Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Studies in acetonitrile solutions. I. The calomel reference electrode. II. The polarographic behaviour… Jayadevappa, Ettigi Sivappa 1955

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1955_A6_7 J2 S9_1.pdf [ 5.59MB ]
Metadata
JSON: 831-1.0062449.json
JSON-LD: 831-1.0062449-ld.json
RDF/XML (Pretty): 831-1.0062449-rdf.xml
RDF/JSON: 831-1.0062449-rdf.json
Turtle: 831-1.0062449-turtle.txt
N-Triples: 831-1.0062449-rdf-ntriples.txt
Original Record: 831-1.0062449-source.json
Full Text
831-1.0062449-fulltext.txt
Citation
831-1.0062449.ris

Full Text

STUDIES IN ACETONITRILE SOLUTIONS .1. THE CALOMEL REFERENCE ELECTRODE II. THE POLAROGRAPHIC BEHAVIOUR OF SOME ORGANIC COMPOUNDS ETTIGI SIVAPPA JAYADEVAPPA A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Chemistry We accept this thesis as conforming to the standard required from candidates for the degree of MASTER OF SCIENCE Members of the Department of Chemistry THE UNIVERSITY CF BRITISH COLUMBIA October, 1955 ABSTRACT The possibility of using a calomel electrode as a reference standard in the polarographic comparisons in acetonitrile has been studied. The results indicate that the time required for attaining a constant potential i s too long to be convenient for such use. The behaviour of some organic nitrocompounds at the dropping mercury electrode in acetonitrile con-taining 0.1 molar tetrabutylammonium iodide has been studied. The nitroanilines and the nitrophenols give distinct double waves on polarographic reduction. There is no formation of any maxima in the case of the nitro-anilines, whereas, maxima are found to develop with time in the case of the nitrophenols. The maxima are found to be non-suppressible. The ease of reduction of the nitrophenols and the nitroanilines, as given by their half-wave potentials, is in agreement with the order expected on the basis of the previously established theories of Shikata and Astle. A possible mechanism of the reduction i s given with the ir r e v e r s i b i l i t y of the reduction in view. ACKNOWLEDGMENT The author wishes to express his gratitude to Dr. H.M. Daggett, Jr. for his invaluable assistance and encouragement throughout the work and to the National Research Council of Canada for financial assistance and equipment. The author also wishes to thank the Standard Oil Company of B r i t i s h Columbia for the fellowship awarded to him. TABLE OF CONTENTS Page PART I. THE CALOMEL REFERENCE ELECTRODE INTRODUCTION 1 General 1 Historical 4 EXPERIMENTAL 7 Materials 7 Apparatus 8 Procedure and results 9 DISCUSSION 13 PART II. THE POLAROGRAPHIC BEHAVIOUR OF SOME ORGANIC COMPOUNDS INTRODUCTION 15 General 15 Theoretical - General Concepts 17 Ilkovic Equation 19 Factors governing diffusion current 21 Residual current 25 Maxima and suppression 28 Historical 32 EXPERIMENTAL 35 Apparatus and Materials 35 Procedure - (i) Characterisation of capillary . . 41 ( i i ) Electrocapillary Curve 42 ( i i i ) Measurement of polarographic wave 43 Page RESULTS 48 (i) Characterisation of capillary 48 ( i i ) Electrocapillary Curve 48 ( i i i ) Residual current 50 (iv) Sodium Iodide 51 (v) o-nitrophenol 55 (vi) m-nitrophenol 62 (vii ) p-nitrophenol 69 ( v i i i ) o-nitroaniline 75 (ix) p-nitroaniline 78 (x) m-nitroaniline 81 DISCUSSION 85 BIBLIOGRAPHY 93 LIST OF TABLES PART I. Page Table I. Variation of potential with time 11 II. Variation of potentials in a solution saturated with calomel 12 PART II. Table I. Characterisation of the capillary 49 II. Effect of potential on m ^ t 1 / 6 50 III. Residual current 51 IV. Measurements with sodium iodide solution . . 53 V. Reduction of o-nitrophenol 57 VI. o-nitrophenol (after 36 hours' standing) . . 59 VII. Reduction of m-nitrophenol solution . . . . 65 VIII. m-nitrophenol (after standing 24 hours) . . 66 IX. Reduction of p-nitrophenol 69 X. p-nitrophenol (after 24 hours ' standing) . . 71 XI. Reduction of o-nitroaniline 78 XII. Reduction of p-nitroaniline 79 XIII. Reduction of m-nitroaniline 84 ILLUSTRATIONS Figure Page 1. Electrocapillary curve of mercury in acetonitrile 44 2. Residual current curve 46 3. Polarographic reduction wave of sodium ion (2.974 millimolar) in acetonitrile . . . . 52 4. Logarithmic plot of V i ^ - i vs. potential for the sodium ion 54 5. Polarographic wave of o-nitrophenol (1.018 millimolar) in acetonitrile immediately after solution 56 6. Logarithmic plot corresponding to Figure 5. 