a i l a b l e . The two equations are now tJop ' I ( </W -') dpi stn . (29) -30-and - ... V'f (30) ruq p At low values of temperature and pressure, assumptions can be made which furt h e r s i m p l i f y the fugacity and a c t i v i t y c o e f f i c i e n t c a l c u l a t i o n s . I f the pressure i s low enough that the vapor phase obeys the perfect gas law, then the. f u g a c i t y i n t h i s phase i s equal to the p a r t i a l pressure, and equation (27) s i m p l i f i e s to (31) or J A ^ B -o/x, And equation (28) becomes - ^ ^ ( M ) The c a l c u l a t i o n of the fugacity of the pure l i q u i d at the temperature and pressure of the mixture i s also made much simpler since the f u g a c i t y of the l i q u i d at i t s vapor pressure i s equal to i t s vapor pressure. Equa-t i o n (30) can therefore be w r i t t e n a™ f - / J^Lln t Pf-vp (33) J Pvnp Rt Very often i t i s assumed that the pressure change from the vapor pressure to the pressure of the s o l u t i o n has a n e g l i g i b l e e f f e c t on the fu g a c i t y of the s o l u t i o n and that the fugacity of the pure l i q u i d i s equal to i t s vapor pressure. When t h i s i s don# the a c t i v i t y c o e f f i c i e n t becomes -31-or a measure of the devia t i o n from Raoult's law. Under these circumstances equation (28) i s w r i t t e n yt, \^^A = ^SZll^J^sA (35) c) A, c U t When the above assumptions are made, i t i s also possible to express the Gibbs-Duhem equation i n terms of t o t a l pressure rather than p a r t i a l pressures. Rewriting equation ( l l ) ~° (36) Rearranging T 3 C'-3> J I 'T (37) Experimental data can now be tested by p l o t t i n g P vs. y and comparing {^pj~> with Up to t h i s point the methods given f o r t e s t i n g vapor l i q u i d e q u i l i -brium data have a l l been based on the d i f f e r e n t i a l form of the Gibbs-Duhem equation. In order to use t h i s form of the equation, the d e r i v a t i v e s must be obtained by measuring slopes or from an equivalent procedure. Since the measurement of slopes i s u s u a l l y subject to a high degree of e r r o r , an integrated form of the equation i s often used. The most accurate type of integrated equation i s the one i n which the value of one a c t i v i t y c o - e f f i c i e n t i s cal c u l a t e d from the measured value of the other and the calculated and experimental values then compared. 47 / One such method i s suggested by Dodge . Equation (11) i s w r i t t e n i n the -32-f orm 7-j c6^i Yt + ( ) d s&t " C (38) Rearranging the equation Integrating between x^ = o and = x At x^ = 0 the a c t i v i t y c o - e f f i c i e n t =/ and the equation bee (39) omes ^ h-- -f*"(-7=krU^r, J X - f% A graphical i n t e g r a t i o n of (39), using values of w i l l y i e l d values of Xx. , to compare with the measured ones. > • ' • Although the value obtained f o r the a c t i v i t y c o e f f i c i e n t from equa-t i o n (39) i s exact under conditions of constant temperature and pressure, i t can be calculated only i f a measured value f o r the other a c t i v i t y co-e f f i c i e n t i s a v a i l a b l e , and the equation therefore i s of l i t t l e value f o r the p r e d i c t i o n of v a p o r - l i q u i d e q u i l i b r i u m . For t h i s reason, many approx-imate solutions of the Gibbs-Duhem equation have been proposed. M a r g u l e s * ^ suggested a s o l u t i o n of the form J-fx*- " ( 4 9 ) •Vr 4 - ( 4 1 ) . I f enough terms are included the s o l u t i o n w i l l be exact but generally, to avoid undue complication, only the f i r s t three terms are used. When -33-the two equations are substituted into the Gibbs-Duhem equation, the f o l l o w i n g r e l a t i o n s h i p s are found 27 Carlson and Colburn rearranged the constants on the basis of these"* e q u a l i t i e s to give • Jn t, = (Z8-n)0-zft Z(»9-0)(!-^) 3 ( 4 2 ) X * Y V , ( Z A - e ) * 1 - + 7.C 6-19) z 3 ( 4 3 ) I t can be e a s i l y seen that at t £m /ft and that at T-^i I f graphs of s&nYi , and VV. are p l o t t e d versus mole f r a c t i o n * - , then the value of the two constants can be found from the end values of the curves. To check the thermodynamic consistency of the experimental data, the a c t i v i t i e s are cal c u l a t e d using equations (42) and (43) with the meas-ured constants A and B, and the. r e s u l t s compared to the experimental values. Since the constants i n the Margules equation are functions of temper-ature, the values ca l c u l a t e d f o r one temperature can not be used at any other temperature. To extend the usefulness of the equation, Robinson and 133 G i l l i l a n d suggest that the constants be taken as proportional to the one-fourth power of the absolute temperature. Thus i f the values of A and B are known at one temperature, the p r o p o r t i o n a l i t y constant can be c a l -culated, and the values found at any other temperature. Probably the best known s o l u t i o n of the Gibbs-Duhem equation ,is that' 97 -I; proposed by Van Laar . The s o l u t i o n was o r i g i n a l l y put forward as the r e s u l t of a'theory based on the Van der Waals, equation of s t a t e d and -34-although the theory i s probably i n e r r o r , the equation i s a useful empiri-c a l representation of the data. The equation has been rearranged by 27 Carlson and Colburn into the form /• y ft ^ }' ' (I + ft*, ^  (44) X n r v = _J> (45) As with the Margules equation, the constants can be evaluated from the values of the a c t i v i t y c o - e f f i c i e n t s at x = 1 and x = 0. Experimental data i s checked f o r thermodynamic consistency with t h i s equation using the pro-cedure described e a r l i e r . A f t e r the constants are calc u l a t e d from the end values of experimental curves, the equation i s solved f o r a c t i v i t y co-e f f i c i e n t s using these constants, and the experimental and calculated values are compared. Because of the form of the Van Laar s o l u t i o n , a very quick q u a l i t a t i v e check of the data can be made. When the mole f r a c t i o n equals .5, equations (44) and (45) can be w r i t t e n A^n r, . ^ K ; . AQ_ ( 4 6 ) I f A equals B then ^ ^ j ^ j ' - equals while i f A = 2B or ^ B, the r a t i o de-2 creases Thus the half-way value on one curve should equal approx-imately \ of the end value on the other curve i f the data i s consistent and i f the Van Laar equation a p p l i e s . 133 Robinson and G i l l i l a n d have modified the Van Laar equation to i n -clude a temperature term and thus extend i t s usefulness. When t h i s i s done, the so l u t i o n s have the form s^K ~- B'/T (47) -35-<&»X = BloiZ. (48) Using a s i m i l a r development to that of Van Laar, Scatchard and 138 Hamer have developed solutions of the Gibbs-Duhem equation i n v o l v i n g the molar volumes of the pure components. The constants have again been 27 rearranged by Carlson and Colburn to give where v^ and v^ are the molar volumes and i, i s the volume f r a c t i o n of component one given by / = „?/ ^  • (51) The constants i n t h i s rearranged form can be found from the end value of the curves and the equation used to check thermodynamic/ consistency i n the same manner as the Margules and Van Laar solutions are used. 164 Wohl has shown that the Margules, Van Laar, and Scatchard and Hamer solutions are a l l p a r t i c u l a r cases of a more general s o l u t i o n hased ID on the excess free energy. The excess free energy, F , has been defined 138 by Scatchard and Hamer as the difference between the free energy of o^t an mixing f o r a r e a l and Aideal s o l u t i o n . The free energy of mixing 4/>, , i s the difference between the free energy of the pure components and that of the s o l u t i o n . Thus 4 F m c ^77; F[ - T Hi (52) Now -36-A Therefore T From the d e f i n i t i o n z A. Ffy, r c o ( - >a (do/ F€ -- RT Z 77< XR,YC (55) The free energy of a mixture can now be w r i t t e n as F= T71i F; i RTT-n; *L / Fe and the chemical p o t e n t i a l as (53) (54) (56) Rearranging Fi " K - R T X , , ^ -f- JF~ Rearranging again <JF': - RTsCr* Yi (57) o> 7)-164 The fo l l o w i n g general equation i s used by Wohl to express the excess free energy _Fl - T. iL ih bih t J 2; i ij 1>L>,- , (58) where « e f f e c t i v e molal volume of component 2 i = e f f e c t i v e volume f r a c t i o n 6 = an empirical constant and each summation represents molecular i n t e r a c t i o n s . D i f f e r e n t i a t i n g with respect to ?7, and ^ f o r a three s u f f i x equation where fl= 9, V * 3 and 8 ^ ( ^ 6 , ^ + 3 ^ , 1 ^ (60) ^ ( R T ) ^ ^ C< L ^ - v . ^ - o y ^ j ( 6 1 ) I t can now be shown that i f - then the two equations reduce to those of Margules. I f ®lB the Van Laar solutions are obtained and i f 9</9 = the equation becomes that proposed by Scatchard and Hamer. 130 Redlich and K i s t e r have developed an expression based on the Gibbs-Duhem equation r e l a t i n g the composition and temperature. The d e r i -vation assumes that the changes of volume accompanying the isothermal mix-ing of the l i q u i d components and of the gaseous components are n e g l i g i b l e and that the equation of state of the gaseous components can be repres-ented i n the form J p where B depends only on temperature. When these assumptions are made they have shown that SLL S (62) where "s" the slope f a c t o r i s given by -38-^ O' • H 3 U 3  or t d t The pressure terms P^ and P^ are f a c t o r s to take into account the change of f u g a c i t y of the components i n the l i q u i d phase with temperature and t o t a l pressure,and a method i s given f o r evaluating them. The authors believe that the equation i s considerably more s e n s i t i v e and more convenient to use than the usual forms of the Gibbs-Duhem equation. 131 The same two authors have also derived a thermodynamically correct equation r e l a t i n g the concentration and a c t i v i t y c o e f f i c i e n t s i n an i n -tegrated form of equation. As shown i n equation (55) f o r a molar s o l u t i o n F e- ATE xt X , ft By d e f i n i t i o n (64) F K For ajbinary misure d Q - s&x* £• dx i -x,d ^ K t x^d ft ( 6 6 ) J h From the Gibbs-Duhem equation and therefore Now at i = 0, ^ = 1, and Q = 0 and at x = 1, Yf = 1, and fi = 0, and i n -t e g r a t i n g (67) between x = 0 and x = 1 gives I /6»<j d x = o (68) -39-I t follows from equation (68) that a graphical i n t e g r a t i o n of with data obtained at constant temperature and pressure must equal zero i f the data i s therraodynamically consistent. 19 Broughten and Brearly* have developed a s i m i l a r expression to that of Redlick and K i s t e r with the change that the Gibbs-Duhem equation i s w r i t t e n as •X, d ( T,£yj X, ) + 7L d C T Joy JfJ =0 to t r y to correct f o r non-isothermal data.