Science, Faculty of
Chemistry, Department of
DSpace
UBCV
Erlebach , Woodland Eustace
Master of Applied Science - MASc
University of British Columbia
The boiling points, densities, and compositions of constant boiling deuterochloric acid have been determined at pressures between 200 and 1000 mm. The boiling points, determined with apparatus similar to that used for the determination of the boiling points of hydrochloric acid, were found to differ to a very small degree from those of hydrochloric acid. The compositions were found to vary from 19.661 percent deuterium chloride at 202.5 mm to 17.146 percent at 998.4 mm; about 2.5 and 2.2 percent lower than hydrochloric acid prepared at those pressures. The densities were found to vary from 1.20034 gms per ml at 202.5 mm to 1.18774 gms per ml at 998.4 mm. The relationship between boiling point and percent composition and between density and percent composition were found to be closely linear over the range investigated. These results were correlated and compared with the data on hydrochloric acid. The difficulties in explaining the difference in the properties of the two azeotropes is pointed out, and the application of theoretical relationships is discussed.
THE BOILING POINTS, COMPOSITIONS, AND DENSITIES OF THE AZEOTROPES OF DEUTEROCHLORIC ACID by WOODLAND EUSTACE ERLEBACH A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in the Department of CHEMISTRY We accept this thesis as conforming to the standard required from candidates for the degree of MASTER OF APPLIED SCIENCE Members of the Department of CHEMISTRY THE UNIVERSITY OF BRITISH COLUMBIA April, 19S3 ABSTRACT The boiling points, densities, and compositions of constant boiling deuterochloric acid have been determined at pressures between 200 and 1000 mm. The boiling points, determined with apparatus similar to that used for the determination of the boiling points of hydrochloric acid, were found to d i f f e r to a very small degree from those of hydrochloric acid. The compositions were found to vary from 19.661 percent deuterium chloride at 202 .5 mm to 17.1U6 percent at 998.h mm; about 2 .5 and 2 .2 percent lower than hydrochloric acid prepared at those pressures. The densities were found to vary from 1.2003U gms per ml at 202 .5 mm to 1.1877U gms per ml at 998.h mm. The relationship between boiling point :and percent composition and between density and percent composition were found to be closely linear over the range investigated. These results were correlated and compared with the data on hydrochloric acid. The d i f f i c u l t i e s in explaining the difference i n the properties of the two azeotropes i s pointed out, and the application of theoretical relationships i s discussed. i i TABLE OF CONTENTS Page ABSTRACT 1 INTRODUCTION :2 A. Historical Basis 2 B.3 Theory lh 1. Vapor-Liquid Equilibrium U 2. Thermodynamic Relationships i n Azeotropism 8 (a) Equations of Reddich and Schutz 8 (b) Equations of Coulson and Herington 12 (c) Equations of Kireev 17 (d) Equations of Carlson and Colburn 18 (e) Comparison of Equations 19 EXPERIMENTAL METHODS AND RESULTS 21 A. Apparatus 21 1. Preparation Apparatus 21 '2. Dry Air Distributor 22 3 . D i s t i l l a t i o n Unit 22 U. Pressure Apparatus 23 5 . Pycnometric Equipment 2k 6. Ebulliometric Apparatus 2h 7. Analytical Equipment 27 8. Dry Box , 27 B. Procedure 27 1. Preparation of the Acid 27 2 . D i s t i l l a t i o n of the Azeotrope 29 3 . Determination of Density 31 h. Determination of Boiling Point 33 5. Determination of Composition 3h C. Results 36 1. Preparation 36 2. Azeotropic Data and Empirical Correlations 37 3. Calibration of Resistance Thermometer 1|1 h. Test Determinations on Hydrochloric Acid U2 5. Isotopic Purity of Deuterium Oxide k3 DISCUSSION hh A. Precision and Accuracy hh 1. Isotopic Purity of the Azeotrope hh 2. Pycnomet ry 1;7 3. Manometry U8 h. Ebulliometry 50 5. Composition 5H B. Comparison of Properties 55 • C. Linearity of Correlations 58 D. Applications of Data 59 REFERENCES 6li APPENDIX 66 I Illustrations 66 ' Fig. 1 Preparation Apparatus 66 Fig. 2A Distillation and B oiling Point Apparatus 2B Pycnometer Fig. 3 Relationship between boiling point and pressure • 68 Fig. h Relationship between boiling point and percent composition 69 Fig. 5 Relationship between pressure and percent composition 70 Fig. 6 Relationship between density and percent composition 71 Fig. 7 Bifference between observed and calculated boiling points 72 Fig. 8 Relationship between logarithm of pressure and reciprocal boiling point 73 Fig. 9 Determination of mean distillation pressure 7h Fig. 10 Temperature and pressure at which the azeotrope disappears 75 II Azeotropic Data 76 III Calculations • 78 1. Composition 78 2. Density 80 3. Correlation of boiling point and composition 81 U. Relation between mole fraction and percent composition 83 5 . Calculation of mean pressure and its mean deviation 8U IV Barometric Corrections 85 V Calibration of Weights 85 VI Notation 87 2. INTRODUCTION A. Historical Basis John Dalton, in 1832, discovered that water and hydrogen chloride of a specific composition had a constant boiling point. This boiling point was found to be so constant that an earlier investigator ' (1) assumed that the mixture formed a compound. Two years later, in i860, Roscoe and Dittmar (33) showed that the composition of this mixture varied v/ith pressure. Almost senenty years later Briggs (5) described a method of using this property to detect and separate constant boiling mixtures. He distilled a mixture of hydrogen chloride and water in a fractionating;1 column;.at an arbitrary pressure. After the bottoms composition became constant, he refractionated the residue at another pressure and examined the final residue. He found that the f i r s t fractionation increased the hydrogen chloride content of the residue, and the second fractiona-tion reduced i t . Aside from the fact that hydrochloric acid was one of the fir s t azeotropes discovered, its use as a volumetric standard has prompted a thorough investigation of its composition when the acid is distilled at approximately atmospheric pressure. After the suggestion by Hulett and Bonner in 1909 (17) that because of its definite composition the constant boiling azeotrope would provide a good volumetric standard, a number of investigators published data on the azeotrope. Foulk and Hollingsworth (13) compared the data of Hulett and Bonner (17) , Morey (28), and Hendrixon (16), with their own on the composition of the azeotrope prepared at 750 mm pressure. They found that after appropriate corrections had been applied to the earlier data agreement in composition was found to within 0 . 0 1 percent. Although constant boiling hydrochloric acid has distinct advantages as a primary standard, Shaw (35) observed that the use of benzoic acid and sodium carbonate for this purpose far exceeded the use of the azeotrope. He assumed that the cause of the lack of use lay in the uncertainty of the s t a b i l i t y of the acid. To investigate this s t a b i l i t y Shaw kept samples of the acid i n well-stoppered bottles i n the dark for periods up to three years. During that time the concentration did not change by more than 5 parts i n 1 0 , 0 0 0 . Bonner, who with Hulett did some of the earliest work on the azeotrope, extended the data by determining precisely the boiling points, densities, and compositions of the azeotropes prepared at various pressures between 50 and 1220 mm. This work was carried out i n conjunction with Titus (3), Branting (2), and Wallace (H). The boiling points of the azeotropes as determined by Bonner and Wallace were later checked by Cadbury (7) and agreement found within ± 0.05°C. The determination, i n 193U, of the vapour pressure of deuterium chloride and the comparison of this data with that of hydrogen chloride by Lev/is, MacDonald, and Schutz (22), -suggests the interest which might attend the comparison of the properties of deuterochloric acid and hydrochloric acid. a. B i. Theory (l) tapor-Liquid Equilibrium In the study of liquid mixtures i t is customary to define an ideal mixture as one that obeys Raoult's Lgw over the whole range of concentration. Raoult's Law states that the partial vapor pressure of a constituent is proportional to its mole fraction in the liquid at a l l concentrations. Applied to a constituent A this may be expressed mathematically as P A = PAXA > W where p^ is the partial pressure, p^ is the vapor pressure of the pure liquid, and x^ is the mole fraction. Any binary solution such that the relationship between total vapor pressure and composition is not linear may be defined as non-ideal. These relationships for non-ideal binary solutions are given by the Duhem-Margules equation (11, 2a) . This was first derived by Gibbs and later, independently, by Duhem, Margules, and Lehfeldt. It may be derived from partial free energy relationships to give the following equation: din p A _ din p B ^ din x^ din Xg This equation assumes only that the vapors of the two components behave ideally. It makes no assumption regarding the ideality or otherwise of the liquid mixture. The validity of this equation was studied by Zavaritzkii (a2) for water and hydrogen chloride solutions from 0 to 30 percent hydrogen chloride and found to be correct. Rosanoff (32) states that the equation is absolutely 5. general and, should hold for a l l actual vapors up to their critical-points. The Duhem-Margules equation may be integrated to the form: o « x n , ° «x2 , . P A = P A X A E B and p B = P B X b e A (3) where <* must be the same constant for both substances. These two equations are generally applicable at constant temperature save where one of the components is highly associated ( lU). The exponential function, e° c x ,^ gives a measure of the deviation of the mixture from Raoult's Law. When the two con-stituents of a mixture differ in (a) internal pressure, (b) polarity, (c) length of hydrocarbon chain, or analogous grouping, or i f (d) one or other of the substances is associated in the liquid state, the system deviates positively from Raoult's Law and the exponential function in the integrated form of the Duhem-Margules equation is greater than one. Further, the total vapor pressure with positive deviation of the partial pressures wil l not be linear with composi-tion. It may also be noted that the closer the vapor pressures of the pure liquids, the smaller the deviation necessary to give a total vapor pressure greater at some point than either of the vapor pressures of the pure constituents. Considering the relationship between vapor pressure and temperature, when the vapor pressure of a mixture equals the total pressure on the mixture the boiling point has been reached. When there is a point of maximum vapor pressure in the curve showing the relation between the vapor pressure and the composition, there must be a corresponding point of minimum temperature in the boiling point 6. and composition curve. That composition exerting a higher vapor pressure w i l l with increase in temperature reach its boiling point before a composition exerting a lower vapor pressure. In an analogous manner i t may be shown that a composition exerting a minimum vapor pressure will have a maximum boiling point. The reduction in vapor pressure is caused by the mutual attraction of the unlike molecules exceeding the attraction of the like molecules. This effect must outweigh the causes of negative deviation already mentioned. For this reason i t is easy to understand why the number of azeotropes exhibiting a maximum boiling point is limited. It is probable that in the system hydrogen chloride and water the acid becomes solvated, that i s , water molecules attach themselves to hydrogen chloride molecules to form a 'complex. This solvation has the effect of lowering the volatility of the solution and raising its boiling point. The solvate formed has a much lower escaping tendency than either the water or the hydrogen chloride. Simmons (36) suggests that the actual boiling point of a binary' acid-water azeotrope depends on the boiling point of the pure solute as well as on its condition in solution. The solute or acid in the solution wi l l be present in ionic or molecular form. Ions raise the boiling point of the azeotrope, whereas, molecules may either raise or lower the boiling point. If the pure solute boils above 100°C, their presence wil l lower the boiling point. With an increasing concentration of hydrogen chloride in water the number of molecules present wil l increase and the boiling point will f a l l . A maximum boiling point elevation is reached when the ionic concentra-7. tion i s a maximum. Simmons further explains that weak solutes do not dissociate sufficiently to produce a maximum unless the boiling point of the pure solute i s close to 100°C. The last statement i s corroborated by other investigators. Temperature has an effect on the chemical relationship of the compliments i n an azeotropic solution. In general a reduction in temperature increases the deviations from ideality. Therefore, a large decrease i n the temperature of a maximum vapor pressure azeotrope may cause the formation of a partly miscible mixture with consequent disappearance of the azeotropic properties. Similarly a miscible mixture which does not normally exhibit azeotropism may with sufficient decrease i n temperature become an azeotrope. In order to show why a maximum in the vapor pressure and composition curve