58 7. Polarographic wave of o-nitrophenol (1.018 millimolar) in acetonitrile after standing for 36 hours 60 8. Logarithmic plot corresponding to Figure 7 . 61 9. Polarographic wave of m-nitrophenol (1.503 millimolar) in acetonitrile, immediately after solution 63 10. Logarithmic plot corresponding to Figure 9. 64 11. Polarographic wave of m-nitrophenol (1.503 millimolar) in acetonitrile after standing for 24 hours 67 12. Logarithmic plot corresponding to Figure 11 68 Figure Page 13. Polarographic wave of p-nitrophenol (3.1'97 millimolar) in acetonitrile, immediately after solution 70 14. Polarographic wave of p-nitrophenol (3.197 millimolar) in acetonitrile, after standing for 24 hours 72 15. Logarithmic plot corresponding to Figure 14. . .74 16. Polarographic wave of o-nitroaniline (1.084 millimolar) in acetonitrile 76 17. Logarithmic plot corresponding to Figure 16. . .77 18. Polarographic wave of p-nitroaniline (2.1 millimolar) in acetonitrile 80 19. Logarithmic plot corresponding to Figure 18. . . 82 20. Polarographic wave of m-nitroaniline (1.062 millimolar) in acetonitrile 83 21. Logarithmic plot corresponding to Figure 20. . .85 • • • • STUDIES IN ACETONITRILE SOLUTIONS I. THE CALOMEL REFERENCE ELECTRODE Introduction In the measurement of the electromotive force of electrochemical c e l l s , the ultimate reference standard has been the hydrogen electrode for a very long time. The use of the standard hydrogen electrode is not always convenient, and hence several subsidiary reference electrodes have been tested and accepted for comparison. There have been several of them in use; the most extensively used being the calomel electrode and the si l v e r - s i l v e r chloride electrode. The latter has been so widely used that i t has acquired consid-erable individual importance as a reference standard (16). However, a l l these electrodes have been tested and their a b i l i t y and r e l i a b i l i t y established only in aqueous solutions. There has been comparatively l i t t l e work done in developing reference electrodes for use in non-aqueous sol-vents. The use of the aqueous hydrogen gas electrode and the other aqueous reference electrodes is not feasible in comparing the potentials in non-aqueous media because of the uncertain liquid-liquid junction potentials involved. The - 2 -evaluation of the latter is not easy. Moreover, where the system is to be absolutely anhydrous, i t i s not desirable to use any such electrodes. The need for reliable reference electrodes has long been f e l t and attempts have been made recently to study them. The c r i t e r i a for a good reference electrode are (1) : i ) The electrode must attain equilibrium quickly and show a stable standard potential for an indefinite length , of time. i i ) The potential must not be altered by the pas-sage of small currents of the order of 50-60 pa. for an hour or more. i i i ) The electrode must be reproducible. Comparison of a newly prepared electrode and an aged one must show no considerable difference. iv) The electrode must be non-polarisable. The relation between current and applied voltage must be a straight line function over a range of zero to two volts, i f i t is -to be of use in polarography. The potential range in the last condition i s the normal polarographic range and for polarographic comparisons, i t is necessary that the electrodes be non-polarisable in that range of potentials. The need for electrodes of this kind in polarography i s great. The mercury pool reference that is often used in the absence of reliable reference electrodes has been found unsatisfactory in several solvents (1,17,21). - 3 -The present attempt is to investigate the possi-b i l i t y of using a calomel electrode in acetonitrile solutions for polarographic work. - 4 -Hist orical Electrolytic studies in non-aqueous media have been rather limited. With the development of organic polarography, the investigations in pure non-aqueous media have increased considerably. But, the work on the stab-i l i t y of electrodes in non-aqueous media i t s e l f has not received too much attention. A survey of the literature shows that the use of the sil v e r - s i l v e r chloride electrode in anhydrous methanol was f i r s t investigated by Nonhebel and Hartley (34), who found that the electrode exhibited a stable and reproducible potential. Woolcock and Hartley (48) showed that the same electrode exhibited good st a b i l i t y and