- When t h i s c o r r e c t i o n i s applied the i n t e g r a l becomes // TA^j *1 dx -O (69) These authors, on the basis of t h i s equation, have derived a r e l a t i o n s h i p f o r c o r r e c t i n g inconsistent experimental data where the inconsistency i s caused by conditions i n the equ i l i b r i u m s t i l l such that <*co*- = ^ o b i (70) where c^C o Y. i s the correct r e l a t i v e v o l a t i l i t y , c / 0 ( ) s i s the observed r e l a t i v e v o l a t i l i t y and s i s the s t i l l f a c t o r . Combining equations (69) and (70) gives 0:(\T^ £)dx ~-ij'(r^h)d* + Lzs 7 - ^ 7 ^ ( 7 1 ) JO <Iu'Co>r -O 'I obi 3 The equation i s solved g r a p h i c a l l y and the value of s to make the equation true c a l c u l a t e d . 15 Benedict et a l have derived a r e l a t i o n s h i p f o r c o r r e l a t i n g vapor-l i q u i d e q u i l i b r i u m data of the f i r m p RT ~ where if^ i s the molar volume of the i t h component i n the l i q u i d s t a t e . -40-The r e l a t i o n s h i p i s dependent upon the assumption that the equation of state of the vapor phase i s and that there i s no change i n volume on mixing the constituents of the l i q u i d phase. The authors recommend evaluation of the a c t i v i t y c o - e f f i c -164 i e n t s by the four s u f f i x equation of Wohl and the use of t h i s r e l a t i o n -ship f o r multicomponent systems. 105 Marek and Standart have found that an attempt to c o r r e l a t e vapor-l i q u i d e q u i l i b r i u m data of mixtures containing a aibstance which p a r t l y associates to form a diiner i n both phases leads to thermodynamically i n -consistent r e s u l t s i f the a s s o c i a t i o n i s not taken into account. The authors have developed an e q u i l i b r i u m r e l a t i o n s h i p , analogous to Raoult's and Dalton's law, f o r such a case which states that f o r the a s s o c i a t i n g com-ponent and f o r the non-associating one *z.^L P ^ 'I ^ (76) where 2( i s a c o r r e c t i o n f a c t o r f o r vapor phase a s s o c i a t i o n of 1 t i s a vapor phase non-idealty f a c t o r f o r 1 o P, i s the hypothetical vapor pressure of pure monomer 1 C i s a c o r r e c t i o n f a c t o r f o r l i q u i d phase a s s o c i a t i o n of 1 X i s a l i q u i d phase non-idealty f a c t o r f o r 1 Equations are given f o r evaluating each of the c o r r e c t i o n f a c t o r s . When equations (75) and (76) are used f o r the e q u i l i b r i u m r e l a t i o n s h i p , the thermodynamic consistency of the data can be checked i n the usual manner -41- " except that and fu !fL are used f o r a c t i v i t y c o - e f f i c i e n t s . Many authors have attempted to c o r r e l a t e v a p o r - l i q u i d e q u i l i b r i u m data by means of e n t i r e l y empirical equations. These equations generally have no t h e o r e t i c a l basis but have the advantage that they are e a s i l y applied and are often used f o r engineering purposes. 32 One of the best known of these empirical equations i s that of Clark . He suggests that the r a t i o of the mole f r a c t i o n i n one phase i s a l i n e a r function of the r a t i o of the mole f r a c t i o n i n the other phase when the r a t i o s are u t i l i z e d such that the component inthe largest amount appears i n the numerator. Thus, when component one i s present i n the lar g e s t amount J*. " ^ and when component two i s present i n the lar g e s t amount 2i r '3= -f B' (78) The point at which equation (78) i s used instead of equation (77) i s given by 3- = J^B/AB' (79) 95 Kretschmer and Wiebe from t h e i r work on the ethanol-toluene and ethanol-iso-octane systems have suggested a r e l a t i o n s h i p f o r alcohols i n hydrocarbons or other symetrical non-polar molecules such as carbon t e t r a -c h l o r i d e . Their equation i s of the form ' /?:8v  z A d M = ( ^ y - c ) ( / - z c + c *,) (80) 122 Prahl has proposed using the equation The three empirical constants can be evaluated using a graphical method -42-r e q u i r i n g one experimental point of known accuracy. 55 Eshaya studied the p o s s i b i l i t y of representing the data i n a power se r i e s of the form He found that normally three but sometimes four terms were necessary f o r accuracy to a few per cent. 169 Yu and C o u l l made use of an expression of the form J L a P (JL\e i (83) This equation has the advantage that the empirical constants can be simply evaluated from a log-log p l o t of the molar r a t i o s . However, the f a c t that molar r a t i o s are involved makes the equation i n v a l i d f o r d i l u t e s o l u -tion£>. 77 H i r a t a found that most e q u i l i b r i u m data could be represented by u x three s t r a i g h t l i n e s on a log-log p l o t . He p l o t t e d -pL versus 7^ on l o g -u log paper and found a s t r a i g h t l i n e of c h a r a c t e r i s t i c slope over the cent-r a l portion of the curve and l i n e s of slope one at each end of the curve. 82 Johnston and Furter developed a s i m i l a r expression to that of Yu 169 and Coull except that only the numerator rather than the e n t i r e mole „* f r a c t i o n i s raised to some c h a r a c t e r i s t i c power. Expressed i n terms of symbols, the r e l a t i o n s h i p becomes _ i = (84) /'J /-*• Norrish and Twig have proposed a r e l a t i o n s h i p f o r binary mixture where water i s not one of the components. The equation recommended i s K k * x> I C (85) V where K i s the r a t i o of the molar volumes, M i s an a r b i t r a r y constant and C i s a known function of the laten t heats and b o i l i n g points of the pure components. A r e l a t i o n s h i p i s given from which M at one pressure can be found from the value at any other pressure. 131 Kedlich and K i s t e r have developed an equation of the form The r e l a t i v e importance of some of the constants has been re l a t e d to the degree of a s s o c i a t i o n of the components. I t can be seen from the discussion to t h i s point, that i f an i n t e -grated form of the Gibbs-Duhem equation i s used to t e s t experimental vapor-l i q u i d e q u i l i b r i u m data, there w i l l be some question as to whether any dev-i a t i o n i n the data from that predicted by the equation i s due to inaccura-c i e s i n the data or to the f a c t that the equation does not apply. For t h i s reason the integrated form has i t s chief importance i n the p r e d i c t i o n of data. However, before using one of the equations f o r the p r e d i c t i o n pur-poses, some check must be made to s e e A i t f i t s the system under considera-t i o n at l e a s t reasonably w e l l and t h i s check i s most e a s i l y made by using 133 the equation to t e s t experimental data. Robinson and G i l l i l a n d have 155 109 given the r e s u l t s of Tucker and Mason of a t e s t of four of the i n t e -grated forms f o r the benzene n-propanol system. The data used, which was 99 probably that of Lee , was f i r s t screened to see that i t gave good agree-ment with the d i f f e r e n t i a l Gibbs-Duhem equation. To give a q u a n t i t a t i v e 109' estimate of the agreement, Mason defined the percentage d e v i a t i o n as Percent Deviation = I ^ IzJ^llt 1 <3-He c l a s s i f i e d the agreement as good when the average percent deviation was l e s s than 5f>, f a i r f o r a d e v i a t i o n of from 5 to 11^ and poor f o r a devia--44-t i o n of greater than 11$. The r e s u l t s of the t e s t s are given as f o l l o w s : C l a s s i f i c a t i o n F a i r Poor Poor Good Shemilt and S i n g h h a v e found the percent d e v i a t i o n i n values c a l -81 culated from the Van Laar equation from that measured by Howey f o r the va p o r - l i q u i d e q u i l i b r i u m of the benzene-n-propanol system at 740 m i l l i -l i t r e s of mercury t o t a l pressure. They obtained, using the d e f i n i t i o n proposed by Mason, a maximum devia t i o n of 31.8$ and an average deviation of 10.3$ when the constants f o r the Van Laar 4equation were evaluated from azeotropic data. The agreement between the data and the equation, accord-ing to the above c l a s s i f i c a t i o n , i s only f a i r . Shemilt and Singh have also tested the data of Howey and the i s o t h e r -mal 40°C. data of Lee with the equation of Broughton and Brearly (equation 7S|). They found a slope f a c t o r of .9688 f o r the former's data and one of 1.0092 f o r the l a t t e r ' s . 159 Weke and Coates have also measured the v a p o r - l i q u i d e q u i l i b r i u m f o r the system benzene n-propanol at a pressure of one atmosphere. The data was checked f o r thermodynamic consistency by comparing the values of ^ consistent with-^Jf, , to the values measured f o r ^ 1 ^ t The two sets of values are plo t t e d on the same graph, and although no f i g u r e i s given f o r the average deviation the agreement between the two seems very good. 96 Kumarkushna et a l have measured the v a p o r - l i q u i d e q u i l i b r i u m of the benzene-propanol system at elevated pressures. Measurements were made at eight pressures ranging from 44.7 to Equation Max. jo Dev. Avg. $ Dev. Margules 53.3 8.6 - Scatchard 68.4 11.6 Van Laar 18.8 17.8 Clark 16.3 4.7 141 -45-309.7 pounds persquare inch and the data obtained was co r r e l a t e d with a three-constant E e d l i c h and K i s t e r equation. -46-Table of Symbols A, A 1 A r b i t r a r y constants i n various equations a a c t i v i t y B a r b i t r a r y constants i n various equations B* a r b i t r a r y constants i n various equations b empirical constant i n Wohl's equation C arbitrary constant i n various equations D a r b i t r a r y constant i n various equations F free energy F p a r t i a l molal free energy excess free energy F* /\ F^ free energy of mixing f f u g a c i t y f° fu g a c i t y of pure component K molar volume r a t i o M a r b i t r a r y constant i n various equations N mole f r a c t i o n n number of moles P t o t a l pressure P vapor pressure vap p p a r t i a l pressure p. K e d l i c h and K i s t e r function q e f f e c t i v e molal volume E gas constant S entropy s E e d l i c h and K i s t e r slope f a c t o r s Broughton and Brearley s t i l l f a c t o r - 4 7 -T temperature V volume V p a r t i a l molal volume v molar volume x mole f r a c t i o n i n the l i q u i d phase y mole f r a c t i o n i n the vapor phase Z e f f e c t i v e volume f r a c t i o n Greek Symbols ^ r e l a t i v e v o l a t i l i t y % a c t i v i t y c o e f f i c i e n t 4 a r b i t r a r y constant i n Margules' equation £ a r b i t r a r y constant i n Margules* equation /U chemical p o t e n t i a l Subscripts I component I 2 component 2 3 component 3 h component h i component i J component j c a l c a l c u l a t e d cor correct exp experimental i d e a l i d e a l f l u i d l i q l i q u i d phase obs observed r e a l r e a l vap vapor -48-MATERIALS Benzene A reagent grade of benzene, supplied by Baker and Adamson, was p u r i -f i e d