indicates the formation of a mixture i n which liquid and vapor compositions are identical, we must refer to an equation derived by Konowaloff (20) from the Duhem-Margules equation. Rewriting the Duhem-Margules equation in the form x Adp A _ xgdpg p Adx A PgdXR and noting that dx A = - d X g , we obtain ? - d p B . *BPA From this equation i t may be seen that as P approaches a maximum or minimum the variation of total vapor pressure P with the mole fraction of component A, x , becomes equal to zero. This means that either the term i n brackets, or the coefficient which describes the change 8. in partial vapor pressure of component B with respect to composition must become equal to zero. The latter is obviously quite unlikely; hence, we may assume that the former is true. This shows that at the point of maximum or minimum vapor pressure the ratio of the partial pressures of ctoponents A and B equals the ratio of their mole fractions. Assuming that the vapor behaves as an ideal gas, the ratio of the partial vapor pressures is also equal to the ratio of the mole fractions of the components in the vapor phase. There-fore, the vapor and liquid compositions become equal as the maximum or minimum vapor pressure is reached. (2) Thermodynamic Relationships in Azeotropism (a) Equations of Reddich and Schutz The thermodynamic relationships derived by Reddich and Schutz (31) and given here in a concise form are limited in application by the large amount of calculation required for their quantitative use. The equations provide important qualitative information on the properties of azeotropes and for this reason warrant reproduction. The relationship between azeotropic pressure and tempera-ture may be represented by writing P = f (xB,T) (6£ Since either a maximum or minimum on the curve of total vapor pressure against composition indicates a fundamental property of an azeotrope we may set the last term equal to zero. Thus we get 9. the equations /dP\ _ VHT/az VaT/: 'xB (8) and /dP \ _ 1^P\ (9) Using the assumption that the vapors behave ideally we may show that ' By applying the Clausius-Clapeyron equation to each component i t can be shown that where AH = the molal heat of vaporisation of the solution and .A H A represents the molal latent heat of vaporisation of component A. T he latter represents the difference between the molal latent heat of vaporisation of the pure component and the relative partial molal heat content. This equation indicates the relationship between azeotropic temperature and pressure. Although i t is applied to a binary system, i t has the form of the Clausius-Clapeyron equation. This supports the "single component" theory presented by Licht and Denzler (23). Further justification of this equation is given by the fact that an azeotrope has, with the exceptionof pressure relationships, the same properties as a one component system. The relationship between azeotropic composition and temper-ature was developed by using another fundamental property of azeotropes. Since the vapor and liquid compositions of an azeotrope are identical, the relative volatility must be equal to one. Reddich and Schutz described the relative volatility as a function (10) dlnP -(11) 10. of three variables, pressure, composition, and temperature. They then proceeded to eliminate one of these variables to give the two-variable relationships that follow. This, as Coulson and Herington (10) pointed out, i s not necessary, since only one of the variables under azeotropic conditions i s independent. Nevertheless, by j applying the Duhem-Margules equation with the general equation of state i n terms of Q, the relative v o l a t i l i t y , the following equation was derived: 1 / & B \ ( d i $ ) R 1 ( a i n * i ) (12) V ^ / a z -AHB - A H A - i r A H x A ^ OXQJ T P By combining equations (11) and (12), the following relationship between pressure and composition may be easily obtained: / ain— \ /"din P L - AH _ . 1_ I ^ ) (13) \&B~/az ^H B -AHA -iTAH x A I a x B / T P In both equations (12) and (13) V* represents the par t i a l molal volume defined by the relation V B - f A f F Considering the identical last terms in equations (12) and (13), i f component B dissociates i n a dilute solution, lim a B = constant (x^ 2 (lij.) x B = 0 lim = = ^ x B = o 3 X b (15) \ 11. Therefore, for a dissociating compound or electrolyte the change of reciprocal temperature with composition and the change of the logarithm of pressure with composition become infinite as the solution becomes infinitely dilute. This explains why azeotropic electrolytes belong to the group with minimum vapor pressures or maximum boiling points. These two equations also indicate that dissociating azeo-tropic systems always have infinite azeotropic ranges. The Integra-, tion of equation 12 between xg - 0 and Xg = 1 shows that i f the denominator at no time becomes equal to zero, the azeotrope exists over a finite temperature interval. The size of this interval may make i t apparent that complete separation of the azeotropic constituents may be accomplished by fractionation. Outside the azeotropic temperature range distillation of the mixture leads to complete separation without the necessity of varying the pressure as the distillation proceeds. If the vapor pressure and temperature curves of two pure liquid compounds cross each other, the temperature at which the two vapor pressures are equal is called the isobaric temperature. If we assume complete miscibility, three forms of vapor pressure and composition curves, indicated by a horizontal straight line where Raoult's Law applies and by concave or convex curves with maxima or minima for non-ideal solutions, are possible. Any such system is azeotropic at the isobaric temperature. The ideal solution presents the limiting case of a system which is azeotropic at any composition. 12. (b) Equations of Coulson and Herington Coulson and Herington (10), using approximate statistical methods, show that azeotropes formed in "strictly regular solutions" would obey empirical rules of azeotropic behaviour. The require-ments of "strictly regular solutions" as defined by Guggenheim and Fowler (l£) are as follows: (a) Components must pack in the same way. (b) Molar volumes are sufficiently alike so that a mixture of two components can also pack in the same way as each of the single liquids. (c) Ratios of free volumes of the two pure liquids should not differ from unity by more than 3° percent. (d) T he molar volumes and free volumes of both components should remain unaltered when the liquids are mixed. (e) In any given configuration of volume, V, given by V = x AV A +- x BV B the potential energy W may be regarded as the sum of contributions from pairs of closest neighbours. In addition to the assumption that only "strictly regular solutions" are applicable, Coulson and Herington assumed, as did Reddich and Schutz, that the vapor phase obeyed the perfect gas laws. T heir method was to compare the properties of azeotropes with assumed properties of ideal solutions and to determine these properties by the amount of deviation from ideal properties. They showed that, on the assumption that the change of entropy of mixing of the "strictly regular solution" is the same as that of an ideal 13. solution, the relationship between the partial pressure of the vapor, the vapor pressure of the pure component, and the mole fraction of the pure component in the liquid is given by: Wabx?7RT pA = p°xAe ^ W (16) Wabxi/RT (17) and pBr-qpgXgej where W ^ equals the mixing energy and is assumed to be invariant with temperature. For such a mixture we may show H» = (XAXB> wab (18) X A + X B ; where H1 is the excess heat over that of an ideal solution. For an ideal solution H1 = 0 and Wab = 0 . The substitution of these two values in equations (16) and (17) reduces the equations to expressions of Raoult's Lav/ for ideal solutions. It may be seen that equations (16) and (17) are of the same form as equations (3), where Wgjy/RT is equal to the constant o C These aquations, which provide a good fi r s t approximation, are used by Coulson and Herington to develop a relationship which shows that azeotropic formation is favored by two pure compounds having vapor pressures which are nearly equal. If we replace the composition term in: Raoult's Law for ideal solutions with the product of the activity coefficient and the composition term, f A x A , we are justified in using the resulting equations for non-ideal solutions. That i s , P A = P°x/ A (19) and P Br p B V B -By> combining the relation Q = P ^ B z 1, (21) as defined by an azeotrope with equations (19) and (20) we find that In + In f A _ 0 o f"~ PB XB A comparison of equations (16) and (19) shows that (22) 2 2 f. - Wab xB and f = WabxA/RT (23) By suitable substitution we may show that x B = | [1 - (RT/V a b)ln p A/p B) . (2U) Since xg is greater than 0 and less than 1, and since the value of is small, the value of the ratio of the vapor pressures of the pure components must be nearly one. In other words their difference must be small. When the solute is solvated the potential energy contribu-tion from pairs of closest neighbors will be negative; when W ^ is negative, negative deviation from Raoult's Law occurs and a maximum boiling-point azeotrope is formed. Again, as pointed out earlier, electrolytes which do form azeotropes are generally solvated; however, the other conditions required of azeotropes are not common among electrolytes. For this reason such azeotropes are relatively rare. 15. To determine the effect of temperature we may differen-tiate equation (2I4.) with respect to temperature. This operation produces the relationship •^B = _ i R ( l n p A y P O + Td ^ po/pg) ( 2 5 ) dT ao a i T he direction of change of composition with temperature depends on the sign of W ^. If we keep the other variables constant, the variation of composition v/ith temperature for minimum boiling-point azeotropes will be opposite to that of maximum boiling-point azeotropes. Since the difference between the vapor pressures of the pure components forming an azeotrope is small, the sign of the bracketed term in equation (20) will be determined by d In P A / P B . dT Since the Clausius-Clapeyron equation tells us that Tdln pA/pg A H J - * H g (26) ST = RT we may rewrite equation (25) in the form dxB = -| R (In p°/p°- ^ - ^Hg ) dT (27) ^ab RT where A E% and A Hg are equal to the latent heats of vaporization of pure components A and B. Examination of this equation shows that the relationships between temperature, composition, and latent heat of vaporization as defined by Wrewsky (ItO) are in agreement. He points out that for a system with a maximum (minimum) vapor pressure the azeotropic concentration of the component with the larger heat of vaporization increases (decreases) with increasing temperature. 16. Since the vapor pressures of the pure components are almost equal, the approximation Tdlnp°/p°. = T/p°(dp7dT - dp°/dT) (28) is made. If, in this approximation, dp^/dT * dpg/dT, the sign of the bracketed quantity in equation (25>) will be positive. For minimum boiling point azeotropes where W ^ is positive dXg/dT will be negative. This shows that the component with the greater value of dp°/dT will increase in concentration as the temperature rises, or decrease as the temperature f a l l s . This is in accordance with an empirical rule derived by Merriman (27) . Coulson and Herington derived from equations ( 1 6 ) , (17) , and ( 2 5 ) , the following approximate relation: By assuming that In P^/Pg in equation (27) may be neglected equation (29) may be written In P = a p A " w D = — + D (30) URT . RT where K and D are constants. TBy a series of similar approximations, they obtained from equations (2U) and (30) this relationship of composition with pressure: x = A' In P + C. (31) 17. (c) Equations of Kireev Kireev (18) approached the problem from the standpoint of free energy by utilising the fundamental relationship AF = -RT(xAln p A/p A -i-x Bln P B V P B ) (32) where A F equals the free energy of formation of pure components. For an ideal mixture of the same composition, that is where Raoult's Law holds, we may write A F 1 = -RT(xA In l / x A + x B In l/xg) . (33) Kireev defined the difference between and ^ F 1 as the excess free energy, given by A F E = RT(xALn P A / x A p A + x B In p-o/x-Qp^ . Oh) By differentiating v/ith respect to x B at constant temperature the following equation is obtained * E) = R T f 1 1 ^ + i n I -:BPA The last term in the equation, In Q is, for an azeotrope, equal to zero. Therefore, we find E x ^ po ^ In fA o PB J (36) The . difficulty in evaluating this relationship exists in the determination of the excess free energy as a function'-of composi-tion and temperature. At this point Kireev suggests that for 18. many solutions the empirical relationship given by A F E = X A X B K ' , (37) where K1 is a constant, independent of x but possibly a function B of T , may be applied. (d) Equations of Carlson and Colburn Carlson and Colburn (8) attacked the problem of extending azeotropic data by evaluating activity coefficients. j-They fi r s t measured the vapor pressure of the pure components•and the total vapor pressure of the azeotrope. They then applied the more general form of Raoult's Law expressing the relationship of these two variables in terms of activity coefficients. With the values so determined they evaluated the Van Laar constants A and B, as defined by the following equations: l Q g f * = (1 + A X A / B X B T 2 — ( 3 8 ) and log f B = B < (39) (1 + Bxg/Ax^ 2 from these constants values of the ratio of f A / f g were calculated and plotted against composition at constant temperature. Com-positions were found at other temperatures from the relationship P § / P £ = F A A * B M and a plot of the ratio of activity coefficients and the composition of component B. It was assumed that the ratios of activity coefficients are independent of temperature. To facilitate the calculatiorfs experimental values of the ratio of vapor pressures 19. of the pure components were plotted against temperature, (e) Comparison of Equations The equations of Reddich and Schutz are not practical, since the measurements required for data in the determination of azeotropic relationships are more difficult to obtain experimental-ly than the relationships desired as the result of the calculation. The final equations obtained from the thermodynamic relationships of Coulson and Herington are not satisfactorily applicable to the azeotropes of hydrochloric acid. The assump-tions that were required to reduce the complexity of the equations cannot be justifiably applied to these azeotropes. Hydrochloric acid is not a "strictly regular solution"; therefore, we cannot expect i t to obey the Margules-Porter relation which provides the starting point of these relations. ,-The relationship between natural logarithm of azeotropic pressure and reciprocal absolute temperature is shown to be approximately linear by Coulson and Herington. They plot the data of Bonner and Wallace and suggest that the result is a straight line. A close examination of the plot indicates that a smooth curve would provide a much better f i t to the data. This plot is reproduced in figure 8 in Appendix I. Supporting evidence on non-linearity is given by equation 11. One would not expect the factor H^/R, given as the slope of the curve to be constant. Reddich and Schutz confirm this non-linearity by calculating the differing slopes at various temperatures and concentrations.. Coulson and Herington indicate that the relationship between azeotropic concentration" and .the logarithm of the pressure 20. is linear i f certain approximations are justified. They do, however, point out that the data of Bonner and Wallace shows that the relation is not linear for hydrochloric acid. Kireev's equations require the evaluation of an empirical constant or the difficult determination of the excess free energy as a function of composition. Since the excess free energy is a function of temperature and since the left-hand side of equation (37) contains no term apparently involving temperature, the empirical constant must be a function of temperature. This fact limits the application of. Kireev's equation. The main drawback of Carlson's and Colburn's method s>£ determining the. effect of pressure on the composition of the azeo-tropes lies in 1 "the assumption that the ratios of activity coeff-icients are independent of temperature. This assumption has no theoretical justification. 21 EXPERIMENTAL METHODS AND RESULTS A. Apparatus (2) Preparation Apparatus The apparatus for the preparation of the gas, deuterium chloride, was designed by Brown and Groot (6) and is shown in figure 1. A capillary tube, 30 cm long, surmounted by a dropping funnel was used to pass deuterium oxide into a litre flask containing benzoyl chloride. The_ capillary tube, "designed to::maintain even flow of the deuterium oxide, passes vertically through a condenser which is sealed to the lit r e flask by means of a ground glass connection. The deuterium chloride produced in the reaction of benzoyl chloride with deuterium oxide passes through the annular space between the condenser and the capillary tubing. An outlet at the upper end of the condenser leads to a trap cooled with ice and watdr. Attached to the trap by a length of glass tubing is a £00-ml flask to contain deuterium oxide in which the gas may be absorbed. A manometer containing about four inches of mercury was used to indicate a block in the small diameter tubing leading into the absorber and to provide a safety valve in the event of a stoppage. The outlets on the absorbing flask and the dropping funnel were connected through a long drying tube containing calcium chloride to a dry air distri-butor. As a precautionary meastire two additional large traps, not shown in figure l,were used. In a test run using ordinary water an excessively large amount was inadvertently added to hot benzoyl chloride. The resulting violent reaction forced benzoyl chloride from the boiling flask to the absorbing flask. Because of this 22 occurrence the two additional traps were added, one large enough to receive a l l the benzoyl chloride and the other large enough to receive a l l the absorbing water. Boiling chips of fritted glass were placed in the boiling flask to minimise bumping during the reaction. (2) Dry Air Distributor Dry air was produced by pumping air through a system of two bubblers containing concentrated sulphuric acid and of two UO-cm glass columns, one containing silica gel and the other contain-ing phosphorous pentoxide distributed on glass wool. The outlet on the second drying column was connected to a distributor, a £00-ml • flask with several openings. These outlets were connected by rubber tubing to the outlets on the main apparatus. (3) Distillation Unit Figure 2A shows at position F the distillation unit. This consists of a 300-ml flask connected through an inclined condenser to a receiving unit. The top of the distillation flask is fitted with a well containing a copper-constantan thermocouple. A Leeds and Northrup potentiometer and galvanometer were used to measure the thermocouple emf. The receiving unit was designed so that the direction of flow of the distillate could be changed without unseal-ing the apparatus. Four !?C—ml flasks, shown at H in figure 2A, were connected to four equally spaced openings on the perimeter of the base of an Erlenmeyer flask. A glass tube from the condenser passes vertically into the Erlenmeyer flask and then at an angle to a point immediately above the center line through these outlets. By rotating the Erlenmeyer in its ground glass fitting the distillate could be 23 directed to any one of the four receiving flasks. An outlet above the receiving unit yras connected through an equilibrium flask shown at B in figure 2A to the pressure controlling pumps. (U) Pressure Apparatus Pressure was controlled by two manually operated Teopler pumps, one to put air into the system and the other to remove i t from the system. The'atmospheric outlets on the Toepler pumps were connected to the dry air reservoir. In order to facilitate large pressure changes the outlet on one of the Toepler pumps was also connected through a stopcock to a water aspirator. The line between the water aspirator and the Toepler pump contained a long drying column of calcium chloride and a large trap to prevent the entry of water vapour into the system. In order to minimise pressure fluctuations due to ebulli-tion a 5-litre equilibrium flask shown at B in figure 2A was connec-ted into the system. This was insulated with rock wool and placed inside a corrugated cardboard box to reduce pressure fluctuations caused by changing ambient temperature. Pressures were determined with a manometer of sufficient length to permit differential pressure readings of 600 mm. This was made with pyrex tubing with an inside diameter of 6 mm. The manometer was f i l l e d with triply distilled mercury. A drying tube was connected to the open arm of the manometer to prevent meniscus irregularities caused by adsorbed water. A cathetometer calibrated with a standard meter from the National Bureau of Standards was used to determine the heights of the mercury columns. A Fortin 2k barometer was used to determine atmospheric pressures. (5) Pycnometric Equipment Figure 28 shows the pycnometer used in the determination of densities. This "U"-shaped pycnometer has two vertical arms made from uniform bore capillary tubing connected to a length of 15-mm tubing with a volume of 35 ml. The two side arms were calibrated i n increments representing 0.01 ml volumes. A small scale was made so that accurate interpolation between divisions could be made. With the scale differences i n volume could be measured to the nearest 0.0005 ml. The pycnometer i s shown i n the figur§ with a 35-ml syringe attached by means of flexible tubing to a ground glass f i t t i n g which covers one of the openings i n the pycnometer. Attached to the other opening i s a l5-cm length of 2-mm tubing i n the shape of an inverted "U". By placing the open end of this tubing beneath the level of the liquid and withdrawing the plunger of the syringe liquid was introduced into the pycnometer. Two ground glass caps were also provided to seal the pycnometer prior to weighing. A second pycno-meter having the same surface area was used as a tare. Auxiliary equipment required for the density determinations included a water bath kept at a constant temperature with a bayonet heater controlled by a mercury thermostat, a short arm analytical balance, and internally calibrated weights. (6) Ebulliometric Apparatus The apparatus used in determining boiling points was similar to that used by Bonner and Wallace (U) to obtain the boiling points of constant boiling hydrochloric acid. This is shown in 25 figure1 2A at A. The ebulliometer may be described as a composite of a number of units in series. The f i r s t , a 50-ml flask is connected by approximtely 30 cm of vertical glass tubing to a trap in the form of a horizontal "S". Vapor from the heated flask loses its superheat in the horizontal section and condenses in the trap. The latent heat of the vapor boils the liquid in the trap. Vapor from this second stage passes up'and condenses on a thermometer well partly enclosed by a vertical tube. The lower half of this tube is cooled by means of a water condenser. The vapor after heating the thermometer and partially condensing passes down the annular space between the well and the surrounding tube. After being completely condensed and partly cooled' the liquid drops into a small chamber. This chamber has four outlets. One, in the upper half of the chamber, is connected to an inclined tube leading to the equilibrium flask; a second, also in the upper half, is connected by a length of vertical tubing, to the space surrounding the thermo-meter well; a third, at the top of the chamber, is connected to the condenser; and the fourth, directly below the condenser outlet, is joined to a "U" shaped trap which returns the liquid to the boiling flask. This design of ebulliometer has certain advantages: super-heating is minimised by the two stages; continued recycling of the liquid ensures that temperature qquilibrium is reached; boiling point elevation due to hydrostatic head is made negligible; and pressure gradients in the ebulliometer are eliminated. The ebulliometer used originally by Bonner and Wallace 26 (U) was modified by placing a condenser in the tubing leading to the equilibrium flask and by replacing a rubber seal used to connect the thermometer to the ebulliometer with a well connected by means of a ground glass joint. A Leeds and Northrup platinum resistance thermometer of approximately 50 ohms resistance encased in a glass tube was used to determine vapor temperatures. To improve heat transfer to the thermometer the well was partly f i l l e d with mercury. A few mi l l i -litres of mineral o i l on top of the mercury provided insulation against heat loss. Although i t is not shown in figure 2, the ebulliometer was insulated with a double thickness of asbestos string on the section above the condenser. The resistance of the thermometer was determined with a five-dial Mueller bridge constructed by Leeds and Northrup.' A four-pole commutator with mercury contacts connected four leads from the thermometer to three leads from the bridge. By changing the commutator the two leads connected from the thermometer to the ratio arms of the bridge could be switched, inv.one position of the commutator one lead resistance is in series with a variable side of the bridge and the other is in series with the fixed side; in o the second position the situation is reversed. By taking the mean of two readings the lead resistances are eliminated. The bridge was designed to measure resistances from 0 to k9.999 ohms. Since the resistance of the thermometer was greater than the maximum resistance measurable by the bridge, a fixed resistance had to be added to the variable side of the bridge. A 27 • manganin resistor of approximately 35 ohms was wound and annealed at lhP°C for 72 hours. The annealing stabilises the resistance of the wire by preventing resistance changes caused by annealing at room temperatures. The resistor was sealed in a brass case containing a high purity v/hite o i l to minimise changes in resistance due to humidity changes. A high sensitivity D'Arsonval galvanometer with a lamp and a scale at one meter from the c o i l was used to indicate bridge balance. (7) Analytical Equipment The apparatus required for the determination of the com-position of the azeotropes included micro f i l t e r beakers with f r i t t e d glass discs, platinum s t i r r i n g rods, micrb-pipettes with syringes, a dessicator, and a vacuum oven;. (8) Dry .Box:: In order to prevent contamination of the deuterium oxide by moisture from the air, a dry box was