reproducibility in anhydrous ethanol also. Yoshida (49) showed that the e.m.f.»s of the c e l l , Cd (amalgam) Cdl 9 (Solid & Saturated) z Solution ) Hg 0I 2"2 Hg had a constant value in methanol, ethanol, propyl alcohol, and acetone and were almost independent of the solvent. Measurements made in liquid sulphur dioxide by Cruse (6) demonstrated the r e l i a b i l i t y of s i l v e r - s i l v e r chloride electrodes as reference electrodes in that solvent; but, he found that the e.m.f. of the mercury-mercurous chloride electrode drifted with time. Scherer and Newton (40) in-vestigated the use of the magnesium electrode in a saturated solution of magnesium bromide in ether and found that there - 5 -was a gradual increase or decrease in the potential while the equilibrium was being attained, followed by a period of constancy lasting from 2 - 7 days; thereafter, i t again showed some irregularities. E l l i o t t and Yost (9) measured the c e l l , Tl (amalgam) T1C1 ZnCl2.10 NH3(s) Zn (amalgam) in liquid ammonia and showed that either h a l f - c e l l could be used as a reference standard in that solvent. The chloranil electrode has also been used as a reference standard by sev-eral workers. Heston and Hall (13) have used i t in glacial acetic acid in the course of the determination of the act-i v i t y of hydrogen chloride in glacial acetic acid. They observed some irregularities in that medium, which they blamed orr possible side reactions. But, several later workers, principally, Swan and Edelman (43), Conant and co-workers (5), and Bergman and James (4), have found i t quite satisfactory in the same solvent. More recently, Arthur and Lyons (1) have studied two reference electrodes in acetone—the acetone calomel electrode (A.C.E. ) and the acetone-saturated calomel electrode (A.S.C.E.) for use in polarographic comparisons. They found that both of them were stable, reproducible and non-polarisable. In acetonitrile i t s e l f , studies of several d i f f e r -ent cells have been made by Uhlich and Spiegel (44), the principal ones being the sil v e r - s i l v e r halide cells and the mercury-mercurous halide c e l l s . They found that, even though the solubility of silver chloride and mercurous chloride in 6 pure acetonitrile was negligible, this was not the case in a solution of lithium chloride in acetonitrile. They as-cribed this to either complex formation between the metal ion and the solvent or the solvation of the metal ion. Later, Cruse, Goertz and Petermuller (7) showed that com-plex formation alone was not sufficient reason for the failure of the electrode, and that the stability of the complex determines the st a b i l i t y of the electrode. This was shown by the improvement of the potentials in the case of mercurous chloride, mercurous hromide and mercurous iodide with the increasing s t a b i l i t y of the complex formed. But, calculations based on complex formation alone did not give potentials in agreement with the observed values, thus in-dicating partial solvation effects. The latter, however, x^ ere found to be least in the case of the calomel h a l f - c e l l . It is our purpose to see i f the calomel half c e l l shows a f a i r l y stable potential, even though i t may be d i f -ferent from the true thermodynamic potential and hence as-certain i f i t may not be suitable for use as a standard polarographic reference electrode. - 7 -Experimental Materials The solvent used was acetonitrile supplied by the Chemical Division of the United States Vanadium Corporation. It was purified before use by the method of Wawzonek and Runner (47), slightly modified. The solvent was kept in a stoppered container with anhydrous potassium carbonate (B.D.H. chemical) for several hours. It was fi l t e r e d through a thick plug of glass wool into a d i s t i l l i n g flask and a few crystals of silver nitrate were added to i t . It was dis-t i l l e d using a four feet long unpacked column. The f i r s t 50 ml of the d i s t i l l a t e were discarded and the d i s t i l l a t e boiling at 81.6°C was collected directly in a solvent bottle. A drying tube containing Drierite was always used to keep the d i s t i l l a t e out of contact with external moisture. The lithium chloride used was the Baker and Adam-son reagent grade. The requisite quantity of i t to make a 0.01 molar solution in acetonitrile was weighed out into a bottle containing the purified solvent, tightly stoppered and