f o r use i n t h i s research. The manufacturers c e r t i f i e d i t as being thiophene-free and meeting ACS s p e c i f i c a t i o n s . Lot properties were given as f o l l o w s : B o i l i n g range 0.5°C. max. B o i l i n g point at 760 mm. of mercury 80.1°C. max. Freezing point 5.2°C. min. Maximum Limit of Impurities Residue a f t e r evaporation JOOlfi Substances darkened by H^SO^ to pass t e s t Thiophene to pass t e s t Sulphur components (as S) 0.005^ Water to pass t e s t The i n i t i a l p u r i f i c a t i o n of the benzene was based on methods reported by Gilmann and G r o s s ^ , Gornowici, Anick and H i x o n ^ , and Tompa*^. One l i t r e of benzene was shaken f o r 10 minutes i n a 2 - l i t r e separatory funnel with 250 m i l l i l i t r e s of Nichols reagent grade s u l f u r i c a c i d* The purpose of t h i s acid wash was to sulphonate and remove any thiophene or t&iophene-l i k e substances present i n the solvent. A f t e r t h i s mixing, the acid was allowed to s e t t l e out f o r 20 minutes, and then drained out through the bot-tom of the funnel. Since the ac i d turned a pale yellow color during the shaking, the above procedure was repeated. The second volume of a c i d , which was l e f t uncolored by the benzene, was also discarded, and the s o l -vent washed twice with 500 m i l l i l i t r e .portions of d i s t i l l e d water. In each case the mixture was ag i t a t e d f o r at l e a s t 10 minutes and allowed to -49-s e t t l e f o r at le a s t 20. The benzene was then shaken with two 500 m i l l i -l i t r e portions of 0.1 normal NaOH. The caustic s o l u t i o n , which was made from Baker and Adamson's reagent grade sodium hydroxide, was used to remove the l a s t traces of s u l f u r i c acid and also any weak acids such as hydrogen sulphide or mercaptans which might be dissolved i n the benzene. A f t e r the benzene was washed twice more with d i s t i l l e d water, i t was shaken with 100 m i l l i l i t r e s of t r i p l y - d i s t i l l e d mercury to remove any remaining s u l f u r compounds. The mercury was l e f t i n contact with the benzene f o r several hours to allow ample time f o r reaction before i t was poured o f f through the bottom. I t was found that a d u l l grey«»colored powder formed oh the mercury-benzene i n t e r f a c e and much of i t remained i n the funnel a f t e r the mercury was removed. F i n a l l y the benzene was washed four times with d i s t i l l e d water, but even a f t e r these washings some of the powder remained i n the s o l -vent. A f t e r the fourth washing,the benzene was poured through the top of the funnel into a glass-stoppered f l a s k . The solvent was poured from the top rather than the bottom to prevent contamination with any water that might remain i n the funnel stem. Care was taken to see that the grey pow-der from the mercury treatment was l e f t i n the funnel and not t r a n s f e r r e d as w e l l . In order to remove any water dissolved i n the benzene, calcium chips were added and the solvent allowed to s i t f o r a week. The stopper on the f l a s k was l e f t p a r t l y open to l e t evolved hydrogen escape. The glass ware used i n the drying and i n a l l subsequent operations was f i r s t c a r e f u l l y cleaned and d r i e d . I t was immersed f o r 24 hours i n chromic a c i d , then rinsed f o r 24 hours with tap water, and rinsed again 3 or 4 times with d i s t i l l e d water. A f t e r cleaning i t was d r i e d , e i t h e r i n an oven set f o r 220°F. or else i n a stream of a i r which was f i r s t passed through a glass wool f i l t e r , then a s i l i c a gel dessicant, and f i n a l l y powdered phosphorous pentoxide. -50-The s t i l l used f o r the d i s t i l l a t ion of the dri e d benzene was an "Ace Glass" 25 m i l l i m e t r e vacuum-jacketed column packed to a depth of 35 inches with 4 mi l l i m e t r e glass h e l i c e s . The glass h e l i c e s were packed 28 into the column a few at a time as recommended by Carney . The s t i l l pot consisted of a 2 - l i t r e roundbottomed f l a s k connected to the s t i l l through a ground glass j o i n t and heated by a "Glass-Col" e l e c t r i c heater. Heat supplied to the s t i l l pot was c o n t r o l l e d by means of a small v a r i a b l e auto transformer. The r e f l u x r a t i o was controlled with a "Galena" brand vacuum-jacketed s t i l l head which was also connected to the column with a ground glass j o i n t . With t h i s head, condensate flow was supposed to be con t r o l l e d by means of an electromagnet and timing device. When the timer turned the magnet on, the condensate was to go to the d i s t i l l a t e receiver, and when the magnet was o f f the flow was to return down the column. How-ever, i t was found that t h i s method of control did not work s a t i s f a c t o r i l y because vapor passed continuously out of the head to the d i s t i l l a t e r e -cei v e r . Best control was given by adjusting the p o s i t i o n of a stopcock placed i n the l i n e to the d i s t i l l a t e r e c e i v e r . Since benzene picks up atmosphere moisture very e a s i l y , a l l parts of the s t i l l which were open to the atmosphere were sealed with a s i l i c a gel dessicant. Two l i t r e s of the p u r i f i e d benzene were charged to the s t i l l pot with f r e s h calcium turnings. This benzene was b o i l e d at t o t a l r e f l u x f o r at leas t 12 hours and then c o l l e c t e d at a r e f l u x r a t i o of 20 to 1. The f i r s t 300 m i l l i l i t r e s were discarded and at le a s t 300 m i l l i l i t r e s were l e f t i n the s t i l l pot at the conclusion of the d i s t i l l a t i o n . The purity'.of the benzene Avas checked i n three ways. Measurements of the b o i l i n g point, the fr e e z i n g point, and the r e f r a c t i v e index were taken on the d i s t i l l e d solvent and compared with values taken from the -51-l i t e r a t u r e and shown i n Table I I . This table does not represent a com-149 plete compilation, but i t does contain those values that Timmermanns 132 and Riddick and Toops selected as most r e l i a b l e as w e l l as many s e l e c -7 ted by the American Petroleum I n s t i t u t e Project 44 . Since the American Petroleum I n s t i t u t e gives a less s e l e c t i v e l i s t of references than e i t h e r of the other sources, references were taken from i t only f o r the period of time not covered by the other two workers. The b o i l i n g and condensation temperatures of the benzene were meas-148 ured with a Swietoslawski d i f f e r e n t i a l ebulliometer. The ebulliometer, which i s s u i t a b l e f o r measuring e b u l l i o m e t r i c degree-of-purity as w e l l as b o i l i n g and condensation temperatures, was constructed according to the 14 standard s p e c i f i c a t i o n s of Barr and Anhorn . I t consisted b a s i c a l l y of a b o i l e r with a thermometer we l l and drop counter, an unpacked r e c t i f y i n g column, a condensation temperature element with a thermometer we l l and drop counter, and a condenser. As with the d i s t i l l a t i o n column, the top of the condenser was sealed with s i l i c a gel dessicant. The ebulliometer, except f o r the drop counter and l e v e l i n d i c a t i n g bulb, was covered f i r s t with asbestos rope and then with wet powdered asbestos f o r i n s u l a t i n g purposes. The b o i l i n g tube was wrapped with a length of nichrome wire and the heat input was c o n t r o l l e d with a v a r i a b l e auto-transformer. The thermometer wel l s on the ebulliometer were f i l l e d with mercury to a depth s u f f i c i e n t to cover the thermometer bulb and then to the top with o i l . These wells were b u i l t up with cork and i n s u l a t i o n so that the thermometer was immer-sed to the bottom of i t s s c a l e , e l i m i n a t i n g the d i f f i c u l t y u s u a l l y found i n making stem corrections i n Beckmann thermometers. The Beckmann thermometer used with the ebulliometer had 100 d i v i s i o n s per degree. I t and a l l other mercury-in-glass thermometers used were -52-c a l i b r a t e d i n a constant temperature bath against a Leeds and Northrup platinum resistance thermometer with a 1955 NBS c e r t i f i c a t e . The constant temperature bath consisted of an o i l - f i l l e d glass vessel covered with min-er a l woo} i n s u l a t i o n . I t s temperature was maintained with two heaters c o n t r o l l e d by v a r i a b l e auto—transformers. One heater was on continuously and was adjusted so that the hea t input was s l i g h t l y l e s s than the heat loss while the other was operated by a mercury thermoregulator and r e l a y combination. The heat input from the second heater was kept as low as possible to give the best control of the bath temperature. The bath was kept at a uniform temperature by means of a small v a r i a b l e speed s t i r r e r . During the c a l i b r a t i o n , the Backmann thermometer was kept immersed to the same depth as i t was i n the ebulliometer. Since the b o i l i n g point of benzene i s s e r i o u s l y affected by traces of moisture, care was taken that as l i t t l e contamination as possible occur-red between the d i s t i l l a t i o n and the b o i l i n g point t e s t . The d i s t i l l e d benzene was c o l l e c t e d from the d i s t i l l a t e r eceiver i n a 500 m i l l i l i t r e f l a s k which had f i r s t been flushed out with dry a i r and which was kept sealed with a tube of s i l i c a gel dessicant while the solvent was stored i n i t . The benzene was transferred to the ebulliometer by d i s p l a c i n g i t from the f l a s k with a i r which had f i r s t passed through the dessicant. To be c e r t a i n that a l l moisture had been removed from the benzene, the b o i l i n g and condensation temperatureswere measured; then 5 m i l l i l i t r e s of the ben-zene d i s t i l l e d o f f and the temperatures measured again. I f there was a s i g n i f i c a n t amount of water i n the benzene,some of i t would be removed and the change i n composition r e f l e c t e d i n a change i n the b o i l i n g point. Although the Beckmann thermometer used can be read to j^oo ne i t h e r the b o i l i n g point nor the d i f f e r e n c e between the b o i l i n g and con-densation temperature can be determined that accurately. In order to correct the b o i l i n g temperature measured to that at one atmosphere, the pressure at which i t i s measured must be known to the nearest .01 m i l l i -l i t r e of mercury. Since the pressure i n the b u i l d i n g can be measured only to the nearest .1 m i l l i m e t r e , the temperature can be corrected only to the 1 o nearest -JOQ C. A s i m i l a r although l e s s serious d i f f i c u l t y occurs with the determination of the differ e n c e between the b o i l i n g and condensation