constructedih 'Tshich solution could be transferred. Air which had been dried by bubbling through sulphuric acid and through columns of calcium chloride was passed continuously into the box. A humidity indicator inside the box was used to test the effectiveness of the drying process. B. Procedure (l) Preparation of the Acid The method of preparation i s that, of Brown and Groot (6). The reaction occurs i n accordance with the equations: C6H£C0C1 H - D2O — C5H5OOOD +• DCl (ll) C6H^C0C1 + C6H^COOD = (C^CO) 2° + ®G1 &2) 28 A measured excess of reagent grade benzoyl chloride was added to the boiling flask shown in figure 1. Before the addition of deuterium oxide, the benzoyl chloride was refluxed at its boiling point for one hour. During this refluxing any benzoic acid present in the benzoyl chloride was converted to the hydrogen free anhydride. Before refluxing ceased dry air was passed into the boiling flask through the dropping funnel and capillary tube. By this means volatile impurities were removed from the benzoyl chloride. When the system cooled the absorbing flask containing 220 gm of doubly distilled deuterium oxide v/ith an isotopic purity of 100.00 i 0,0.02 percent was connected to the gas outlet. Then 17 gm of the same deuterium oxide were added to the dropping funnel. The trap provided to remove any benzoyl chloride carried over in the reaction was cooled with ice. Two drops of deuterium oxide were added through the capillary to the benzoyl chloride. The benzoyl chloride was then slowly heated to °5°C, the temperature at which satisfactory evolution of deuterium chloride was obtained. After a l l the deuterium oxide was added, the temperature of the benzoyl chloride was slowly raised to the boiling point (197°C), and maintained at this temperature until the gas evolution notice-ably slackened. At this time a stream of dry air was passed into the boiling flask to flush out the deuterium chloride in the system. The flask containing the acid was removed, wealed, and set aside for distillation. 29 (2) Distillation of the Azeotrope The prepared acid was transferred to the distillation flask shown at F in figure 2 through a glass tube connecting, by means of a pair of ground glass fittings, the mouths of the distillation and absorption flasks. In this way contamination by moisture in the air was prevented. Contamination by moisture inthe complete system was reduced to a minimum by flushing i t with dry air for twenty hours before the distillation was commenced. When the distillation pressure was below atmospheric, i t was adjusted to approximately the correct value by means of the aspirator.. An approximate pressure above atmospheric was obtained by forcing dry air into the system. Final pressure adjustment , was made and maintained with the Toepler pumps. After the d i s t i l l -ation was commenced, care was taken to keep the pressure within 0 . 2 mm of the desired pressure. To reduce pressure fluctuations caused by bumping, fritted glass fragments which had been dried for two hours at 120°C in a vacuum oven were placed in the distillation flask. The rate of distillation was kept constant at 2.5 ml per minute by adjusting the temperature of the heating mantle with an auto-transformer. During the distillation repeated pressure readings were taken. The manometer readings included mercury colmmn heights, meniscus heights, manometer temperature, and cathetometer tempera-ture. Corrections as indicated by the last three measurements were applied to the mercury column heights. The correction for the local effect of gravity was also applied. Atmospheric pressure 30 was determined with the Fortin barometer. Corrections similar to those applied to the manometer readings were made to the barometer readings. A further correction had to be applied to the barometer readings to obtain the pressure in the laboratory which was one floor bglow the room containing the barometer. A table of these corrections is given for different air temperatures in Appendix IV. After 130 ml of acid had been distilled, a sample of I4O ml was collected in the last of four receiving flasks shown in figure 2 at H. To make sure that the azeotropic composition had been reached, the boiling temperature was approximately determined with the copper-constantan thermocouple. Repeated readings were taken until the maximum boiling point was reached. The cold junction of the thermocouple was placed in an ice-bath to eliminate fluctua-tions in emf caused by changing room temperatures. After the sample was collected a pressure slightly higher than atmospheric was immediately applied to the system. The sample-bottle was then removed and capped. The opening inthe system created by the removal of the sample bottle was also covered. The outflow of air from the system ini t i a l l y above atmospheric pressure reduced the possibility of moist air diffusing into the system. The series of manometer and barometer readings obtained -during the distillation were plotted against time, and the average pressure over the time of the distillationof the selected sample determined. At the same time the mean deviationof the pressure during distillation was calculated. A sample plot showing the method of determination is given in Appendix I, figure 7. The 31 corresponding calculation is given in Appendix III, part 5. The distillations were made at approximately 100 mm pressure intervals from 200 mm up to 1,000 mm. An additional distillation was carried out at 760 mm. Since the approach to azeotropic composition is more rapid for the acid containing excess deuterium chloride than for the acid containing excess deuterium oxide, the distillations were carried out in the order given. The excess deuterium chloride was drawn off through the drying column and through the water aspirator. Consequently, for the i n i t i a l portion of the distillation one of the Toepler pumps was used merely as a vacuum reservoir connected to the water aspirator. When the azeotropic composition was reached the aspirator was disconnected and the Toepler pump brought into use. The hydrochloric acid prepared to test the method of preparation was distilled using the above procedure at a pressure of 600 mm. (3) Determination of Density The next stage in the procedure was the determination of the density, of the azeotrope. After being cleaned in chromic-sulphuric acid, the pycnometer, and tare were flushed with distilled water for a period of hours and then placed in a bath of distilled water overnight. The pycnometer and tare vrere then dried under vacuum and flushed for half an hour with balance room air. Before the empty weight of the pycnometer was determined both pieces were wiped with a moist chamois cloth and left in the balance case for twenty minutes. The pycnometer was then f i l l e d with acid in the dry 32 box to an arbitrary level. The syringe and f i l l i n g tip were removed, and the ground glass tops of the pycnometer wiped off with tissue paper. Then the walls of the capillary tube above, the liquid level were moistened by tipping the pycnometer. The pycnometer was then capped, removed from the dry box, and placed in the constant tempera-ture bath. The water bath, kept at 2!? .CO t 0,01°C, was used to bring the solution up to the desired temperature. The bath thermometer, calibrated at the ice point and'the boiling point of water, was calculated by linear interpolation to be 0.035° low at 25°C. This correction was applied and the bath temperature adjusted accordingly. After approximately one half hour in the bath the positions of the surface of the acid in the capillary arms of the pycnometer were determined. Readings were taken at three minute intervals to insure at the end of the half hour that the solution had reached the bath temperature. The pycnometer was then removed from the bath and wiped with a piece of moist chamois leather. The same treatment was given to the tare pycnometer. The weight was subsequently determined as before. Following this the acid was returned to the receiving flask. The pycnometer was then washed with distilled water and dried by passing through air for half an hour. The empty pycnometer was then reweighed. Since the pycnometer could be f i l l e d with water- without the encumbrance of a dry box, an attempt was made to use the same volume of water as acid. In this way the correction applied to the water volume was made small. The calibration with water was 33 made at each determination to minimise the errors i n corrections for air buoyancy. Carrying out the determination and calibration at the same temperature and humidity reduces the error i n the assumption of an average a i r density. The density of the hydrochloric acid prepared as a test determination and that of the deuterium oxide used for the acid were determined in.addition to the densities of the deuterochloric acids. The deuterium oxide density provided a measure of isotopic purity. (h) Determination of Boiling Point The boiling point of the azeotrope was next determined. The f i r s t step leading to this goal was the calibration of the resistance thermometer. This was done by comparing measured resist-ances with the boiling points of water at nine different pressures between 1,000 and 500 mm. After doubly d i s t i l l e d water had been added, to the dry ebulliometer the pressure was adjusted to within 0.5 mm of the desired pressure and the water brought to i t s boiling point. The pressure was then readjusted to within 0.1 mm of the same pressure. After boiling for one hour a series of readings of resistance and pressure were taken for approximately one hour. These readings were plotted'against time and simultaneous pressure and resistance readings were interpolated. As i n the acid d i s t i l l a t i o n , f r i t t e d glass boiling chips were used to minimise bumping. The data obtained i n this manner were correlated by means of a second degree polynomial with temperature as the independent variable. The method of least squares was used to obtain the best equation. The boiling points of the azeotropes were determined i n a similar manner. The same d i s t i l l a t e was poured into the ebulliometer 3U by means of a coupling tube. Condensing water was provided at a constant head to maintain a constant heat output. The heat input supplied by a heating mantle was regulated by means of an auto-transformer. (5) Determination of Composition Each time the pycnometer was filled with acid, a milli-litre sample was taken to determine its composition. These samples were removed from the receiving flask and put into tubes with a capacity of just over one ml. These were then set aside to be analysed on completion of the determination of densities and boiling points. The proven stability of the azeotrope of hydrochloric acid permitted this procedure (35>) • The chloride was determined gravimetrically by precipi-tation as silver chloride. Four mico filter beakers and their accompanying platinum stirring rods were dried in a vacuum oven at 120°C for an hour and a half. They were immediately placed in a dessicator and allowed to cool. i'Two of the flasks, one a tare, were removed at the end of thirty minutes and wiped with a moist chamois. After standing for twenty minutes in the balance case the difference between the empty weight of one flask and the tare was determined. The third and fourth flasks were removed forty and fifty minutes respectively after being placed in the dessicator. These were also weighed after being wiped and after having stood for twenty minutes. Within five minutes of the determination of empty weights approxi-mately one half milliliter of the acid was added to the empty flasks and weighed. The acid was immediately diluted to six times its volume to await precipitation as the chloride. Doubly distilled 35 water was used to dilute the samples. Approximately 1.5 M silver nitrate, prepared from the reagent grade salt after three crystallis-ations, was then added dropva.se with repeated stirring until no further precipitation occurred. The solutions were then heated to their boiling point. A further drop of silver nitrate solution was added to test for the completeness of the precipitation. After the precipitates had stood in the dark for two hours, they were sucked dry on the sintered glass f i l t e r . They were then washed with five one ml portions of 0.02 N nitric acid prepared from re-distilled nitric acid and doubly distilled water. A second washing with two five-ml portions of absolute alcohol was followed by a wash with the same volume of anhydrous ether. The precipitate was then sucked dry for ten minutes. The micro beaker was wiped with a damp chamois, and the tip of the platinum stirring rod was cleaned with a small amount of ether:.to remove grease applied by handling. All three micro beakers containing the precipitates and the tare micro beaker were placed again inthe vacuum oven and dried for an hour and a half at 120°C. The procedure of handling the empty micro beakers was then followed. Care was taken that the times elapsed between steps of the drying and weighing processes were the same for the weight determination both with and without silver chloride. An attempt was made to determine the rest point change of the empty micro beaker between the time the empty weight was deter-mined and the time the weight of the acid; was determined. A series of readings of rest points with the empty vessel showed no signifi-36 c ant difference. The weights of acid and silver chloride were corrected for the buoyancy of air. A further correction^was required for the loss of silver chloride due to its solubility in water. The assay of anhydrous ether indicated a small correction should be made for the solids in the ether. This partly compensated for the solubility correction; consequently, an approximate total correction was made. Any correction which might have been required to compensate for the photodecomposition of the silver chloride was made negligible by carrying out the procedure in a subdued light. A preliminary determination was made on the composition of the hydrochloric acid which had been prepared and distilled by the above procedures. G. Results (1) Preparation The method of preparation devised by Brown and Groot (6) proved very satisfactory. The reaction went smoothly with good yield. The absorption of deuterium chloride in deuterium oxide was nearly complete. Evidence of very small losses during a test prepara-tion of hydrochloric acid was obtained by connecting the outlet tube of the absorbing flask to a second absorber containing a silver nitrate solution. A slight cloudiness was observed at the end of the preparation. A small amount of benzoyl chloride was carried -over during the refluxing period; this, however, was quickly removed as benzoic acid during the f i r s t distillation. 