shaken in a mechanical agitator for several hours. This 0.01 molar solution was used as the electrolyte solution. The calomel used was of reagent grade supplied by the Merck and Co. Throughout the study the same sample was used. A paste of the calomel was made with t r i p l y d i s t i l l e d mercury in an agate mortar and pestle together with a small - 8 -quantity of the pure acetonitrile. A tiny spoon made of platinum was used to handle this paste in preparing the calomel electrodes. Apparatus The electrode vessel consisted of a 500 ml. cap-acity f l a t bottomed flask provided with a central neck with a ground glass joint of size ^ 24/40 and six other necks with ground glass joints, each of size $ 19/38 arranged at the periphery. The central neck was fi t t e d with a sintered glass bubbler projecting into the centre of the vessel, through which nitrogen gas could be bubbled. The other necks at the periphery were designed for the electrodes. The electrodes were cylindrical glass tubes, about 18 cms. in length f i t t e d with ground glass joints to f i t into the necks of the electrode vessel. A short piece of platinum wire about 3 cms. in length was fused into the other end, so as to project partly out of the tube into a small cup attached at that end. The cup carried a small hole in i t to bring i t into contact with the external solution. E l e c t r i c a l connections were made by means of mercury in contact with the platinum wire fused into the tube. The potential measuring equipment consisted of a Leeds and Northrup student type potentiometer. A mirror - 9 -type galvanometer supplied by the same company, Type 2420 C having a sensitivity of 0.025 microampere per mm. division was used as the null point detector. Comparisons, could be made between the different electrodes by a switching ar-rangement. Procedure The calomel electrodes were prepared by placing a few mis. of t r i p l y d i s t i l l e d mercury in the electrode cups and covering i t with a paste of mercurous chloride with mercury prepared as described before. The paste was handled by means of a tiny platinum spoon throughout. The electrode was then introduced into the electrode vessel containing the electrolyte solution. The electrolyte solution was a 0.01 molar solution of lithium chloride in acetonitrile. The electrode vessel containing this solution was kept immersed in a constant temperature bath, whose temperature was maintained at 25 - 0.1°C. Dry nitrogen gas was passed through the bubbler for about half an hour before the newly prepared electrodes were introduced into i t . No special precautions for purify-ing the nitrogen were taken. After the gas has been passed through the solution for about half an hour, some t r i p l y d i s t i l l e d mercury was poured into the vessel to form a pool at the bottom. The pool could be used as an electrode by dipping into i t a platinum electrode, similar in form to the other electrodes. - 10 -Measurements were made with electrodes ( T ) and (2) prepared as above. A straight platinum electrode (4) was taken to serve as a reference electrode. It consisted of a cylindrical glass tube, about the same length as the calomel electrodes, f i t t e d with a ground glass joint at one end and a short length of platinum wire fused into the other. A platinum f o i l , about 4.5 sq. cms. in area, was attached to the platinum wire. The platinum wire was previously cleaned thoroughly by electrolysing in a dilute hydrochloric acid solution and washing i t thoroughly with d i s t i l l e d water. The potentials were measured between electrodes ( l ) and (4), (2) and (4), and ( T ) and @ at different intervals of time. The i n i t i a l measurements showed a difference of 0.6 - 0.7 m i l l i v o l t between the calomels (l) and (2) whereas the platinum electrode was found to be about 16 - 17 millivolts m o r e ; negative with respect to the calomel electrodes. There was found to be a rather haphazard variation with time but the calomels showed a tendency to attain the same potential. At the end of 24 hours, ( T ) and (2) differed by 0.2 m i l l i v o l t . A new calomel electrode (3) was then prepared in the same manner as before and i t s potential was compared with the potentials of the two aged ones. The new one was found to be slightly more negative than the aged ones. Measurements made at different intervals of time showed again an irregular variation of the potential. At the - 11 -end of 40 hours they were found to settle down to a constant potential and were uniformly 1.4 millivolts more positive than the platinum electrode. There was observed no change in the potential for several hours after this was reached. The results are shown in Table I. (a) (Electrodes prepared at the same time) Time (Hours) Measured potentials (volts) © + vs. ® © + vs. © © + vs. 0.5 0.0009 0.0175 0.0166 1.0 0.0006 0.0046 0.0040 2.0 0.0012 0.0043 0.0031 3.5 0.0010 0.0042 0.0032 4.5 0.0007 0.0046 0.0039 5.5 0.0003 0.0042 0.0039 6.5 0.0002 0.0042 0.0040 24.0 0.0002 0.0026 0.0024 (b) After the introduction of the newly prepared calomel (3), the potentials were as follows: Time (Hours) Measured potentials (volts) %?• ®dr ®dr %vs-1.0 2.0 10.0 15.0 16.0 33.0 34.0 35.0 0.0005 0.0005 0.0004 0.0002 0.0002 0.0001 0.0000 0.0000 0.0026 0.0025 0.0020 0.0018 0.0015 0.0016 0.0014 0.0014 0.0002 0.0002 0.0002 0.0005 0.0009 0.0002 0.0001 0.0000 0.0028 0.0027 0.0022 0.0023 0.0024 0.0017 0.0014 0.0014 0.0007 0.0008 0.0006 0.0007 0.0011 0.0001 0.0001 0.0000 0.0021 0.0020 0.0016 0.0016 0.0013 0.0018 0.0015 0.0014 Note; + sign indicates the electrode found positive to the other. - 12 -A set of new calomel electrodes was prepared in the same manner as described above to examine their behaviour in a solution previously saturated with calomel. Calomel was thrown on top of the mercury pool to serve as a larger calomel electrode. An electrode, with a short length of electrode and measurements of the potentials of the small (6J were made as before. The results showed that there was a more systematic variation of the potential with a tendency for equalisation. There was, at f i r s t , a fast decay in the potentials and later a slower decay. After about 20 hours, the potentials were a l l within a milli v o l t of each other. The results in Table II show the tendency of variation. Table II. Variation of potentials in a solution saturated platinum wire fused'i nto i t was thrust into this larger (2) and (3) against the larger calomel electrode with calomel Time(Hours) Measured potentials (volts) 7.0 8.5 17.5 18.5 20.0 22.5 0.0011 0.0013 0.0037 0.0002 0.0026 0.0009 0.0014 0.0030 0.0005 0.0021 0.0003 0.0002 0.0009 0.0001 0.0006 0.0001 0.0000 0.0007 0.0000 0.0006 0.0001 0.0001 0.0007 0.0000 0.0006 0.0001 0.0001 0.0006 0.0000 0.0005 0.0024 0.0016 0.0007 0.0007 0.0006 0.0005 - 13 -Discussion The work of Cruse, Goertz and Petermtlller (7) has d e f i n i t e l y shown that i n the case of calomel i n acetonit-r i l e , complex formation i s the most important f a c t o r . I t i s also clear that solvation does have some e f f e c t , however small in comparison. The two factors operate side by side and the e f f e c t of the complex formation i s to a greater or smaller extent counteracted by the e f f e c t of solvation. The complex formation tends to disturb the d i s -sociation equilibrium of mercurous chloride i n the d i r e c t i o n of the complex forming substance as indicated by the follow-ing equations: HgCl + CI" —• HgCl 2" (complex formation step) HgCl 2" ^ Hg + + 2 CI"" This goes on u n t i l the saturation concentration of the com-plex forming substance i s reached. There i s thus a v a r i a t i o n of the mercurous ion concentration causing the v a r i a t i o n of pote n t i a l u n t i l the saturation concentration of the complex i s reached, when there i s an equilibrium i n the mercurous ion concentration; the electrode then shows a constant poten-t i a l . The i n i t i a l v a r i a t i o n of the potential and ultim-ate equalisation observed i n the re s u l t s i s , therefore, not - 14 -s u r p r i s i n g . The constant value of the potential i s s t i l l not the r i g h t thermodynamic potential calculated on the basis of complex formation alone, as a r e s u l t of the accom-panying solvation e f f e c t . In considering the f e a s i b i l i t y of the use of the calomel electrode as a reference electrode in a c e t o n i t r i l e , on the basis of the constancy i t attains with time, one has to take into account the fact that upwards of 50 hours are required f o r a t t a i n i n g that state. As a consequence, i t must be concluded that the use of the calomel electrode in a c e t o n i t r i l e i s far from s a t i s f a c t o r y . Accordingly, i n the polarographic studies that followed, i t was decided to use the mercury pool as the anode, a f t e r the work of Wawzonek and Runner (47). - 