temperatures. The two temperatures are determined with the same thermo-meter and thus cannot be measured at the same time. I t i s assumed that the atmospheric pressure remains constant during the ten minutes that are required to determine the two temperatures, but i t i s l i k e l y that the pres-sure does change enough to cause a s i g n i f i c a n t e rror i n the d i f f e r e n c e . The f o l l o w i n g are the r e s u l t s obtained with the ebulliometer described above: B o i l i n g point 80.07°C. Difference between b o i l i n g and condensation temperatures .005 C. Af t e r d i s t i l l i n g o f f 5 m i l l i l i t r e s of solvent B o i l i n g point 80.08°C. Difference between b o i l i n g and condensation temperatures .004 C. The f r e e z i n g point of the benzene was determined with a f r e e z i n g 104 point apparatus s i m i l a r to that used by Rossini and co-workers and i s shown i n Figure 1. An unsilvered double-walled dewar f l a s k which could be evacuated through a stopcock was centred i n a brass c y l i n d e r by means of cork c o l l a r s placed at the top and bottom of the f l a s k . The c y l i n d e r was supported i n s i d e a glass vessel by a metal stand. Heat t r a n s f e r through the glass ware was reduced by 2 inches of mineral wool i n s u l a t i o n . The -54-temperature i n s i d e the dewar was measured with a platinum thermometer held i n place by a cork s e a l i n g the end of the f l a s k . Heavy wire, bent i n the form of a s p i r a l around the thermometer, was used to s t i r the benzene when readings of temperature were made. When the f r e e z i n g point of the benzene was measured, the space bet-ween the brass c y l i n d e r and glass vessel was f i l l e d with crushed i c e . Benzene was then added to the dewar f l a s k to a depth s u f f i c i e n t to cover the c o i l e d portion of the resistance thermometer. Care was taken that there was as l i t t l e opportunity as possible f o r the benzene to absorb moisture while i t was being poured into the f l a s k . I t was found that a s a t i s f a c t o r y rate of cooling was obtained with the f l a s k l e f t unevacuated. The benzene was cooled at approximately ,06°C. per minute, and readings of resistance were started about 40 minutes before the benzene began to freeze and taken f o r about 20 minutes afterwards. The f r e e z i n g point, determined by ex t r a -p o l a t i n g the cooling curve f o r the f r e e z i n g benzene back to the one f o r the l i q u i d , was found to be 5.49°C. The r e f r a c t i v e index of the p u r i f i e d benzene was measured with a P u l -f r i c h RefTactometer. This refTactometer, supplied by Adam Helger L t d . , i s , according to the manufacturers, accurate to one u n i t i n the fourt h decimal place of the r e f r a c t i v e index. The temperature was maintained by water pumped through the prism and around the benzene from a constant temperature bath. The temperature of the water stream was measured with a mercury-in-glass thermometer c a l i b r a t e d against a platinum resistance thermometer. The value determined f o r the r e f r a c t i v e index was 1.4879 at 25°C. Mercury A t e c h n i c a l grade of commercial mercury was p u r i f i e d using procedures 79 recommended by Sanderson 1 3 6 and the Handbook of Chemistry and Physics . -55-The f i r s t step i n the p u r i f i c a t i o n was the removal of surface d i r t by passing the mercury through a funnel i n which the stem.had been drawn out to form a small j e t . The cleaned mercury was then placed i n a f l a s k and f i l t e r e d a i r bubbled through i t f o r 24 hours to o x i d i z e any dissolved mat-e r i a l s such as the a l k a l i metals, z i n c , copper, or lead. The surface of the mercury was kept covered with a frequently changed 1$ s o l u t i o n of n i t r i c a c i d during t h i s operation to a i d i n the removal of the i m p u r i t i e s . The oxides formed rose to the surface as a scum, and were removed by again passing the mercury through a small j e t . The mercury was*next washed three times i n a 10$ NaOH scrubber to d i s s o l v e any grease. The scrubber consis-ted of a column of glass tubing 3 centimeters i n diameter and 110 c e n t i -meters high,which was sealed at the bottom with a mercury t r a p . The mer-cury was poured into the top of the column through a length of c a p i l l a r y tubing so that i t f e l l through the caustic s o l u t i o n i n a f i n e spray. A f t e r being washed i n the NaOH tower,it was passed through a s i m i l a r one contain-ing 10$ HN0o to remove the l a s t traces of the base metals, and f i n a l l y o through one containing d i s t i l l e d water. The mercury from the water scrub-ber was b l o t t e d with f i l t e r paperto remove any surface water and t r a n s f e r -red to a vacuum s t i l l i n which i t was d i s t i l l e d three times to remove any traces of the noble metals or t i n . n-Propanol The n-propanol that w i l l be used i n t h i s research was supplied by the Fisher S c i e n t i f i c Company. I t was c e r t i f i e d as being of reagent grade, and l o t properties were given as f o l l o w s : A c i d i t y (CHgCOOH) 0.002$ B o i l i n g Range 96.0° - 97.5°C. Non-Volatile Matter 0.000$ Substances p r e c i p i t a t e d by H20 None -56-Th e procedure recommended f o r the p u r i f i c a t i o n of n-propanol i s based 93 17 91 on techniques used by Kertschmer , Berner , and Keyes and Winninghoff Kretschmer found that the p r i n c i p a l impurity i n commercial n-propanol was a l l y l alcohol and that i t can be removed by shaking each l i t r e of the s o l -vent with 15 m i l l i l i t r e s of bromine. I f a separatory funnel i s used f o r t h i s operation, the l i q u i d s can be separated by running the bromine out of the bottom of the funnel and pouring the n-propanol from the top as was done i n separating the benzene-water mixture. The propanol should be stored i n glassware cleaned and dried as previously described, and l e f t over anhydrous potassium carbonate f o r several days to remove any dissolved water. The alcohol can be furt h e r p u r i f i e d by d i s t i l l a t i o n and the same pro-cedure and column can be used as were i n the benzene p u r i f i c a t i o n . When changing the alcohol to the s t i l l pot,fresh anhydrous potassium carbonate should be added as w e l l . Since pure n-propanol i s e a s i l y o x i dized to the 22 aldehyde , nitrogen must be bubbled through the column during the d i s -t i l l a t i o n . Commercial grade nitrogen can be p u r i f i e d f o r t h i s purpose by passing i t f i r s t through two bubblers containing a l k a l i n e sodium hydro-sulp h i t e with a trace of sodium anthroquinone -sulphonite (Fieser's s o l -u t i o n ) , then through one containing concentrated H^SO^ to remove any water vapor or caustic s o l u t i o n entrained i n the gas, and f i n a l l y through a glass wool t r a p . The deoxygenating s o l u t i o n i s prepared by d i s s o l v i n g 150 grams of caustic soda i n a l i t r e of water, adding 2 grams of sodium anth-roquinone -sulphonate and allowing the mixture to cool i n a stream of nitrogen. A f t e r i t cools 100 grams of sodium hydrosulphite are added and the s o l u t i o n shaken w e l l . The middle s i x t y percent of the propanol i s c o l l e c t e d and any moisture -57-s t i l l remaining i n the solvent i s removed by s t o r i n g i t over magnesium ribbon f r e s h l y polished with s t e e l wool. Any aldehyde produced by the bromine treatment and not removed i n the d i s t i l l a t i o n , a s w e l l as any formed subsequently, can be removed by adding a l i t t l e 2,4-dinitrophenylhydrazine to the a l c o h o l . A f t e r the a d d i t i o n of t h i s compound*samples of n-propanol must be removed from the storage f l a s k by vacuum rather than atmospheric d i s t i l l a t i o n because of the explosion hazard. The solvent can not be t r a n s -ferred by pouring, of course, because of the danger of also t r a n s f e r r i n g the 2,4-dinitrophenylhydrazine. Both the b o i l i n g point and the r e f r a c t i v e index of the n-propanol should be measured as a check on the p u r i t y . The equipment f o r and the method of making these measurements was described e a r l i e r . Table I I gives recently-measured values f o r these two q u a n t i t i e s which were obtained from the same sources as the corresponding values f o r benzene. The values 132 Riddick and Toops recommend as best are indicated by un d e r l i n i n g . -58-Author Barbaudy Timinermans and Martin Zmaczynski Lowry and Allsopp Puschin and Matavulj Deffet Davies Cohen and B u i j Wojciechowski Smith and Matheson Grosse and Wackher Scatchard, Wood, and Mochel Linton Maryott, Hobbs, and Gross Smith S t r e i f f and Rossini Davison F o r z i a t i , Glasgow Gibbons, Thompson Glasgow, Murphy TABLE I I Physical Data f o r Benzene from the L i t e r a t u r e B o i l i n g Point Freezing Date at 760 mm.Hg. Point R e f r a c t i v e Index Reference — ^ ^_ nD nD 1926 1926 1930 1931 1932 1935 1936 1937 1937 1938 1939 1939 1940 1940 1941 1944 1945 1946 1946 1946 80.106 C. 80.105 C. 80.07 C. 80.098 C. 80.08 C. 5.50 C. 5.50°C. 80.094°C. 5.51°C. 80.094 C. 5.50°C. 1.5009 1.5010 1.50115 1.5011 1.49795 1.4979 1.49807 5.530°C. 5.496°C. 80.103 C. 5.533 C. 1.50110 1.49790 5.50 C. 5.533°C. 1.5011 64 13 151 171 103 125 44 41 35 165 144 72 139 102 108 143 143 147 60 68 -58-TABLE I I (cont'd.) Author B o i l i n g Point Date at 760 mm.Kg. Harrison and Berg 1946 1946 Marschner and Cropper Simonsen and Washburn 1946 Campbell and M i l l e r 1947 Fenske, Braun 1947 Coulson, Hales 1948 O l i v e r , Eaton, and Hauffman 1948 Tomps 1948 Dew and Smith 1949 F o r z i a t i , N o r r i s , and Rossini 1949 F o r z i a t i and Rossini 1949 Steinhauser and White 1949 F o r z i a t i 1950 Waldichuk 1950 La Rochelle and Vernon 1950 Al-Mahde and Ubbelohde 1953 Chang and Moulton 1953 Trew 1953 80.1°C. 80.099 C. 80.099 C. 80.2°C. Brown and Smith 1954 80.1°C. 80.07°C. Freezing Point Refractive Index Reference n. 20 *D 1.5010 1.5009 n 25 D 5.53 C. 1.49797 1.5012 1.5011 5.511 C. 5.54°C. 1.4981 1.4981 1.50112 1.49792 1.4979 1.50112 1.49792 5.454°C. 1.5010 1.4979 5.454 C. 5.53°C. 1.4977 1.50119 1.4978 1.49803 76 107 1.49807 142 24 58 40 118 152 57 61 62 146 59 98 4 30 153 21 -60-TABLE I I (cont'd.) Author B o i l i n g Point Freezing Bate at 760 mm.Hg. Point Dixon and Schiester 1954 Grunberg 1954 Sandquist and Lyons 1954 Week and Hunt 1954 Brown and Jungk 1955 Neff and Hickman 1955 L i c h t e n f e l s , Fleck, and Burow 1955 White and K i l p a t r i c k 1955 Canjar, Horni, and Rothfus 1956 Whittle 1957 80.10°C. 80.12°C. 80.1 ° C . I.10°C. 80.07 C. 5.34"C. 5.492 C. 5.49 c. Refractive Index Reference "2TT nD 1.50110 1.5009 1.5012 1.5009 n "25" D 1.4975 1.4979 46 73 137 158 20 115 101 