37 (2) Azeotropic Data and Empirical Correlations The data obtained on the boiling points, compositions, and densities of the.azeotropes of deuterochloric acid distilled at various pressure are listed in Table I. These data are plotted TABLE I Azeotropic Data for Deuterochloric Acid Pressure, Boiling Point, Density, Composition, mm. Hg. C. GM/ML. mole fraction DCl percent DCl 202.5 76.83 1 . 2003U 1 0.11567 19.661 301.1 85.52 1.197U9 ^ 0.11199 19.090 397.5 91.97 1.1953U 0.10927 18.667 503.2 97.76 1.19352 0.10691 18.298 598.0 102 .07 1.19209 0.10510 18.01U 698.9 106.18 I .19078 0.103a3 17 .752 760 .5 108.52 1.18999 0.102a2 17 .595 1 800.7 109.89 1 . 1 8 9 5 3 1 0.10187 17 .506 2 902.7 113.26 1.188UU o.iooas 1 7 . 2 8 6 1 998. U 115.U32 1.1877U 0.09959 17.1U6 Calculated from the correlation of composition and density 2 Calculated from the correlation of composition and boiling point in figures.3, a, 5 , and 6, of Appendix I v/ith the equivalent data for hydrochloric acid as given by Bonner and Wallace (a) . This data is reproduced in Appendix II. T he boiling points of the deuterochloric azeotropes were related to the corresponding total pressures by means of the 38 following polynomial: t - 100 = 8.U1D2 + 3.5U13(10" 2 ) (P - 760) - 1.5203(10-5) (p - 7 6 O ) 2 + 3.5575(10" 8 ) (P - 760) 3 (^3) This equation was obtained by using the data between 301.1 mm and 902.7 ram inclusive. T he deviations from this equation of the data obtained on the deuterochloric acids and of the data of Bonner and Wallace on the hydrochloric acids are given in Tables II and III respectively. They are plotted in figure 7, Appendix I, Table II also shows the mean deviation of the pressure from the mean value during the distillation and the corresponding temperature deviation calculated from the slopes of the boiling point and pressure curve. TABLE II Deviations of B oiling Point of Deuterochloric Acid from Mean and Calculated Values Pressure, Boiling point Mean Mean Temperature mm. Hg. C. pressure temperature deviation deviation deviation from during calculated calculated distillation from pressure temperature, deviation. mm °C °C 202.5 76.83 ± 0 . 3 ±0.03 - 0 . 9 8 301.1 85.52 ± 0 . 2 ± 0 . 0 2 - 0 . 0 3 397.5 91.97 ± 0 . 2 ± 0 . 0 1 +0.06 503.2 97.76 ± 0 . 2 ± 0 . 0 1 + 0 . 0 2 598.0 102.07 ± 0 . 2 ± 0 . 0 1 -0.08 698.9 106.18 ± 0 . 2 ± 0 . 0 1 - 0 . 0 3 760.5 108.52 ± 0 . 2 ± 0 . 0 1 + 0.06 8OO.7 109.89 ±0.1t ± 0 . 0 1 + 0 . 0 3 902.7 113.26 ±0.8 ±0.02 - 0 . 0 3 998. h 115.^3 ± 0 . 6 ±0.02 - 1 . 0 7 39 TABLE III Deviations of Boiling Point of Hydrochloric Acid from Datum Equation ( l ) * Pressure, Boiling point, Temperature mm C deviation from calculated .temperature °C. 250 81.205 -0.501 370 90.237 +0.031 hoo 92.080 +0.019 U5o 95.029 +0.088 500 97.578 -0.002 5U0 99.(63 +0.118 600 102.209 -0.030 ' 6U0 103.967 +0.057 680 105.56U +0.072 700 106.1*21* +0.171 7h0 107.859 +0.13U 760 108.581* +0.11*1* 800 110.007 +0.172 1000 116.185 -0.370 1220 122.98 -1.996 boiling point data .is that of Bonner and Wallace (1*). The relationship between boiling point and percent composi-tion i s linear over the range considered. The.experimental data i s 1*0 closely fitted by the line corresponding to the equation t = 378.899 - l5 .3665c D C 1 (hh) where t is the boiling point of the azeotrope in degrees Centigrade, and c-pQ-^ is the percent by weight of deuterium chloride in the azeo-trope. The mean deviation determined from the temperature residuals is 0.0U°C. .'This corresponds to a deviation in-composition of 0.003 percent. The boiling point and"composition data are shown plotted with the corresponding data for'hydrochloric acid in figure 5,' Appendix I. When equation (Uk)"is rewritten in terms of mole frac-tion of deuterium chloride, x^^, and degrees Absolute, T, i t takes the following form: T = - 3301.00 xpci + 6£ 2 <o6 (U5) X DC1 + l.lUblb For this conversion the molecular weight of deuterium chloride was taken to be 37.1i72, and that of deuterium oxide was taken to be 20.0281;. The difference between degrees Absolute and degrees Centi-grade was taken to be 273.16 degrees. The data on density and composition are related by the following empirical equation: c _ dg? - 1.10185' D C 1 0.0050096 (U6) where cnci is the percent by weight of deuterium chloride in the azeotrope and d|p is the density of the azeotropeat 25°C. relative to water at U°C. The mean deviation of composition from the composi-tion calculated from equation (h6) is 0.002 percent. T his corres-ponds to a density deviation of 1.0(10 -£). Bonner and Titus give the following equation relating the i l l density and percent composition of hydrochloric acid: c s d g * - 0.9966 H C 1 0.001*912 (U7) where c H C 1 is the percent by weight of hydrogen chloride and d 2^ is the density of the azeotrope relative to water at U°C. Both sets of data on composition and density are plotted in figure 6, Appendix I. (3) Calibration of the Resistance Thermometer The resistance of the platinum resistance thermometer can be related to temperature by the following equation: r = 69.30^2 + 0 . l 8 9 1 9(t - 100) - 0.00020U58(t - 100) 2 (1*8) where r is the resistance of the thermometer in ohms, and t is the temperature of the thermometer in degrees Centigrade. The data given in Table IV showing the boiling points of water at various pressures and the corresponding resistances of the thermometer were used to calculate equation (1*8). These resistances include the resistance of the resistor used to change the range of the Mueller bridge. The value of the latter was found to be 35.01*1*3. The boiling points in Table IV were computed from the following equation: t = 100 + 0.0368578(P - 760) - 0.000020l59(P - 760) 2 (1*9) + 0.00000001621(P - 7 6 0 ) 3 , where t is the boiling point of pure water in degrees Centigrade and P is the pressure in mm. This equation was fitted to the data selected by Swietoslawski (39) . h2 TABLE IV Data for the Calibration of the Thermometer Resistance Pressure, mm Boiling point, of water, °C Resistance of thermometer, ohms 501.8 88.860 67.166 582.5 92.732 67.932 699.5 97.692 68.867 7$k.k 99.79k 69.256 755.6 99.839 69.275 823.6 102.272 69.737 881.8 10it.219 70.098 913.6 105.2HU 70 . 292 999.8 107.903 70.789 The mean deviation obtained from the resistance residuals by means of equation (U8) was found to be O.OOI4.3. ohms. This devia-tion corresponds to a temperature change of 0.023°C. (U) Test Determinations on Hydrochloric Acid The boiling point of hydrochloric acid prepared at 600 mm was found to be 102.21°C. Bonner and Wallace (U) give a value of 102.209°C. The density of the test determinationuusing hydrochloric acid was found to be I.O98OO and is given the same value by Bonner and Titus (3) . The composition of this same sample was determined to be 20.637 percent hydrogen chloride. The value given by Bonner and Titus is 20.638 percent. U3 {$) • Isotopic Purity of Deuterium Oxide The density of the deuterium oxide used in the acids relative to water at 2f?°C was found to be 1.10766 gms per ml. That of pure ' deuterium oxide is now generally accepted as being I .IO76U (19) . This value of density indicates a deuterium oxide content of 0 .019 percent greater than 100 percent. hh DISCUSSION (A) Precision and Accuracy (l) Isotopic Purity of the Azeotrope The purity of the deuterium oxide used to prepare the acid was determined from the specific gravity, S, using the equa-tion Atom percent D = 92$.k9AS 1 - 0.0339*S (50) where AS = S(sample) - S(D20 free H20) and dijjj = I.IO76U for deuter-ium oxide. The constants in the equation are those used by Vosukyl and privately communicated to Kirshenbaum (19). S(D20 free H2O) is taken as 16 ppm less than that for ordinary water, that is, 0.99998U. Kirshenbaum points out that the equation can be varied to f i t another value of d|| by a simple inverse-proportional adjustment of the coefficients. The equation involves the assumption that the volume change on mixing deuterium oxide and water is negligible. Deviations from the normal distribution of the isotopes of oxygen in the water were assumed to be negligible. The relative density of the deuterium oxide was determined as being 2(10""->) greater than the value selected by Kirshenbaum (19) after a critical survey of available data. Equation (50) gives a change of 0.02 atom percent deuterium for this difference. This deviation in density gives a measure of the accuracy of the density determinations. Further evidence on precision should, however, be considered before any conclusion about the isotopic purity can be made. Evidence of the accuracy of the density measurements h5 indicated by the determination on deuterium oxide is supported by the limitation in the differential volume measurements made with the pycnometer and the precision determined from azeotropic data. The volumes in the capillary side arms of the pycnometer were measurable to within 0 .0005 ml. An error of 0 .0005 ml in the 35 ml volume of the pycnometer corresponds to an error in density of 1.5(10"^). This is approximately the same as the deviation indicated by the density determination of deuterium oxide. A mean deviation of 1(10-5) was obtained from the correlation of density and composi-tion determined by the method of least squares from the data on the azeotropes of deuterochloric acid. The density determination made on the hydrochloric acid is unfortunately of l i t t l e assistance in estimating the accuracy. The published density measurements of Bonner and Titus (3) are given only four significant figures after the decimal point. The data of Tronstad and Stokland (37) on the change in density with temperature of deuterium oxide and the corresponding data on water taken from the Smithsonian Tables show that i f the difference between the measured and the correct temperature of the water bath were 0.07°C. the measured relative density would be in error by only 5(10"^). This same data shows that this same error would be present i f the calibration of the pycnometer with water and the determination with deuterium oxide were done at two tempera-tures separated by 0.02°Q. Since the water bath was kept constant to within 0.01°C. these thermal errors were negligible. U 6 The use of an average air density in the buoyancy correct-ions does not introduce significant error. If i t is assumed that the relative humidity is constant at- 60 percent, a temperature change of approximately 3°C is required between the calibration and determina-tion steps of the procedure to cause an error of 1.5(10 -^). At a constant temperature humidity changes of nearly 50 percent are required to produce the same error. Since precautions were taken to ensure that the two weighings were carried out under similar conditions, i t is reasonable to assume that the errors due to atmospheric changes are unimportant. Other means of introducing errors were foreseen and eliminated. Electrostatic charges on the glass surfaces and in the balance case were neutralised by placing a radioactive source in the case. The same weights, where possible, were used inbboth weighings to diminish any small errors in the calibration of the weights. The change in apparent weight of the pycnometer caused by evaporation of moisture from the glass surface became negligible within fifteen minutes after the pycnometer was wiped with a moist chamois cloth. The conclusion may be made that the largest source of error was the measurement of the volumes of liquid in the pycnometer. Other possible errors even i f they were cumulative would not exceed the possible volume error. This indicates that we may reasonably assume an isotopic purity of deuterium oxide within 0 .02 percent of 100 percent. An approximation of the change In properties of the deuterochloric acids caused by small amounts of ordinary water in the deuterium oxide may be obtained by a linear interpolation of hi the data on deuterochloric and hydrochloric acids. The boiling points of the azeotrope considered independently will not be measura-bly affected by even large amounts of ordinary water since they are so remarkably close. The effect on the composition of the azeotrope will not become significant until the deuterium oxide content drops below 99.95 percent. At this isotopic concentration a change in the composition of 0 . 