15 -STUDIES IN ACETONITRILE SOLUTIONS II. POLAROGRAPHIC BEHAVIOUR OF SOME ORGANIC NITRO-COMPOUNDS Introduction Polarographic investigations for a long time were limited to substances which are soluble in water and do not react with i t . The large group of organic compounds, which do not belong to this class could not therefore be studied in the aqueous medium. However, the use of organic solvents as polarographic media has brought a large number of such compounds within the scope of polarographic inves-tigations. Several factors deterred the early investigators from using non-aqueous solvents as polarographic media. In general, most of the non-aqueous solvents show far greater c e l l resistance than the aqueous solutions, so that the cor-rections for the IR-drop become imperative. The diffusion coefficients of the ions in most non-aqueous solvents are considerably lower than those in aqueous solutions which makes the diffusion currents far smaller compared to those in aqueous solutions. The non-aqueous solvents are generally non-polar and in most cases, only a limited number of sub-stances could be employed as supporting electrolytes, i.e. substances that are added to conduct the current. Moreover, - 16 -the solubility of oxygen is considerably greater in non-aqueous than in aqueous medium which results in undesirable interfering waves. A l l these factors make the analytical conditions not too ideal and for a long time polarography in non-aqueous sol-vents suffered neglect. The advantages of developing non-aqueous solvents suitable for use in polarography was, however, realised quite early. The foremost advantage i s , of course, the possibility of studying a large number of organic reactions which could not be studied in aqueous media. In the study of reactions, where the presence of moisture i s undesirable, recourse to anhydrous solvents i s unavoidable. Hence, several attempts have been made to study the polarographic behaviour of non-aqueous solvents in recent years. Some of the solvents re-ported suitable in literature, are methanol, ethanol, ethylene glycol, acetic acid, liquid ammonia, acetone and formamide. These have been considered 'well behaved1 whereas other sol-vents such as benzene, toluene, and aniline have been con-sidered unsatisfactory because most of the inorganic electro-lytes are insoluble in them while most organic electrolytes have relatively large resistances. - 17 Polarography General Concepts 'Polarography' is essentially a study of the relation-ship between the current and applied voltage, when the rele-vant reaction i s taking place at a dropping mercury electrode. The electrode reaction takes place at the electrode under the influence of the applied potential and is characterised by a transfer of electrons, which produce a flow of current. The current then is the result of the electrode reaction and not i t s cause. When no polarisation is taking place, the current-voltage curve is linear. In the present work, only polarographic reduction processes have been studied at the dropping mercury electrode. However, oxidation reactions can also be studied in a similar way. In the following discussion, reference w i l l be made only to polarographic reductions. In polarography, a relatively large concentration of a non-reducible electrolyte i s usually added to carry the greater part of the current and prevent the elec t r i c a l migrat-ion of the reducible ions, which are present in a very low concentration. This indifferent added electrolyte i s called the 'supporting electrolyte'. The transfer of the reducible ions to the electrode when a large concentration of ions of the supporting electrolyte are present, takes place solely through diffusion. If we consider an electrode reaction of - 18 -the type Ag + e ^± Ag as taking place at an electrode and assume a very small concentration of Ag + ions in an excess of the ions of the supporting electrolyte, then even a small reduction current w i l l cause the Ag + ion concentration at the electrode surface to decrease considerably with respect to the concentration in the bulk of the solution. This sets up a concentration gradient and hence brings about a diffusion of that ion from the bulk of the solution to the electrode. The diffusion current that flows, arises as a result of con-centration polarisation. It is far greater in magnitude than the residual current, which i s the current that flows when no electro-reducible