161 25 This research -61-TABLE I I I Phy s i c a l Data f o r Normal Propyl Alcohol from the L i t e r a t u r e Author Young and Fortey Dorochewsky Dorochewsky Mundel Brunei, Crenshaw, and 'fobin Brunei Grimm and P a t r i c k Trew and Watkins Timmermans and Delcourt Wojciechowski Zepalova-Mikhailova Addison Vogel Carley and Bertelsen Mumford and P h i l l i p s Howey McKenna, Tartar, and L i n g a f e l t e r Wetzel, M i l l e r , and Day Pu r n e l l and Bowden B o i l i n g Point Date at 760 mm.Kg, 1903 1909 1911 1913 1921 1923 1923 1933 1934 1936 1937 1945 1948 1949 1950 1951 1953 1953 1954 97.19 C. 97.20 C. 97.26 C. 97.1°C. 97.19 C. 97.15°C. 97.19 C. 97.15 C. 97.209 C. 97.15 C. 98.0°C. 97.19 C. 97.2°C. 97.2 C. Refractive Index -h^2D npZ5-1.3833 1.3833 1.38343 1.3856 1.38556 1.3862 1.3858 1.3838 1.3837 Reference 168 49 50 114 123 22 70 154 150 166 170 1 156 26 113 1.3841 97.2°C. 1.3840 111 160 124 -62-APPARATUS The apparatus designed i n t h i s research c o n s i s t s , b a s i c a l l y , of two pressure bombs, ( l and 2)*, placed one above the other. The top bomb, ( l ) , which serves as an equ i l i b r i u m c e l l , ' i s machined from s o l i d 304 s t a i n l e s s s t e e l bar stock and i s 2 inches i n inside diameter, 3 inches i n outside diameter, and 9 inches deep. The volume of t h i s c e l l i s about 450 cubic centimetres. The bottom bomb, (2), which i s used as a mercury storage c e l l , i s s i m i l a r i n design but i s 2 inches shorter and has a v o l -ume of about 350 cubic centimetres. The two bombs are placed, the top one i n a constant temperature bath one and the bottom Abelow i t outside the bath, so that the open end of one faces the open end of the other. Each of these ends i s sealed by means of a cap (3) and a s t a i n l e s s s t e e l head (4 ) . The caps are machined from hexa-gonal stock and are tapped to thread over the ends of the bombs. Each one i s also d r i l l e d and threaded f o r s i x y j ~ i n c h set screws (13) which are used to apply pressure on a hardened s t e e l r i n g (10). This r i i g , i n turn , presses the head against the end of the bomb. The seal between the head and the bomb i s completed with a standard 0-ring (12) which f i t s i n a groove machined into the end of the bomb according to manufacturer's s p e c i -f i c a t i o n s . Since the 0-ring used i n the top bomb i s subjected to elevated temperatures, i t i s made of t e f l o n . The one used with the bottom bomb, which i s located outside the constant temperature bath and kept at room temperature, i s of synthetic rubber. The head sealing each bomb i s d r i l l e d to allow a -£-inch s t a i n l e s s s t e e l rod (14) to pass from one bomb in t o the other. A s t u f f i n g gland 1 The number which follows each part r e f e r s to the part number on the d e t a i l and assembly drawings included at the end of the t e x t . - 6 3 -T 3 1 o i s also d r i l l e d i n each and i s packed with -r x -r- x — x 90 t e f l o n Vee-o o 16 r i n g packings to prevent leakage around the rod. The packings are supp-orted on s t a i n l e s s s t e e l packing r i n g s , ( 5 , 6 , 7 , 8 , 9 ) , the ones ( 5 , 6 , 7 ) i n the top bomb placed so a-s that the Vee-rings w i l l seal under e i t h e r press-ure or vacuum and the ones ( 8 , 9 ) i n the bottom bomb so that they w i l l seal under pressure. The packing support rings and the packing glands are 53 designed according to the Vee-ringj& manufacturer's s p e c i f i c a t i o n s . The two bombs are joined by means of pressure tubing -J—inch i n out-side diameter and g^-inch i n ins i d e diameter. This tubing i s connected to the bombs by means of standard f i t t i n g s , which thread into wells tapped into the sides of the bombs, and extends from the bottom of the eq u i l i b r i u m c e l l to the bottom of the mercury storage c e l l . The top of the storage c e l l i s also connected with s i m i l a r tubing to a nitrogen c y l i n d e r . Apply-ing pressure from t h i s c y l i n d e r to the lower bomb t r a n s f e r s mercury from i t to the eq u i l i b r i u m c e l l . In t h i s way both the pressure on and the volume of the eq u i l i b r i u m c e l l can be c o n t r o l l e d . The pressure i n the e q u i l i b r i u m c e l l i s measured with a "Barnet" dead weight t e s t e r . This dead weight t e s t e r , which w i l l measure from 0-4000 pounds per square^with an accuracy of greater than 0 * 1 $ , i s connec-ted through a l e v e l i n d i c a t o r to the tubing j o i n i n g the two bombs. The l e v e l i n d i c a t o r consists of a length of glass pressure tubing i n which the p o s i t i o n of the in t e r f a c e between the o i l from the dead weight t e s t e r and the mercury from the bomb can be seen. An accurate determination of the p o s i t i o n of the inter f a c e i s necessary i n order that the s t a t i c head between the dead weight t e s t e r and the eq u i l i b r i u m c e l l can be determined, and thus a pressure c o r r e c t i o n c a l c u l a t e d . This c o r r e c t i o n i s p a r t i c u l a r l y important at lower values of the t o t a l pressure. -64-In order to determine the e f f e c t i v e volume of the e q u i l i b r i u m c e l l , the height of mercury i n i t must be known and t h i s height i s measured with a resistance c i r c u i t . The rod extending from the storage bomb to the e q u i l i b r i u m c e l l i s divided i n t o two part s , the lower part c o n s i s t i n g of a s o l i d rod and the upper part,of a hollow tube. A measuring head (assembly drawing 2) i s f i t t e d on to the end of the tube in s i d e the equi-l i b r i u m c e l l . A wire, sheathed i n t e f l o n , passes up the tube and through a t e f l o n seal (16) i n the head, into the bomb. The head i s designed so that t i g h t e n i n g the cover (15) over the top of i t compresses the t e f l o n seal and prevents vapor from leaking out, e i t h e r along the wire or around the edge of the t e f l o n . The pressure from the cover i s transmitted to the seal through a s t a i n l e s s s t e e l c o l l a r (17) which i s held from r o t a t i n g by a small key (23). The wire which i s sealed i n to the measuring.head i n t h i s manner i s made of two mate r i a l s . The upper end,that passes through the seal and i n t o the head, i s of 22 B & S gauge platinum, while the length i n the tube i s of the same s i z e copper. The two pieces are joined j u s t below the t e f l o n s e a l . g Two T - - ~ i l l c n holes are d r i l l e d v e r t i c a l l y i n the measuring head cover* 16 and into each hole i s f i t t e d a t e f l o n sleeve (19 and 21). The sleeves are kept i n place by a flange at the bottom and a wire clamp (25) at the top. A s t a i n l e s s s t e e l p i n (18 and 20) i s f i t t e d i n s i d e each sleeve i n such a manner as to be e l e c t r i c a l l y insulated from the head and held there by a- nut (24) threaded on to the top. The wire passing up through the measuring head i s joined to one of the pins (20) so that*when the wire out-side the c e l l i s connected through a resistance bridge to the bomb,the p i n may be used as mercury l e v e l i n d i c a t o r . Since the resistance of the c i r -c u i t i s d i f f e r e n t when the pin i s i n the mercury than when i t i s not, the -65-point where the contact j u s t touches the surface i s indicated by a change i n the bridge balance. The p o s i t i o n of the measuring head,and thus of the mercury l e v e l i n the bomb,is determined by measuring the height of a graduation on the rod extending between the two bombs. A zero p o s i t i o n of the graduation i s defined by having the contact touch the bottom of the bomb, and heights are measured with a cathetometer from t h i s p o s i t i o n . Since the rod extends into both bombs, and since the bombs are connected by pressure tubing, r a i s i n g of the rod causes mercury to flow from the top bomb into the bottom one and lowering of the rod causes the reverse flow. This t r a n s f e r of mer-cury means that the e f f e c t i v e volume of the c e l l remains constant despite the p o s i t i o n of the head. The measuring head i s ra i s e d and lowered by r o t a t i n g the rodil extend-ing between the two bombs. A length of rod equal to the length that the head w i l l be ra i s e d or lowered i s threaded through a c o l l a r which i s held r i g i d l y i n a permanent p o s i t i o n . Since the c o l l a r cannot move, r o t a t i n g the rod causes i t to go up or down. The c o l l a r i s formed from a threaded brass cone which i s s l o t t e d v e r t i c a l l y to allow i t to expand or contract. This cone i s forced into a s l i g h t l y smaller s t e e l one by a cap, causing the brass to close t i g h t l y around the rod and thus preventing any play bet-ween the two threads. Since, i f the top h a l f of the rod were rotated, i t would break the wire extending from the i n t e r i o r of the bomb through the measuring head, the rod i s divided into two sections. These sections are joined by a r o l l e r bearing which allows any v e r t i c a l force to be transmitted from one section to the other but no ho r i z o n t a l one. to be, and therefore r o t a t i n g the bottom h a l f of the rod gives only v e r t i c a l motion to the top one. The bearing also allows the top half to be rotated s l i g h t l y f o r s t i r r i n g purposes, without changing i t s l e v e l . A h o r i z o n t a l length of §-inch rod i s threaded into a c o l l a r on each section so that each i s e a s i l y turned. The l e v e l of the liquid-vapor i n t e r f a c e (and thus of the volume of the two phases) i s measured with a hot wire anemometer, s i m i l a r to that used 134 by Sage and Lacey . A length of .003-inch diamter platinum wire i s spark-welded on to the mercury l e v e l i n d i c a t o r p i n , stretched across the end of the second p i n , and spark-welded to a t h i r d p in (22) threads! into the measuring head cover. The post extending down from the second p i n i s placed a l i t t l e o f f centre so that by r o t a t i n g t h i s p i n the tension i n the wire may be varied. A current s u f f i c i e n t to heat the wire a few degrees above the temperature of the surroundings i s passed through the wire,using the measuring head rod as one of the leads. Since the conduction of heat away from the wire i s d i f f e r e n t i n the l i q u i d phase from that i n the gas phase, the temperature and thus the resistance w i l l also be d i f f e r e n t i n the two phases. The resistance of the platinum wire i s measured with a Wheatstone bridge. I f the bridge i s balanced with the platinum wire i n the gas phase and the measuring head i s slowly lowered, the point where the wire passes into the l i q u i d phase i s indicated by the bridge suddenly being out of balance. The