0 0 1 percent could be expected. These figures indi-cate that any significant contamination occurring during the experi-mental work would show up as a discontinuity in the curves relating the composition to other properties. The deviations of composition from the values calculated from the boiling points show no trend which might indicate such a discontinuity in the data. Furthermore, the density values themselves show no sharp deviation from their relationship with composition. The possibility of contamination during the preparation should be considered. A search of the literature vras made to determine whether the nuclear hydrogen in the benzoyl chloride could be expected to exchange with the deuterium in the deuterium chloride. Although no relevant information was found on this compound i t was assumed that any such exchange would be relatively slow and would not occur to a significant extent. (2) Pychometry The possible sources of error in the determination of the density of the .azeotrope are the same as those indicated in the earlier discussion on the determination of the density of the deuterium oxide. In extending to the azeotrope the approximate results of the U8 calculations showing the magnitude of the errors caused by changing experimental, conditions one must assume that the coefficient of thermal expansion is approximately the same for the acid as i t is for the deuterium oxide. (3) Manometry • During the distillations of the azeotropes i t was found difficult to maintain the pressure to within 0.1 mm of the desired value. Early in each distillation a considerable volume of deuterium chloride was evolved; this, of course tended to increase the pressure rapidly. Bumping could not be eliminated at the lower pressures. Generally a short interval during which boiling stopped preceded violent ebullition. During this time the pressure dropped suddenly, as much as 2 mm. In a period of seconds, however, the pressure returned to its previous value. Further small pressure fluctuations were caused by changing laboratory temperatures. A simple calcula-tion shows that a change in laboratory temperature of approximately 0.ls.°C. would be accompanied by a change in pressure of 0.1 mm near atmospheric pressure. These changes in pressure were accompanied by changes in meniscus heights. Because of these fluctuations the method of averaging a number of readings over the time of d i s t i l l a -tion was found necessary. At higher pressures i t was found even more difficult to maintain a constant pressure. The ground glass fitting between the condenser and the distillation flask was the most probable source of trouble at higher pressures. At this point the hot acid vapors heated the connection and the stopcock lubricant used to retain the seal. Leakage at this point may easily explain the large mean U9 deviation values at pressures above 800 mm. It is interesting to note that the meniscus corrections which had to be applied to correct the manometer readings were occasionally as large as 1.3 mm. The corrections used were those given by Weissberger (39), selected from the work of Cawood and Patterson (9) and others. At the time Bonner and Wallace (h) produced their results the meniscus corrections probably used were those of Mendeleef and Gutkowski (25). The latter corrections are s t i l l quoted in the Handbook of Physics and Chemistry (thirty-second edition). Because of these corrections i t is quite probable that the pressure readings of Bonner and Wallace will be in error. A true estimation of the error cannot be made since neither the meniscus heights at the pressure readings nor the inside diameter of the manometer are known to the writer. If a small manometer were used, say h mm inside diameter, and one meniscus were 0.8 mm and the other zero, the recommended correction is 2.12 mm. The older data indicates a correction of 1.5U mm. The pressure differ-ence of 0.58 mm corresponds to a temperature difference of approxi-mately 0.02°C. near atmospheric pressure and 0.0ii°C. at 200 mm pressure. The agreement between the measurements of the boiling points of hydrochloric acid distilled at 600 mm indicates that, for this pressure at least, the error in meniscus correction was not significant. The temperature and pressure data are both dependent on the correctness of the barometer used.- A certain doubt exists regarding the accuracy -of the measurement of atmospheric pressure. 50 The barometer used was not in the same laboratory as the remaining equipment. Although the pressure readings were corrected for the difference iii:'altitude of the two rooms, a difference of U.3 meters, (see Appendix IV), the effect of draughts may introduce error. The magnitude of such an error would be difficult to determine' since relevant conditions are not constant. An attempt was made to mini-mise such an effect by keeping the doors and windows of both labora-tories closed. Fortunately an error of 0 . 1 mm in pressure has a very small effect on the azeotropic properties determined. Boiling point is in error by 0.003°C. near atmospheric pressure and less at higher pressures, density by l ( 1 0"6) } and composition by 2(10 - ^) percent. At pressure below U00 mm the same error in pressure would be equivalent to an error in boiling point of 0 .01°C, in composi-tion of 6(10-U) percent, and in density 3(10"^) . From these figures i t can be seen that composition and density are relatively insensitive to pressure changes. This is a definite advantageimpreparing an acid of known composition. The establishment of pressure to the nearest millimeter would be generally quite satisfactory. (U) Ebulliometry The data selected by Swietoslawski (39) from a group of references and the data published in the Handbook of Physics and Chemistry based on values given by Keyes in the International Critical Tables on the boiling points of water below 600 mm pressure were found to differ substantially. At 600 mm pressure the boiling point given by Swietoslawski is 0.01°C. higher than the value given in the handbook. At 500 mm the difference is 0.08°C. in the same 51 direction. The deviation becomes greater as the pressure decreases; at U60 mm the difference is 0 .26°C. It was assumed that the data of Swietoslawski was correct. The possibility of similar differences in the calibration done by the National Bureau of Standards on the resistance thermometer used by Bonner and Wallace and the calibration of the resistance thermometer used in the determination on deutero-chloric acid should not be dismissed. If the differences were of the same magnitude, the boiling points of hydrochloric acid would be about 0.3°G. lower than shown in figure 7 at 300 mm pressure. Since the differences in the two sets of data on the boiling points of water above atmospheric pressure are negligible this possibility does not affect the boiling point differences of the acids above 550 mm. The limit of precision set on the measurement of boiling point by the measurement and control of pressure was approximately equal to the limit of precision set by the measurement of resistance. The.sensitivity of the galvanometer used with the Mueller bridge was considerably reduced by the addition of an auxiliary resistance to the variable side of the bridge. This modification limited the precision of the resistance readings to ± 0 . 0 0 1 ohms. The corres-ponding temperature deviation would be approximately ± 0.006°C. One of the most satisfactory methods of Binding the preci-sion of the temperature measurements is_ to determine i t from a good correlation between temperature and a variable which has been measured to a much greater relative precision than temperature. Unfortunately, pressure, the variable which would best provide information, has not 52 has not been successfully correlated with temperature. Figure 7 shows a definite trend i n the deviations of boiling point from the values calculated with the correlating equation. It i s apparent from the deviation curve that the equation does not adequately describe the relationship between temperature and pressure. An attempt to correlate a l l the data including the boiling points at 202.5 and 998.U mm pressure resulted in a mean deviation of 0 . l5°C. This is much higher than the deviation of 0.0l4°C obtained with the equation f i n a l l y used. The correlation of the logarithm of pressure with reciprocal absolute temperature results i n an even larger mean deviation. Because of the complexity of the relationship between the azeotropic boiling point and pressure we must turn to the proven quadratic relationship between resistance of platinum and tempera-ture to determine the deviations i n the boiling point measurements. Since a precise relationship between pressure and the boiling points of water exists ( 3 9 ) , the calibration of the resistance thermometer offers a method of finding the deviation of individual temperature readings from those obtained from the resistance-temperature correlation. We may assume that the experimental errors i n measuring resistances during the calibration of the thermometer w i l l be present to some extent during the determination of the b o i l -ing points - of the azeotropes. Since the results of the calibration were obtained from nine measurements we can reasonably expect that the smoothed data w i l l be more accurate than any of the individual readings. For this reason the errors i n boiling point measurement occurring during the calibration and those occurring during the determination w i l l not be cumulative. It i s suggested that the 53 errors in the determination will be even smaller than those in the calibration. The application of the experience in technique gained in calibrating the resistance thermometer to the subsequent measure-ment of azeotropic boiling points will probably reduce the error. During their experimental work on the boiling points of hydrochloric acid Bonner and Wallace (h) observed that i t was much easier to obtain reproducible measurements on the acid than with either water or benzene. This evidence indicates that the mean deviation of 0.0[|°C. calculated from the calibration data is greater than the mean deviation to be Expected in the boiling points of the azeotrope. Further information on the precision of the boiling points is obtained from the linear correlation between boiling point and percent composition. The results from this correlation are unfavor-able since the composition measurements were not determined to the same relative precision as the temperature measurements. If i t is assumed that regardless of the experimental deviations in the varia-bles controlled or measured the acids were always azeotropic, a composition change of 0 . 0 0 1 percent corresponds to a temperature change of 0 . 0 2 ° C . From the correlation between density and composi-tion the mean deviation of percent composition is 0 . 0 0 2 percent, the corresponding temperature deviation is 0 . 0 l|°C. This value helps to establish an upper limit on the temperature error. Qualitative evidence on the precision of the measurement of the boiling points is given by the curve shown in figure 7, Appendix I, which best fits the points representing the deviations of the boiling points from those indicated by the equation (U3) . 5k The evidence of two maxima in the curve is supported by the data for hydrochloric acid (h) given on the same graph. If the precision on the boiling points of deuterochloric acid was as large as 0.0U°C., one could not expect the experimental points to li e on the smooth curve clearly defining the two maxima. This plot indicates, that the mean deviation is less than 0.02°C. The final evidence on the extent of the error in the measurement of boiling points is obtained from the results of the test determination in which hydrochloric acid was used. The difference between the boiling point given by Bonner and Wallace and the boiling point determined in the test experiment was less than 0.01°C. It should be noted in conjunction with this informa-tion that the deviation of the boiling point determined by Bonner and Wallace falls very close to the deviation curve of hydrochloric acid given in figure 7. It can be concluded that the boiling points of the azeo-tropes are precise to within ± 0.02°C. This temperature deviation corresponds to a pressxire deviation of between 0 . 3 and 0 . 6 mm, indicating that the degree of pressure control was adequate. (50 Composition The conclusions regarding the errors in measuring boiling point and density and the success in the correlation of these variables with composition show that the precision in the measure-ment of composition is between ± 0 . 0 0 2 and t 0 . 