substance is present in solution. The process, therefore, becomes mainly diffusion controlled. If we consider the variation of the current with i n -creasing potential we find that, at f i r s t , (i.e., before the electrode attains the reduction potential of the reducible ion) the current is wholly due to the electrical potential gradient; on increasing the potential beyond the reduction potential, the depletion of the concentration of the ions at the electrode surface starts and hence diffusion of the ions takes place to-wards the electrode surface. The ions get reduced on reaching the electrode surface. The diffusion current, then, is dir-ectly proportional to the concentration gradient between the bulk of the solution and the electrode surface. As the poten-t i a l is made more negative, reduction takes place at a faster rate, causing an increasing concentration gradient and hence, - 19 -the diffusion current is proportionately increased. This takes place u n t i l , at a particular potential, the concentrat-ion of the ions at the electrode surface i s negligibly small, or virtually zero. A further increase in potential does not produce any further decrease in the concentration and hence, the diffusion current is then proportional to the concentration in the bulk of the solution only. Thus, a constant limiting current results. This represents virtually a state of complete concentration polarisation. The fact that the limiting cur-rent is directly proportional to the concentration forms the basis of quantitative determinations from current-voltage curves. Polarography is based on this phenomenon of concen-tration polarisation. A polarographic diffusion current i s , as shown above, mainly diffusion controlled and hence, the theoretical treatment of the diffusion current i s based on the theories of diffusion. Since the electrode surface in this case i s not stationary, but shows a periodic growth, suitable modifications have to be made in deriving the diffusion equation. Ilkovic equation The derivation of an expression for the diffusion current was f i r s t made by Ilkovic (14). It was later re-derived by MacGillavry and Rideal (30). Ilkovic derived the equation for the diffusion current by employing the fundamental equation for a symmetrical spherical diffusion up to a - 20 -stationary surface: » C / M = D fyT2 • 2 / r « / J The stationary co-ordinate r was replaced by a mov-ing co-ordinate p defined by 4/3 TT r 3 - 4/3 TT r Q 3 = 4/3 TT p 3 where r represents the r a d i a l distance from a point i n the solution to the centre of the drop and r the radius of the o drop at any instant. The expression derived thus f o r the d i f f u s i o n current at any instant t i n the l i f e of the drop i s given by: i + = 0.732 nP D* Cm2/3 t 1 / 6 where F = Faraday i n coulombs. D = Di f f u s i o n c o e f f i c i e n t of the ion in cm.2 s e c . - 1 . 3 C = Concentration i n moles per cm. . m = gms. per sec. of mercury t •= time i n seconds. n = Number of electrons involved i n the reduction. The constant 0.732 i s a combination of several numerical constants. I t i s more convenient to express the current i n microamperes, the concentration i n terms of millimoles per l i t r e , and m as rag. sec."" 1. The expression then becomes, i . =706 nD^ Cm2/3 t 1 / 6 - 21 -The maximum current in the l i f e of a drop i s given by i ^ . when t is maximum. The average current in the l i f e of a drop is given by performing the integration over the drop time and i s given by I = 607 nD* C m2/3 t ^ This i s referred to as the 'Ilkovic equation'. For the lim i t -ing diffusion current, therefore, i d = 607 nD* C m 2 / 3 t l / 6 Thus, i d/Cm 2/ 3 t1^6 = 607 nD ¥ = Constant for any parti-cular ion. It is only approximately so in cases where the m 2/3 .f.l/6 v a i u e s for two capillaries differ by more than 0.5 mg2/3 sec 1/ 6; otherwise they agree to within 2%. The original equation of Ilkovic has been subject to criticism because of certain omissions and oversimplifications (27) and corrections have been introduced. Even the corrected equations have not been found completely satisfactory from the theoretical standpoint, even though they hold to a f a i r degree of accuracy in practice. Factors that govern diffusion current From a study of the Ilkovic equation, one immediately recognises the factors that govern the diffusion current: a) The diffusion current i s directly proportional to the concentration