resistance bridge used i n the l e v e l i n d i c a t o r c i r c u i t i s made up of three wire-wound r e s i s t o r s each of approximately the same resistance as the platinum wire and leads. A d i a l resistance box i s placed i n p a r a l l e l with each r e s i s t o r f o r f i n a l balancing of the bridge. The current f o r the bridge i s supplied from a 6-volt storage battery, and the balance measured with a s e n s i t i v e b a l l i o f r i c galvanometer. For coarse balancing of the bridge the galvanometer can be protected by a 33000, a 2200 or a 270 ohm -67-s e r i e s r e s i s t o r . Since only the change i n resistance and not the actual resistance of the platinum wire i s desired, the bridge has not been c a l i -brated. Once the p o s i t i o n of the i n t e r f a c e i s found, the volume of each phase can be calculated i n the same manner as the e f f e c t i v e t o t a l volume i s found from a knowledge of the height of mercury l e v e l . The temperature of the e q u i l i b r i u m c e l l i s c o n t r o l l e d by immersing i t i n a constant temperature bath. The bath i s 28 inches i n diameter and 30 inches high and i s constructed of — - i n c h s t a i n l e s s s t e e l p l a t e . In order to reduce heat losses from i t , the sides are covered with 4 inches of glass wool, ^ - i n c h of a powdered asbestos and water glass mixture, and wrapped with cotton canvas. The top and bottom are covered with 4 inches of glass wool held i n place by a plywood frame. The bath i s f i l l e d with "Mobile super c y l i n d e r extra hecla mineral o i l " , which has a f l a s h point of 600°F., f o r high temperature use, and with a l i g h t straw o i l f o r lower temperatures. Pressure tubing, valve stems, and the measuring rod are brought out of the bath through glands packed with g r a p h i t e - l u b r i c a t e d cot-ton. The temperature of the bath i s c o n t r o l l e d by three immersion heaters and a cooling c o i l . Two of the heaters, both of 2500-watts power, are tapped into the bottom of the bath and are used to supply enough heat to almost balance the heat losses. A t h i r d 500-watt one, which i s suspended from the top, i s used i n an off-on control c i r c u i t . A l l three heaters are c o n t r o l l e d by variacs from a common panel board and the larger two are connected to voltmeters and ammeters so that the heat losses from the bath may be c a l c u l a t e d . The 500-watt heater i s c o n t r o l l e d by a r e l a y operating from a magnetically adjustable " P h i l a d e l p h i a Roto-stat" mercury thermo-regulator. This thermoregulator extends from the top of the bath 4 inches -68-down past the top of the bomb, and thus controls the temperature at the bomb l e v e l . I f necessary, the bath can be cooled by means of cooling water passed through a c o i l of -§—inch copper tubing wrapped around the ins i d e of the bath. The bath i s s t i r r e d to ensure constant temperature throughout with a £ h.p. "Greey" constant speed s t i r r e r suspended from above the bath. Since there i s some danger that the o i l may smoke, a vent of 3-inch stove pipe i s connected from the top of the bath through a win-dow to the outside. The bath i s maintained at a pressure s l i g h t l y l e s s than atmospheric by means of an a i r j e t placed a few feet from the end of the pipe which operates from a 15 pound per square inch l i n e . The temperature of the bath and thus that of the eq u i l i b r i u m c e l l i s measured i n three ways. A continuous record of the temperature i s given by a "Leeds and Northrup" thermohm. This thermohm, which measures tempera-ture by means of a platinum resistance, has an accuracy of ±0.5° up to 250°F. and + 1° up to 1000°F. I t i s tapped into the bottom of the bath and connected through a transformer to a Micromax recording Wheatsbone bridge. The transformer i s needed to reduce the resistance of the thermohm to a value which the bridge can measure. I t has three taps on the second-ary winding to enable the recorder to cover the e n t i r e range of tempera-ture over which the bath w i l l be used. The transformer i s kept i n an a i r bath, the temperature of which i s regulated by a b i m e t a l l i c thermoregulator coupled to a 15-watt heater. I t s temperature i s c o n t r o l l e d to ensure rep-roducible bath temperature readings regardless of room temperature. This continuous record of temperature, although very convenient, i s not accurate enough f o r measuring the eq u i l i b r i u m c e l l temperature. For t h i s reason two other measuring devices are used, a "Leeds and Northrup" platinum thermometer and an iron-constantan thermocouple. Since the temp-69-erature can be read at only one l e v e l using the thermometer, i t i s used i n conjunction with the thermocouple which can be raised or lowered to read the temperature at various l e v e l s . In t h i s manner the uniformity of temperature throughout the bath can be checked. In order to determine the composition of the l i q u i d and of the vapor i n e q u ilibrium i n the upper bomb, samples must be obtained from each phase. A method by which t h i s sampling may be done under conditions of constant temperature and pressure has been devised f o r t h i s apparatus. Pressure 9 5 tubing, y^-inches * n outside diameter by y-r-inches i n in s i d e diameter and 17 inches long,has been i n s t a l l e d i n the bath p a r a l l e l to the axis of the equi l i b r i u m c e l l . This tubing i s connected to the equ i l i b r i u m c e l l i n three places with inch tubing. Oneelength of the -^-inch tubing connects the top of i t to the top of the c e l l , another j o i n s the bottom of i t to the bottom of the c e l l , and a t h i r d connects a point 3^ inches from the bottom of the c e l l to a point 7 inches from the bottom of the large tubing. The sampling tube i s placed so that the point 3^ inches from the bottom of the bomb i s l e v e l with the point 7 inches from the bottom of the tubing. A valve i s placed i n each of the ^ — i n c h l i n e s and each valve i s positioned so that i t i s at a s l i g h t l y lower l e v e l than the corresponding connection to the bomb. The valves are placed i n t h i s manner so that once the l i n e s connected to the bomb have been f i l l e d with mercury, the mercury w i l l r e -main i n the c e l l and not run into the bomb as long as the valves are closed. In operation, the sample tube and ^ - i n c h l i n e s are f i l l e d with mercury and pressure i s applied from a nitrogen c y l i n d e r u n t i l i t i s the same as i n the e q u i l i b r i u m c e l l . When a sample of the gas phase i s to be c o l l e c t e d , the valve to the top of the eq u i l i b r i u m c e l l i s opened and then the one to the bottom. Mercury flows from the sampling tube into the eq u i l i b r i u m -70-c e l l , d i s p l a c i n g vapor from the c e l l i n to the tube. The two valves are then closed^and the vapor c o l l e c t e d from the sampling tube by vacuum d i s -t i l l a t i o n . In order to sample the l i q u i d phase, the sampling tube i s r e f i l l e d with mercury and the procedure repeated using the two lower valves. In order that the mercury i n the sampling tube may be replaced a f t e r the removal of a sample, the bottom of the tube i s connected to the bottom of an a u x i l l i a r y storage c e l l . This storage c e l l i s inches i n in s i d e diameter, 2-| inches i n outside diameter, and 3 inches deep. The top i s sealed with a cap and plate i n the same manner as f o r the other two bombs. Two l i n e s are connected to the top of the c e l l through a tee; one f o r the ad d i t i o n of f r e s h mercury, and the other f o r applying pressure from the nitrogen c y l i n d e r . In order to assure that e q u i l i b r i u m i s reached between the gas and l i q u i d phases, the contents of the e q u i l i b r i u m c e l l are agitated with a magnetic s t i r r e r . A plate (25), which i s 2 inches i n diameter, -f-inch t h i c k , and made from 304 s t a i n l e s s s t e e l , i s held i n s i d e the e q u i l i b r i u m c e l l near the upper end. A l l but the minimum area required f o r mechanical strength has been machined out i n order to allow c i r c u l a t i o n of the vapor phase around i t . I t i s held i n p o s i t i o n with a tapered screw (27) which, when tightened, expands the edge of the plate t i g h t l y against the wa l l of the bomb. Three small pins (31) are screwed into the upper face of the p l a t e . Since these pins are the same length as the distance that the plate i s to be held from the top of the bomb, i t i s positioned by pushing i t up u n t i l they butt against the top. The plate i s used to support an "Alnico" permanent magnet. The magnet seats on a t e f l o n washer (30) and i s held i n p o s i t i o n by a shaft (28) which extends from i t through the p l a t e . A s t a i n l e s s s t e e l s t i r r i n g arm (26) i s bolted to the bottom of the shaft, -71-on the underneath side of the p l a t e , so that r o t a t i n g the magnet rotates t h i s arm. The magnet i s turned by means of a much stronger one suspended from the top of the bath to the top of the bomb. This magnet i s rotated by a small v a r i a b l e speed motor and i s encased by a thin-walled copper c y l i n d e r so that i t does not have to operate i n the bath o i l . The use of the copper c y l i n d e r also allows the magnet to be positioned e a s i l y . A u x i l -l i a r y s t i r r i n g , p a r t i c u l a r l y of the l i q u i d phase, can be accomplished by moving the measuring head back and f o r t h . Because of the r o l l e r - b e a r i n g connection on the measuring head rod, r o t a t i n g the head does not change i t s l e v e l . -72-HIOCEDURE FOR MAKING MEASUREMENTS Before any measurements can be made with the equ i l i b r i u m apparatus described above, a c a l i b r a t i o n r e l a t i n g the height of the mercury contact on the measuring head to the volume of the eq u i l i b r i u m cell,measured must be obtained. This determination can be made by f i r s t evacuating the equi-l i b r i u m c e l l and then completely f i l l i n g i t with mercury. ^he measuring head i s raised as f a r in s i d e the bomb as i t w i l l go, and the mercury then allowed to run out of the c e l l u n t i l i t s upper surface j u s t touches the t i p of the mercury contact. The volume of c e l l measured i n t h i s manner i s the minimum volume which can be determined with the measuring head. The mercury removed from the c e l l i s c o l l e c t e d i n a weighing bottle,and i t s volume determined from a knowledge of i t s weight and density. A ho r i z o n t a l l i n e i s machined on the measuring rod i n such a manner that, regardless of the p o s i t i o n of the measuring head, i t i s always v i s i b l e