0 0 3 percent. The accuracy of the determination of composition is indicated by the close agreement between the value found for the composition of the azeotrope of hydrochloric acid prepared at 600 mm and the value given by Bonner and Wallace (It) . B. Comparison Properties The boiling points of the .azeotropes distilled at the same pressures are remarkably close. The scale of figure 3 is not large enough to indicate any difference. Figure 7, however, giving the deviations of the boiling points from equation (h3) does show that the boiling points of hydrochloric acid are slightly higher at pressures above 600 mm. The curves y/ere drgvm both to-fit the observed data and to show a uniform change in the difference of the boiling points of the two acids. It is interesting to note that the concentration of deuterium chloride in the heavy ageotrope is lower than the concentration of hydrogen chloride in the light azeotrope. If Simmons' (36) hypothesis that the maximum ionic concentration at a given temperature occurs at the azeotropic composition is correct, i t can be stated that the maximum ionic concentration in the heavy azeotrope occurs at a lower solute concentration in the heavy acid than in the light acid at the same temperature. The increase in the difference between the boiling points of hydrochloric acid and deuterochloric acid with increase in pressure as shown in figure 7 may be explained by the increase in the difference between the compositions of the two acids as shown in figure U. A decrease in concentration of solute produces a decrease in the number of ions present. Thus the more rapid decrease in the concentration of deuterium chloride in the heavy acid would indicate an increasing separation of boiling points. 56 It is difficult to make specific comparisons of the proper-ties of the two azeotropes because of the differences in the variables. If the vapor pressures are compared at the same compositions, the unknown effect of temperature places doubt on any conclusions which may be drawn. Similarly, the boiling points at the same compositions should not be compared without taking into account the large total pressure difference. Some information may be obtained by considering the compositions at approximately the same temperatures and pressures. A necessary assumption could be made that the differences in the boiling points at the same pressure were negligible. Under these conditions two problems become apparent, the first, the reasonsfor the same total vapor pressures at these compositions, the second, the reason for the lower concentration of chloride in the heavy acid. The first problem could be answered by a knowledge of the relative contributions of the different species in the two solutions to the total vapor pressure. An equilibrium probably exists in concentrated solutions as indicated by the following equations: HCI + H20 = H 2 0 H C 1 - H 30 + * Cl~ (5l) DCl + D20 = D 2 0 D C 1 = D30+-»- CI". (52) The concentration of hydrogen chloride is probably small; nevertheless, its very high volatility would have significant effect on the total vapor pressure. Although the vapor pressure of pure deuterium chloride is known to be less than that of hydrogen chloride (22), no conclusion about their relative effect in the acids can be made until their relative concentrations have been determined. Similar problems exist in estimating the effective contribution of the water. In addition to the effect on the vapor pressures of water and deuterium 57 oxide of hydrogen and deuterium bonding, the effect of deuteration and hydration of ions should be considered. Hydration of ions at the boiling temperatures of the azeotropes wi l l not be extensive; however, the differences between hydration and deuteration may be significant. Although the other species shown in the equilibrium may not contri-bute directly to the vapor pressure of the solution, they partly control ' the concentrations of the constituents which are transferred to the vapor state. If i t could be assumed that there was a smaller degree of ionization in the heavy acid than in the light acid, the equilibrium in the second-step shown in the above equations would be shifted to the right for the light acid. This information alone does not indicate that the f i r s t step of the equilibrium is also shifted in the same direction. The complex D2ODCI may have less tendency to dissociate than the corresponding HgOHCl. Although i t might appear that the low concentration of deuterium chloride in the heavy acid could be explained by a smaller ionization constant for the complex, this condition of concentration would not necessarily be predicted because of the smaller constant. The only justified conclusion that could be made would be that at the same concentration and temperature there would be fewer ions in the heavy acid. No conclusions about the concentration of the molecular compound or the dissociated constituents of compound could be made. The equilibrium suggested above neglects a number of factors which may be important in governing the difference in proper-ties of these acids. The formation of ion pairs and ion triplets will occur to a different extent in the two solutions since their 58 formation is partly regulated by ionic size. The possibility that the ion pair and the molecular compound are indistinguishable may be a justified assumption. Any" attempt to explain the results in this manner is limited by the lack of available information both theoretical and empirical on concentrated solutions. C. Linearity of Correlations The linearity of the plots of density vs composition (figure 6) is a condition arising out of the relatively short range of compositions and densities compared. If the curve for deutero-chloric acid is extrapolated to zero concentration of deuterium chloride, a value for the relative density of deuterium oxide of 1.10185 is obtained; the correct value is 1.1014$. The slope of the straight line joining the density of the azeotrope containing 17.752 percent to the density of pure deuterium oxides, different by 3 percent from the determined slope. Using this line through the mid-point of the data rather than the one determined experimentally will produce a deviation in composition of 0 . 0 3 5 percent at each of the two extremities of the range. Since this deviation is far greater than the maximum deviation i t is apparent that the curve is actually concave upwards. The linear relationship between the temperature and the composition as shown in figure It may seem rather surprising. Unfortunately, no simple test such as that applied to the relation between composition and density can be made. An extrapolation to zero composition gives a boiling point of 378.8°9°C for pure deuterium oxide. An extrapolation of the pressure to zero composition to determine whether this temperature is the boiling point of deuterium oxide at the extrapolated pressure cannot be made because the pressure increases 59 very rapidly v/ith decrease in composition as shown in figure 5 . An independent method of determining the temperature at which the concentration of deuterium chloride becomes zero has been suggested by Nutting and Horsely (29). The use of a Cox chart with deuterium oxide as the reference liquid to plot the boiling points of the azeotrope of. deuterochloric acid indicates that the azeotrope disappears at approximately 200°C. The plot is shown in figure 10.. The assumption.of continued linearity of the azeotropic data on the Cox chart is justified by the fact that the azeotrope in becoming more dilute approaches more closely the properties of pure deuterium oxide. The vapor pressure data for liquid deuterium oxide were selected by Kirshenbaum (19) from the results of several independent investigations. Although the temperature at which the azeotrope disappears is only approximate, i t indicates that the relationship between boiling point and composition is not truly linear. The extrapolated value of the boiling point according to figure h is nearly double the value obtained on the Cox chart. The slope of the line shown in figure h thus tends to get less negative as the composition decreases. When i t is realised that the plotted data cover only one tenth of the composition range from zero concentration of deuterium chloride to the compositions investigated, the linearity of the plot does not appear unusual. D. Applications of the Data The determination of the activity coefficients of deuterium chloride from azeotropic data by.means of equations (19) and (20) is prevented by the non-existence of a vapor pressure of pure deuterium chloride at the temperatures of the determinations. These 60 temperatures were a l l higher than the critical temperature of hydro-gen chloride. Although the activity coefficient of deuterium oxide can be computed for each temperature at the respective composition of the azeotrope by this method, the determination from them of the activity coefficients of the deuterium chloride is s t i l l not possible. The Duhem equation of an integration of the Duhem equation which is normally used to relate activity coefficients was derived on the assumption of isothermal conditions. The single value of the activity coefficient of deuterium oxide at each temperature is, therefore, not sufficient to permit either the evaluation of the integration constants or to put to use a graphical method of integra-tion. Nevertheless, some of the thermodynamic relationships_given by Reddich and Schutg do provide a method of checking the consistency of the results from the determination of activity coefficients by other methods. Equation (11) and the coincidence of pressure and temperature relationships indicates that the molal heat of vaporiza-tion of deuterochloric acid is equal to the molal heat of valoriza-tion of that hydrochloric acid which is g reater in mole fraction, df the chloride by 10 percent. This statement is based on the assumption that the slopes of the curves of the logarithm of pressure vs reciprocal temperature are the same over the range investigated. Figure 8 supports this assumption. By calculating the latent heats of vaporization of deuterium oxide and deuterium chloride from the data of Kirshenbaum (19) and Lewis, Macdonald and Schutz (22) respectively, one can obtain from equation (ll) an equation relating > 61 the relative partial molal enthalpies of the two components. Since activities are simply related to these partial molal quantities, this equation would prove useful in substantiating the values of the form-er. Equation (12) , relating composition with temperature could also be utilised to check the value of 3 l n ! B T,P. 7^ by equating i t to equation (H5) . The partial molal volume, v, can be determined from the density measurements. It was remarked earlier that the correlations of Coulson and Herington were limited by their use of the Margules-Porter equation. A check can be made on the constancy of by calculating it at two temperatures using equation (23). The value of the activity coefficient of deuterium oxide may be calculated with the relation P= fAp° (53) obtained from equation (19) by assuming that the vapor is ideal. The vapor pressure of deuterium oxide at the two temperatures selected, 85.52, and 113.26°C was determined by a linear interpolation using the logarithm of vapor pressure and the reciprocal temperature. The data used was that selected by Kirshenbaum (19). The two values of WaD were found to be 18235 Gal/mole at 85.52°C. and 18279 CaC/mole at 113.26°C. This indicates, that the Margules-Porter integration.'is reliable for the determination of the activity coefficients of the deuterium oxide. The small difference in \1ao is shown to be S negligible by calculating the total pressure of 113.26°C. with the 62 value of Wab determined at 8£.52°C. The error introduced is only O.I4. mm. Nevertheless, the adequacy of the equation should not be accepted without a test using the available data on the chloride. This check may be accomplished by evaluating the vapor pressure of deuterium chloride with equations (20) and (23). The value of p° determined in this manner at 85.52°C. is 1.7£l(l(P-)mm. The value of p§ for hydrogen chloride found at the same temperature by extra-polating on a plot of logarithm of pressure vs reciprocal tempera-ture is 1.139(10^) mm. Since the extrapolation beyond the critical temperature (5l.U°C) is relatively short, the empirical value should be fairly reliable. The assumption that the vapor pressures of deuterium chloride and hydrogen chloride are the same at the same temperature introduces a negligible error. The substantial difference between the vapor pressures determined in these two ways casts serious doubt on the applicability of the Margules-Porter integration to deuterochloric acid. Since the relationship between mole fraction and percent composition is nearly linear over the short range of azeotropic compositions examined, the relationship between temperature and mole fraction is approximately linear. This can be shown by using a mean value in the denominator of equation (U5) relating temperature to mole fraction. Such an approximation introduces a maximum error of 0 . 6 percent in the slope of the line relating these two variables. This approximate linearity indicates from equation (2U) that the 63 factor -IL- In Pa/pS should be constant and independent of tempera-\'{ , A D ab ture. Furthermore, the constant in the empirical relationship between mole fraction and temperature is 0.2U8; whereas, the constant in the theoretical equation is 0.5>. Thus the empirical equation shows the inadequacy of the theoretical equation for this acid. Since the derivation of equation (30) relating pressure and tempera-ture utilises the relationship between temperature and mole fraction, the adequacy of the former is in doubt. This doubt must also be extended to the relationship between mole fraction and pressure. The azeotropic data on the acids as pointed out earlier does not conform to the last equation. 