between the bottom of the bath and the top of the mercury storage c e l l . The v e r t i c a l distance between t h i s l i n e and a s i m i l a r one on the bath frame i s determined with a cathetometer. The measuring head i s then lowered a quarter of an inch, and mercury again removed u n t i i the contact j u s t touches i t s Sur-face. This procedure i s repeated u n t i l the bottom of the eq u i l i b r i u m c e l l i s reached. This l a s t p o s i t i o n i s the zero p o s i t i o n of the measuring head and the distance between the l i n e on the measuring head rod and the one on the bath frame defined i n t h i s manner i s subtracted from the other readings to obtain the height of mercury i n the c e l l . The values obtained i n the above manner are then p l o t t e d g r a p h i c a l l y with the volume of c e l l as ordinate and the p o s i t i o n of the rod as abscissa. The volume of c e l l above the liquid-vapor i n t e r f a c e wire can be determined from the graph and a knowledge of the distance between the wire and the contact point. The c a l i b r a t i o n determined i n t h i s manner, of course, i s v a l i d only f o r the temperature at which i t was made. However, i t can be corrected f o r use at other temperatures from a knowledge of the temperature co-e f f i c i e n t of expansion f o r the bomb bath and measuring head rod. The c a l i b r a t i o n should be made at at lea s t one other temperature, such as 200°C, to check the accuracy of the corrected values. When the measurements of mercury l e v e l i n the eq u i l i b r i u m c e l l are made, care must be taken that, at at lea s t one p o s i t i o n , the height of mercury i n the glass l e v e l i n d i c a t o r i s determined as w e l l . This deter-mination i s necessary i n order that a s t a t i c head c o r r e c t i o n f o r the pres-sure measurements can be ca l c u l a t e d . The next step i n the use of the apparatus i s to introduce both the mercury and the sample to be tested i n an a i r free c o n d i t i o n . The mercury i s d i s t i l l e d , under vacuum, into the f l a s k shown i n f i g u r e 2 and, while s t i l l under vacuum, the stopcock on the f l a s k closed. This t r a n s f e r f l a s k i s then removed from the s t i l l and connected through a ground glass j o i n t , shown i n fi g u r e 4, to the mercury storage c e l l . Once the eq u i l i b r i u m apparatus has been evacuated, the stopcock on the f l a s k i s opened and the mercury allowed to run into the storage c e l l . I f i t i s necessary to add more mercury, the three-way stopcock, ( f i g u r e 4), on the t r a n s f e r l i n e i s positioned so that the f l a s k i s i s o l a t e d from the rest of the system. The f l a s k i s then removed and r e f i l l e d with mercury from the vacuum s t i l l . When f i l l e d , i t i s replaced and the three-way stopcock positioned so that the tubing between t h i s stopcock and the one on the f l a s k i s connected to the vacuum pump. Af t e r the tubing i s evacuated, the mercury i s allowed to run into the storage c e l l as before. - 7 4 -The method of t r a n s f e r r i n g the solvent to be studied into the e q u i l i -87 brium apparatus i s based on the procedure used by Kaye and Donham , and the apparatus used f o r the t r a n s f e r i s shown i n f i g u r e 3. The p u r i f i e d benzene i s displaced with dry a i r from the solvent t r a n s f e r f l a s k through I I stopcock "e" into f l a s k "B". I t i s then frozen with a mixture of dry ice i n acetone and the glass ware completely evacuated. A f t e r evacuating, a l l stopcocks but "a" and "d" are closed, the Dewar f l a s k containing the dry i c e mixture removed from around "B", and a small percentage of the benzene allowed to d i s t i l l i n t o the cold t r a p . Stopcock "a" i s then closed, the f r e e z i n g mixture placed around f l a s k "A", and stopcock "c" opened. A f l a s k of warm water i s placed around "B" and the benzene d i s t i -l l e d under vacuum into f l a s k "A". When a l l but about 5$ of the benzene has been d i s t i l l e d , stopcock "c" i s closed, stopcock "a" opened, and the rest of the benzene d i s t i l l e d i n to the cold t r a p . Since the p a r t i a l pre-ssure of a i r over the d i s t i l l i n g solvent w i l l be very much less than one atmosphere, the amount of a i r occluded i n the frozen benzene w i l l be quite small. The benzene i s d i s t i l l e d back and f o r t h between f l a s k s "A" and "B" i n t h i s manner, reevacuating (the apparatus between each d i s t i l l a t i o n , u n t i l a l l the dissolved a i r i s removed. Since, i n each case, the p a r t i a l press-ure of a i r over the benzene i s due almost e n t i r e l y to that occluded during the previous d i s t i l l a t i o n and f r e e z i n g , i t w i l l r a p i d l y drop to a n e g l i -g i b l e value. A f t e r a l l the a i r has been removed, the solvent i s d i s t i l l e d i n t o f l a s k "E" i n preparation f o r the t r a n s f e r to the e q u i l i b r i u m c e l l . A s i m i l a r procedure i s used with the normal propyl alcohol with the excep-t i o n that the i n i t i a l t r a n s f e r from the storage f l a s k must be made by vacuum d i s t i l l a t i o n f o r the reasons discussed e a r l i e r . I I Unless otherwise noted, a l l references to stopcocks and f l a s k s r e f e r to f i g u r e 3 and a l l references to valves r e f e r to f i g u r e 4. -75-When both solvents are i n f l a s k E i n the desired proportions, the bath surrounding the equ i l i b r i u m c e l l i s cooled as much as possible by running cold water through the copper cooling c o i l . The mercury l e v e l i n the c e l l i s then lowered u n t i l the c e l l w i l l hold the solvent mixture, and the mixture i s tra n s f e r r e d from f l a s k E to the c e l l by vacuum d i s -t i l l a t i o n through stopcock j and valve D. Since, during the t r a n s f e r , the constant temperature bath i s kept as cold as pos s i b l e , the next step i s to heat i t to the temperature at which the equ i l i b r i u m measurements w i l l be made. In order to heat the bath q u i c k l y , both 2500-watt heaters are i n i t i a l l y turned up to the point where they are working at the maximum permissible watt density. When the desired temperature i s reached, the heaters are turned down u n t i l the temperature s t a r t s to drop slowly. The 500-watt heater i s then turned on and adjusted u n t i l the temperature s t a r t s to r i s e again. Once t h i s adjust-ment has been made, the thermoregulator relay combination i s turned on to control the bath temperature. The bath, and thus the eq u i l i b r i u m c e l l temperature, i s measured, using both the platinum resistance thermometer and the iron-constantan thermocouple. I t i s determined f i r s t with the resistance thermometer and then at the same l e v e l i n the bath with the thermocouple. The thermocouple i s next lowered two inches and the temperature again measured. This pro-cedure i s repeated u n t i l a temperature p r o f i l e i s obtained f o r the e n t i r e bath. I f there i s a s i g n i f i c a n t change i n temperature through the bath, the p o s i t i o n of the s t i r r e r i s adjusted and the measurements repeated. While taking readings with the thermocouple, care i s taken to obtain a reading at the l e v e l of the platinum thermohm because the manufacturers do not supply a temperature-resistance c a l i b r a t i o n with t h i s thermometer. -76-The pressure applied to the e q u i l i b r i u m c e l l at any p a r t i c u l a r temp-erature i s varied by changing the volume occupied by the solvent mixture. To decrease the volume and thus increase the pressure, mercury i s tr a n s -fe r r e d to the eq u i l i b r i u m c e l l from the storage c e l l below. • This t r a n s f e r i s accomplished by d i s p l a c i n g the mercury with nitrogen from the nitrogen c y l i n d e r . To increase the volume and thus decrease the pressure, some of the nitrogen i n the storage c e l l can be vented to the atmosphere through valve M. The pressure on the e q u i l i b r i u m c e l l i s measured with the dead weight t e s t e r described e a r l i e r . Valve "F" i s closed, i s o l a t i n g the equi-l i b r i u m c e l l , and valve "E" opened. The p o s i t i o n of the i n t e r f a c e between the mercury from the bomb and the o i l from the dead weight t e s t e r i s ad-justed so that i t i s v i s i b l e i n the glass l e v e l i n d i c a t o r . The pressure i s then measured by pla c i n g weights of known value on the dead weight t e s t e r piston u n t i l they j u s t balance the upward pressure of the o i l . The pis t o n i s spun continuously during the balancing to reduce the f r i c t i o n between i t and the containing sides. Since the minimum weight a v a i l a b l e f o r the t e s t e r represents one pound per square inch at pressures up to 400 pounds per square inch, and 5 pounds per square inch at pressures up to 4000 pounds per square inch, i t i s u n l i k e l y that an exact balance can be made using the weights alone. Values w i t h i n these i n t e r v a l s can be obtained, however, by r a i s i n g or lowering the p o s i t i o n of the mercury o i l i n t e r f a c e and thus varying the s t a t i c head between the c e l l and the t e s t e r . Chang-ing the p o s i t i o n of the i n t e r f a c e w i l l change the pressure at the c e l l as w e l l as that at the t e s t e r , of course, because the e f f e c t i v e volume of the c e l l w i i l change. Since t h i s change occurs, i t i s necessary to allow time f o r the solvent to come to a new e q u i l i b r i u m before making the f i n a l pres-sure measurement. I f a f u r t h e r change i n the p o s i t i o n of the i n t e r f a c e -77-i s necessary, the above procedure must be repeated. The dead weight t e s t e r i s designed so that small amounts of o i l leak out around the p i s -ton when measuring the pressure. For t h i s reason the valve connecting the t e s t e r to the bomb should be kept closed except when a c t u a l l y making measurements. The pressure measured by the dead weight t e s t e r i s not, of course, the pressure i n the equi l i b r i u m c e l l . A c o r r e c t i o n must be applied to the value obtained i n order to take into account the s t a t i c head of o i l and mercury i n the l i n e connecting the c e l l and the t e s t e r . In order to ca l c u l a t e t h i s c o r r e c t i o n , the differ e n c e i n height between the mercury surface i n the bomb and the oil-mercury i n t e r f a c e i n the l e v e l i n d i c a t o r must be known, as must the difference between the i n t e r f a c e and the dead weight t e s t e r . The second difference can be measured d i r e c t l y with a cathetometer, but the f i r s t can not, as the mercury i n the bomb can not be seen. However, during the c a l i b r a t i o n of the measuring head, the height of mercury i n the l e v e l i n d i c a t o r required to balance a known height of mercury i n the eq u i l i b r i u m c e l l was determined. Therefore, by measuring the change i n l e v e l of the mercury surface i n the bomb from that at the time the c a l i b r a t i o n was madejand also i n l e v e l of mercury i n the i n d i c a t o r from the l e v e l at the time of c a l i b r a t i o n , the head due to mer-cury can be ca l c u l a t e d . A c o r r e c t i o n f o r the depression of the mercury surface i n the l e v e l i n d i c a t o r caused by c a p i l l a r y action w i l l not be nec-essary, as a s i m i l a r depression w i l l have occurred when i n i t i a l l y deter-mining the balance between the two l e v e l s . The sum of the pressures due to the head of o i l and to the head of mercury w i l l give the t o t a l correc-t i o n to apply to the dead weight t e s t e r reading. Before any fu r t h e r measurements can be made on the solvent mixture, the gas and l i q u i d phases must be i n equil i b r i u m . In order to reduce the time required to reach t h i s e q u i l i b r i u m , the mixture i s s t i r r e d with both the magnetic s t i r r e r and the measuring head. As soon as the sample i s transferred into the bomb, the magnetic s t i r r e r i s turned on and i t i s l e f t running u n t i l a l l the needed measurements have been made. Unless the l i q u i d l e v e l i n the bomb i s quite high, t h i s s t i r r e r w i l l operate only i n the gas phase, and therefore, to s t i r the l i q u i d phase, the measuring head i s rotated back and f o r t h o c c a s i o n a l l y as w e l l . When the two phases are i n eq u i l i b r i u m , the volume of each i s deter-mined with the measuring head. The p o s i t i o n of the head i s adjusted so that the mercury contact j u s t touches the mercury surface and the volume of c e l l occupied by the solvent then found from the c a l i b r a t i o n determined e a r l i e r . With the head l e f t i n t h i s p o s i t i o n , the resistance of the gas-l i q u i d i n t e r f a c e wire i s balanced with the Wheatstone bridge described e a r l i e r . Once the balance has been obtained, the head i s slowly raised u n t i l the point where the resistance of the wire suddenly changed i s found. This change i n resistance of the wire indicates that i t has passed from the l i q u i d to the gas phase. The volume of gas above the wire i s again found from the c a l i b r a t i o n curve. When the measuring head i s raised or lowered, care must be taken that valve "F" on the l i n e j o i n i n g the two bombs i s open. I f i t i s not, moving the head w i l l change the volume of the c e l l occupied by the solvent and thus the eq u i l i b r i u m between the two phases. Before an attempt to obtain samples of the two phases can be made, the sampling chamber must be completely f i l l e d with mercury. F i r s t the a u x i l i a r y mercury storage c e l l i s f i l l e d by the same method as was used -79-i n f i l l i n g the main mercury storage bomb. The mercury t r a n s f e r f l a s k i s f i l l e d with mercury at the vacuum s t i l l and then connected to the equi-l i b r i u m apparatus. A f t e r the f l a s k i s placed i n p o s i t i o n , valve "Kn i s opened and the three-way stopcock positioned so that the a u x i l l i a r y s t o r -age c e l l and tubing connecting i t to the t r a n s f e r f l a s k are evacuated. The mercury i s then allowed to run into the c e l l and valve "K" closed. When the c e l l has been f i l l e d , nitrogen pressure i s applied to the mercury surface through valve "H" u n t i l the pressure i s approximately the same as i n the e q u i l i b r i u m c e l l . Before mercury can be t r a n s f e r r e d from the s t o r -age c e l l into the sampling tube, the tube i s evacuated through the l i n e connecting the vacuum pump to the sample c o l l e c t i o n vessel ( f i g u r e 2) by opening valve "L". When a l l the a i r has been removed, valve "L" i s closed, valve " J " opened, and mercury forced by the nitrogen pressure from the storage c e l l into the sampling chamber. Once i t has been f i l l e d , valves "A" and "B" are opened very s l i g h t l y and a l i t t l e mercury forced through the valves into the e q u i l i b r i u m c e l l . The tubes connecting the e q u i l i b r i u m c e l l to the sampling chamber are flushed with mercury i n t h i s manner i n order to remove any solvent that might have c o l l e c t e d i n them. I f any solvent accumulated i n these tubes, as would be almost sure to happen i f they were not completely f i l l e d with mercury, then, when samples were taken of each phase, t h i s material would be c o l l e c t e d as w e l l . The error introduced i n t h i s manner could be very serious, p a r t i c u l a r l y when sampling the vapor phase at r e l a t i v e l y low pressures. The volume of vapor c o l l e c t e d when sampling v a r i e s with the height of mercury i n the bomb, but i t i s l e s s than 20 m i l l i l d t r e s . I f a small amount of l i q u i d had condensed or been splashed into the vapor l i n e and was c o l l e c t e d with the vapor sample, i t , when vaporized, could have a larger volume than that of the - 8 0 -sample. The composition of the l i q u i d would be that of the l i q u i d phase and not that of the vapor, and therefore the error introduced would be very large. When enough mercury has been forced through the l i n e s to c l e a r them, valves "A" and "B" are shut very slowly so as to leave the tubes f i l l e d with mercury. I f they are not l e f t f i l l e d , then as soon as the valves are closed solvent w i l l accumulate i n them again. Since, i n order to c l e a r the tubes, mercury i s forced into the bombs, the volume of c e l l occupied by the solvent w i l l be reduced s l i g h t l y and thus the pressure and e q u i l i b r i u m between the two phases changed. For t h i s reason, enough time f o r the phase to return to e q u i l i b r i u m mustbe allowed before sampling. The gas phase can then be sampled by f i r s t opening valve "A" and then valve "C". The mercury i n the sample chamber runs out through valve "C! i n t o the c e l l and i n doing so, forces an equal volume of gas through valve "A" into the sample chamber. Since t h i s displacement involves no change i n volume and thus none i n pressure, the e q u i l i b r i u m between the phases i s not affected and a representative sample i s obtained. Once the sample has been transferred to the sampling chamber i t i s i s o l a t e d there by c l o s i n g the two valves. The sample i s removed from the chamber by vacuum d i s t i l l i n g i n to sample c o l l e c t i o n f l a s k "F" ( f i g u r e 2). In order that t h i s d i s t i l l a t i o n can be made, both f l a s k s "F" and "G" ( f i g u r e 2) are evacuated, the valve connecting the two closed, and the three-way stopcock positioned so as to connect the f l a s k s to the sampling chamber. A cold trap i s then placed around f l a s k "F" and valve "L" opened s l i g h t l y . The sample i n the sampling tube and, depending upon the bath temperature, p o s s i b l y the mercury as w e l l , w i l l d i s t i l l i n t o f l a s k "F". Care must be taken that valve "L" i s opened very slowly since the sample i n the tube i s under the same pressure as i n the e q u i l i b r i u m c e l l , and a sudden release -81-of t h i s pressure could shatter the glass r e c e i v i n g f l a s k . Once the sample i s i n f l a s k "F", valve "L" i s closed, the cold trap i s tr a n s f e r r e d to f l a s k "G", and the solvent vacuum d i s t i l l e d i n to "G". Since the tempera-ture required to t r a n s f e r the sample from "F" to "G" i s only s l i g h t l y above room temperature, any mercury present w i l l remain i n "F". The stop-cock between the two f l a s k s i s then closed, f l a s k "G" removed, and the sample analyzed. The procedure f o r obtaining a sample of the l i q u i d phase i s p r a c t i c a l l y i d e n t i c a l . Once the vapor sample has been removed, the sampling chamber i s evacuated and r e f i l l e d with mercury. Samples of the l i q u i d phase are then obtained by opening valves "B" and "C", thus allowing the mercury to run into the c e l l through "C" and the sample out into the sample chamber through "B". One difference that does a r i s e i n the sampling i s that the po s i t i o n s of the mercury-liquid i n t e r f a c e and v a p o r - l i q u i d i n t e r f a c e i n the bomb must be known. In order to obtain a sample of the l i q u i d phase, the mercury l e v e l must be below that of the point where the tubing from valve "B" enters the bomb and the l e v e l of the va p o r - l i q u i d i n t e r f a c e must be above t h i s point. I f the phase boundaries are not i n the correct p o s i t i o n , then e i t h e r the amount of solvent i n the c e l l or else the press-ure applied to the solvent must be changed. Once the samples have been obtained, they must of course be analyzed. The composition of the l i q u i d phase i s determined from the r e f r a c t i v e index of the l i q u i d sample. The r e l a t i o n s h i p between r e f r a c t i v e index and composition i s determined by using the P u l f r i c h refTactometer described e a r l i e r f o r mixtures of known composition and the indek f o r the l i q u i d sample i s compared to t h i s c a l i -b r a t i o n . The sample from the vapor phase i s analyzed using a gas f r a c t o -meter. This sample can not be analyzed by using the refTactometer because -82-the volume of l i q u i d obtained by condensing the gas i s i n s u f f i c i e n t f o r a measurement. The sampling procedure described above can be used to obtain as many samples from each solvent mixture as desired, the only r e s t r i c t i o n being that s u f f i c i e n t solvent remains i n the c e l l that the two i n t e r f a c e s are i n the correct p o s i t i o n . In general, i t i s suggested that two samples of each phase be taken at each temperature and pressure before changing these v a r i a b l e s . -83-BIBLIOGRAPHY 1. Addison, C.C., J . Chem. S o c , 1945, 98. 2. Akers, W.W., Attwell} L.L., and Robinson, J.A., Ind. Eng. Chem., 46: 2539, 1954. 3. Akers, W.W., Burns, J.F., and F a i r c l i i l d , W.R., Ind. Eng. Chein., 46: 2531, 1954. 4. Al-Mahde, A.A.K. and Ubbelodhe, A.R., Proc. Roy. Soc. (London), A220: 143, 1953. 5. Amer, H.H., Paxton, R.R., and Van Winkle, M., Ind. Eng. °hem., 48: 142, 1956. 6. American Instrument Company, Superpressure and C a t a l y t i c Hydrogena-t i o n Apparatus, Catalog 406. American 1 Instrument Company, 1947. 7. 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