61* REFERENCES 1. Bineau, M.A., Ann. Chim. Phys., 68, 1*17, (1858) 2. Bonner, W.D., Branting, B.F., J . Am. Chem. Soc, 1*8, 3093,(1926) 3. Bonner, W.D., Titus, A.C., J . Am. Chem. Soc, 52, 633, (1930) h. Bonner, W.D., Wallace, R.E., J . Am. Chem. Soc, 52, 17^7, (1930) 5. Briggs, T.R., J . Phys. Chem., 28, 61*l*-50, (1929) 6. Brown, H.C., Groot, C.J. Am. Chem. Soc, 6U, 2223, (19U2) 7. Cadbury, W.E., J . Chem. Edn., 12, 292, (1933) 8. Carlson, H.C., Colburn, A.P., Ind. Eng. Chem., £, 581, (193U) 9. Cawood, W., Patterson, H.S., Trans. Faraday Soc, 29, 5Hl, (1933) 10. Coulson, E.A., Herington, E.F.G., Nature, 158, 198, (19U6) 11. Duhem, P., Ann. a L'Ecole Normale Sup.", 3, U-9, (1887) 12. Elliot, C, "Distillation Principles", Ernst Ben Ltd., London, (1925) 13. Foulk, C.W., Hollingsworth, M., J . Am. Chem. Soc, 1*5, 1221*, "(1923) l l * . Glasstone, S., "Textbook of Physical Chemistry", D. Van Nostrand ! Co., of New York (19h6) 15. Gugganheim, E.A., Fowler, R.H., "Statistical Thermodynamics", ' MacMillan Co., New York (1939) 16. Hendrixon, W.S., J . Am. Chem. Soc, 37, 2352, (1915) 17. Hulett, G.A., Bonner, W.D., J . Am. Chem. Soc, 31, 390, (1909) 18. Kireev, V.A., Acat Physicochem., U.S.S.R., l l * , 371-86, (19l*l) 19. - Kirshenbaum, I., AECD-1991, United States Atomic Energy Commission, (19U3) 20. Konowaloff, D.P., Ann. Phys., l l * , 3h, (l88l) 21 . Lecat, M., Compt. Rend., 189, 990-2, (1929) 22. Lewis, G.N., Macdonald, R.T., Schutz, P.W., J . Am. Chem. Soc, 56, 1*9U, (193U) 65 23. Licht, W.J., Denzler, C.G., Chem. Eng. Progress, 1+1+, 8, 627, ~ (191+8) 2k. Margules, M. Lehfeldt, R.A., Sitzungsber der Y/iener Akad., 10k, 121*3, (1895) 25. Mendeleef, D.I., Gutkowski, E.K., J. Russ. Phys. Chem. Soc, 8, 212, (1876) 26. Merriman, R.W., J. Chem. Soc, 103, 1798, (1901) 27. Merriman, R.W., Wade, J., J. Chem. Soc, 101, 997, (1911) 28. Morey, J., J. Am. Chem. Soc, 3k, 1027, (1912) 29. Nutting, H.S., Horseley, L.H., Ind. Eng. Chem., 19, 602, (191+7) 30. Porter, N., Trans. Farad. Soc, 16, 336, (1920) 31. Reddich, 0 . , Schutz, P.W., J. Am. Chem. Soc, 66, 1007-11, (19kk) 32. Rosanoff, P., J. Am. Chem. Soc, 36, 2, 1993, (19U+) 33. Roscoe, H.E., Dittmar, W.D., J. Chem. Soc, 12, 136, (i860) 3k. Randall, M., Young, L.E., J. Am. Chem. Soc, 5p_, 989, (1928) 35. Shaw, J.A., Ind. Eng. Chem. 18, 1065, (1926) 36. Simmons, L.M., J. Proc. Roy. Coc, N.S. Wales, 80, 196-202, (191+7) 37. Stokland, K., Ronaess, E., Tronstad, L., Trans. Farad. Soc. 35, 312, (1939) "~ 38. Vogel, A.I., "A Textbook of Quantitative Inorganic Analysis", Longmans, Green and. Co., London, (191+3) 39. Yfeissberger, A. (Editor) , "Physical Methods of Organic Chemistry" Vol. I, Part I, Interscience Publishers Inc. New York(19U5) kO. Wrewsky, M.S., Z. Physik. Chem., §3, 551,(1913) and 37, 283,(1932) 1+1. Young, S., "Distillation Principles and Processes"', MacMillan Co. Ltd., London, (1922) 1+2. Zavaritzkii, N., J. Russ. Phys. Chem. Soc, 56, 367-71, (1925) Fig. h. T E M P E R A T U R E (C) 75 8 0 9 0 100 110 120 Fig. 5. P R E S S U R E ImmHg) 0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 Fig.6. Relationship between density and^percent composition.. 109 2 I-094 1096 1098 1100 1102 1104 ± 1 0 6 1186 1188 I 190 1192 1194 1196 1198 1*2 0 0 Fig. 7. Difference between observed and calculated boiling Mg. V Relationship between Logarithm of Pressure and Reciprocal Boiling Point Fig. 10. Temperature and pressure at which the azeotrope disappears. o o o 76 APPENDIX II TABLE I Azeotropic Data for Hydrochloric Acid^ Pressure, mm Boiling Pt., °C Density Composition Mole fraction HCl percent HCl 50 U8.72U 1.1118 0.131263 23. k2 150 69.956 1.1073 0.125570 22.520a 250 81.205 1.10U2 0.121576 21.883 370 90 . 237 1.1016a 0.1183U9 21.365a Uoo 92.080 1.1010 21.235a 95.029 1.1002 0.116551 21.075a 500 97.578 1.0993 20.916 5Uo 99.653 1.0987 20.777a 600 102.209 I.O98O 20 . 6 3 8 a 6U0 103.967 1.0973 0.1130U6 20.507 680 105.56U 1.0 968 a 20.U13 700 106.U2U I .0966 20.360 a 7U0 107.859 1.0962a 0.111578 20.268 760 108.58U 1.0959 20.222 800 IIO.007 1.0955 0.110885 20.155 1000 116.185 1.0933 0.106025 19.73U 1220 112.98a 1.0915 19.358 Interpolated From the data given by Bonner and Wallace (U) 77 APPENDIX II TABLE II Converted data for figures 8 and 10 Boiling Pt., °C 76.83 85.52 91.97 97.76 102.07 106.18 108.52 109.89 113.26 116.12 Boiling Pt., °K 3h9.99 358.68 365.13 370.92 375.23 379.3k 381.68 383.05 386.U2 389.28 1000/T°K 2.85722 2.78800 2.73875 2.69600 2.66503 2.63616 2.62000 2.61063 2.58786 2.56885 Pressure mm 202.5 301.1 397.5 503.2 598.0 698.9 760 . 5 8OO .7 902.7 998. U Log Pressure 2.306U3 2.U7871 2.5993U 2.7017U 2.77670 2 . 81M2 2.88110 2.9031+7 2.9555U 2.99930 78 APPENDIX III Calculations 1. Composition (a) Calculation of the chemical factors Percent DCl = 37.U72-a * 1 0 0 = 26.lU28a/b 1U3.337 b where a = gms of AgCl and b = gms of acid Percent HCl = 2?.Ul02a'/b' (b) Calculation of percent composition Data taken'from sample entry DATA Room temperature 27.0°C. Empty weight Out of dessicator 13:55 hrs weight empty - weight of tare (Nominal weights) 1, 0.050, 0.020a, 0.020b, 0.010, 0.005, 0.002a 0.002b, 0.001 Rest point 10.68 at 1U:22 hrs Sample weight weight with approximately 5 ml deuterochloric acid - weight of tare 1, 0.500, 0.050, 0.020a, 0.020b, 0.005 Rest point 8.35 at lU:29 hrs Precipitate weight Out of dessicator 2k:25 hrs weight with precipitate - weight of tare 1, 0.200, 0.100a, 0.100b, 0.020a, 0.010, 0.005, 0.002a, 0.002b Rest point .13.01 at 2lu52 hrs. 79 CALCULATION Apparent weight of sample 0.^ 00 - 0.010 - 0.002a - 0.002b - 0.001 = 0.1+8351+ gms (see Appendix V, Table I) Rest point correction -2.33(0.00016) = -0.00037 Apparent weight = 0.1+8317 gms. Apparent weight of precipitate Wa 0.200 + 0.100a + 0.100b - 0.050 - 0.020b - 0.001 = 0.32797 fems Rest point correction + 2.33(0.00016) = 0.00037 Apparent weight = O.3283I+ gms Reduction to vacuo of weight of deuterochloric acid From Table XXIV p 212 Vogel's Quantitative Analysis (38) d = 1.19 (for deuterochloric acid) k = 0.865 where Wv = Wa + i^a 1000 Wv = 0.U8317 * 0.865(0.0001+8) = 0.1+3359 gms Reduction to- vacuo of weight of silver chloride d = 5.56 k = 0.08 Wv = O.3283I+ + 0.08(0.00033) = 0.32837 gms Loss of precipitate by washings - gain in precipitate weight due to impurities = + 0.00001 gms Percent DCl = 0-3.2838(0.2611+28)100 0.1+8359 = 17.752 percent. 80 2. Density-Data taken from sample entry DATA ' Room temperature 27.0°C. Determination Weight of empty pycnometer - weight of the tare (nominal weights, see Appendix V) 0.100a, 0.020a, 0.010, 0.005, 0.001. Rest Point 8.1* , Weight f i l l e d with acid - weight of the tare 20, 10a, 10b, 2a, 0.500, 0.200, 0.100, 0.020a, 0.005, 0.002a, Rest point 12.7 acid level in capillaries left level 0.1200 right level -0.0025 Calibration Weight of empty pycnometer - weight of the tare 0.100a, 0.020a, 0.010 0.005, 0.001 Rest point 8.9 Weight f i l l e d with water - weight of the tare 20, 30a, 5, 0.500, 0.200, 0.100a, 0.020a, 0.020b, 0.002a, 0.001 Rest point 8.1* water level in capillaries left level 0.115.0 ^ right level -0.0030 CALCULATION Determination Apparent weight of acid 20, + 10a, + 10b, + 2a, + 0.500, + 0.200, + 0.002a, -0.0300 -0.001 = 1*2.57527 gms Rest point correction + 1*.3(0.00026) = +0.00112 Apparent weight = 1*2.57639 gms Calibration Apparent weight of water 20 -»- 10a, + 5 + 0.500 + 0.200 + 0.020b - 0.010 - 0.001 = 35.61003 Rest point correction - 0 . 5 ( 0 . 0 0 0 2 6 ) = -0 .00013 Apparent weight = 35.60990 Volume difference correction - ( 0 . 0 0 5 0 +• 0 .0005)0.9971 = -0.005U8 Apparent weight of same volume of water as acid = 3$.&Okh2 gms Reduction to vacuo of weight of acid Volume of liquid x density of air = 35*60(1.2) 10 Reduction to vacuo of weight and water Volume of liquid x density of air = 1+2.9(10"3) 0.9971 = U2.57 (1.2) 10-3 8 . 5 Net correction for vacuo -+-O.O3688 gms Vacuum weight of acid •? 1+2.61327 gms Volume of weights x density of air = 35.61(1.2)10-3 8 .5 = 5.03(10-3) gms Net correction for vacuo = +O.O3787 Vacuum weight of water = 35.61+229 gms dp^ = 1+2.61327(0.997075) - = -1.19209 u 35.61+229 Correlation of boiling point and composition The form of the equation desired is y = a -+• bx. 82 For the sum of the squares of the residuals to be a minimum na -f- b^x - y, and (2) aZx+ bZx 2 = 21 xy. (3) Let x = c - 18, (It) and y = t - 100, where c = percent DCl in azeotrope and t = boiling point, degrees C. The data for the solution of the constants in equation (l) by the simultaneous solution of equations (2) and (3) is given in the following table: TABLE I Determination of constants t c y X X 2 - xy 76.83 19.661 -23.17 1.661 2.758921 -38,1*8537 85.52 19.090 -1U.U8 1.090 1.188100 -15.78320 91.97 18.667 -8.03 0.667 O.UUU889 - 5.35601 97.76 18.298 -2.2U 0.298 0.088801* - 0.66752 102.07 18.01U 2.07 0 . o i l ; 0.000196 + 0.02898 106.18 17.752 6.18 -0.2U8 0.06150U . - 1.53261* 108.52 17.595 8.52 -0.1*05 0.161*025 - 3.1+5060 113.26 17.286 13.26 -0.71U 0.509796 - 9.U6761* Sums -17.89 +2.363 5.216235 ' -7U.71UOO By substituting the values, in the table of /Ly, Xx,5x^, and^xy in equations (2) and (3) we obtain the following equations: 8a +- 2.363B 17.89 = 0 (5) and 2.363a+ 5.216235b + 7U.71UO =0 (6) From equations (5) and (6) the values of a and b are found to be as 83 follows: a = 2.30262 b = -15.3661*6 From the values of the constants and the relationships given in equation (ii) i t can be shown that c = 378.8989 - t ( J ) 15.3665 By rewriting the equation so that i t is explicit in y, the boiling point deviations can be calculated with greater facility. The calcu-lation is done in tabular form in the following table: TABLE Ii:: Calculation of mean deviation ^obs bx y calc A t -23.17 -25.5237 -23.22 +-0.05 -0.003 -1U.U8 -16.7U9U -1U.U5 -0.03 +0.002 - 8.O3 • -10.2U9U - 7.95 -0.08 +0.005 - 2.2U - k.5792 - 2.28 + 0.01* -0.002 2.07 - 0.2151 2.09 -0.02 + 0.001 6.18 +-3.810 9 6.11 + 0.07 -0.005 8.52 +-6.223U 8.53 -0.01 0.000 13.26 +•10.9717 13.27 -0.01 0.000 o.ol* 0.0025 The value for A c was calculated from the slope of equation mean deviations are given at the foot of the deviation columns. 1*. Relation between mole fraction and percent composition Let y^ gms DCl/gm acid = c/100 The 8U where c = percent DCl in the azeotrope, and let x = moles DCl/mole acid. Then x = y/MDci y/M D C 1 -»- (1 - y) /MD20 - 2Q.028Uy • " 37.U72 - 17.UU36y . _ 0.20028Uc 37.U72 - 0.17UU36c J>. Calculation of mean pressure and its deviation from the , pressure data ^ Table III gives the barometric and differential pressures taken from figure 9, Appendix I, and the total pressures and deviations from the mean. TABLE III Time, minutes Barometer, mm Manometer, mm Net pressure, mm ' Deviation, mm 0 755.55 k.93 760. U8 -0.01 6 755.U8 5.0U 760.52 -0.03 12 755.U3 U.90 760.33 -0.16 18 755.38 5.33 760.71 +0.22 2k 755.32 k.Qk 760.16 -0.33 30 755.26 5.68 760.9k +0.U5 36 755.20 5.io 760.30 -0.19 Mean values 760. k9 ±0.19 85 APPENDIX IV TABLE I Barometric corrections to be added to compensate for differences in;..altitude of floor 2 and floor 1 of the Chemistry B uilding, University of British Columbia^ (Corrections in mm) Barometric pressure Air Column Temperature mm 8°C. 12°C. 16°C. 20°C. 2U°C 28°C. 780 o . U l o . U l o.Uo o.Uo 0.39 0«39 760 o.Uo o . l p 0.39 0.38 0.38 0.37 7lP o.39 0.39 0.38 0.37 • 0.37 0.36 Data calculated from corrections given on pp 2081 to 2083 Handbook of Physics and Chemistry, 32nd Edition, Editor, D. Hodgman, Chemical Rubber Publishing Co., Cleveland, Ohio, (1950) Altitude of Floor 1 115.8 meters above mean sea level. Altitude of Floor 2 120.1 meters above mean sea level. APPENDIX V . TABLE I Calibration«f weights^ • Nominal weight • Calibrated weight • gms gms 20 19.9U590 10 a 9.97287 10 b 9.97297 5 U.986U3 2a 1.99U57 2b 1.99U62 1 0.99729 0.500 0.U9859 (continued) 86 TABLE I (cont'd) Nominal weight Calibrated weight gms gms 0.200 0.199U3 0.100a 0.09969 0.100b 0.0997U 0.050 O.0U988 0.020a 0.01995 0.020b 0.01995 0.010 0.01000 0.005 0 . 0 0 5 m 0.002a 0.00200 0.002b 0.00199 0.001 0.00106 The calibration assumes that the weight of the 0.010 gm weight i s correct. 87 APPENDIX VI TABLE I Notation a^ = activity of component A az = azeotrope A - Van Laar constant A' = constant i n equation, x = f(P) c = percent composition C = number of components C = constant i n equation, xr f(P) dj^ > = density, gms per ml, at 25>°C. referred to water at U°C. D - constant i n equation lnP= f (l/T) F Z number of degrees of freedom f A = activity coefficient of component A F r free energy of formation of pure components A F 1 = free energy of mixing of pure components excess free energy of solution oyer that of ideal H1 = excess heat content over that of ideal A H ; molal heat of vaporization of solution z P H ° - molal heat of vaporization of pure component K = constant i n the equation InP f (hj K' = constant i n equation F E = f (x) P ^ = part i a l vapor pressure of component A PA = vapor pressure of pure component P - total pressure and number of phases X 88 r resistance, ohms R gas constant S specific gravity T temperature, degrees Kelvin t temperature, degrees Centigrade molal volume, defined by relationship VB " VA z RT P v^ a partial molal volume of component A in the liquid phase TF A = partial molal volume of component A in the gas phase Wab = e n e r g y required for mixing pure components x^ = Mole fraction of component A. = constant in the integrated form of the Margules equation
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The boiling points, compositions, and densities of the azeotropes of deuterochloric acid
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