of the reducible ion. This forms the basis of a l l analytical work in polarography. - 22 -b) The d i f f u s i o n current i s proportional to the square root of the d i f f u s i o n c o e f f i c i e n t of the ion. c) The d i f f u s i o n current i s proportional to m2/3 t l / 6 ^ T h i s f a c t o r m2/3 t l / 6 i g d e p e n d e n t o n t h e c a p i i _ l a r y c h a r a c t e r i s t i c s . Hence i t i s necessary to determine the c a p i l l a r y c h a r a c t e r i s t i c s before the polarographic i n -vestigations are made. Ca p i l l a r y c h a r a c t e r i s t i c s In the case of c a p i l l a r i e s , d e f i n i t e l y known to be of uniform c i r c u l a r bore, the radius and the length of the c a p i l l a r y constitute the c h a r a c t e r i s t i c s . But with most c a p i l l a r i e s , there i s a f a i r degree of uncertainty as re-gards the uniformity of the bore. In such cases, "C a p i l -l a r y Constant'" designated by 'K' i s employed to characterise the c a p i l l a r y . By a form of the P o i s e u i l l e ' s equation, we have, m = ft r 4 d P c 8lT) where m = milligrams of mercury flowing per second r c = radius of c a p i l l a r y i n cm. d = density of mercury i n gms./cm. . 1 = length of c a p i l l a r y i n cm. T| = Co e f f i c i e n t of v i s c o s i t y of mercury and P = 'Effective* pressure on the dropping mercury i n -2 dynes, cm. 23 -When the mercury i s dropping slowly from the c a p i l l a r y , the ef f e c t i v e pressure P i s smaller than the t o t a l hydrostatic pressure of the mercury column because the i n t e r f a c i a l ten-sion at the surface of the drop exerts a back pressure. The back pressure has been shown by Kucera (19) to be given by P_,. = 2<r/rH where o~ = I n t e r f a c i a l tension i n dynes -1 cm and r d = radius of the drop i n cm. When expressed in terms of m and t, the average value of Pback i s S i v e n b v < 1 7» P aS e 8 1> Average P b a c k = 3.1/m1/3 t 1 / 3 P i s therefore given by: p — T3 _ q -1 / 1/3 + l / 3 P - * applied d ' 1 / m t From the P o i s e u i l l e ' s formula above, we have, c • K ' can be used to characterise a l l kinds of c a p i l l a r i e s , P/m = 8T|l/flr 4 d = K whether or not they are uniform. d) The influence of the potential of the dropping electrode on the d i f f u s i o n current i s another factor of im-portance . It has been shown (28) that the i n t e r f a c i a l tension at a mercury-electrolyte solution interface depends on the potential of the mercury. On applying increasingly negative potentials at the dropping electrode, the i n t e r f a c i a l tension i s found at f i r s t to increase, pass through a maximum and - 24 -then ste a d i l y decrease. The potential corresponding to the maximum i s c a l l e d the ' E l e c t r o c a p i l l a r y Zero* and the para-b o l i c curve obtained by p l o t t i n g the i n t e r f a c i a l tension against the potential i s c a l l e d the ' E l e c t r o c a p i l l a r y Curve* of mercury. The drop time i s found to be d i r e c t l y proportional to the i n t e r f a c i a l tension as given by the expression, mt = c where g = g r a v i t a t i o n a l force g and the others have t h e i r usual meaning. Hence, t varies i n a s i m i l a r manner to 0" with the p o t e n t i a l of the dropping electrode. As has already been seen the d i f f u s i o n current i s d i r e c t l y proportional to m2/3 t 1 ^ . If i n the same way, the values of m2^3 t 1 / 6 are plotted against the potential of the cathodically polarised mercury, a para-b o l i c curve s i m i l a r to the above i s obtained, the maxima i n a l l the cases corresponding to the same po t e n t i a l . Thus the d i f f u s i o n current i s s i g n i f i c a n t l y changed due to the e f f e c t of p o t e n t i a l , Kblthoff and Orlemann (18) have shown the importance and method of making corrections f o r this e f f e c t . The corrections are based on the observat-ion that the r e l a t i v e values of the drop time at two d i f f e r e n t potentials are independent of the c h a r a c t e r i s t i c s of the c a p i l l a r y f o r any given supporting e l e c t r o l y t e . - 25 -e) The e f f e c t of temperature on the d i f f u s i o n cur-rent involves the variation of the d i f f u s i o n c o e f f i c i e n t with temperature, the variation of density with temperature and f i n a l l y the temperature c o e f f i c i e n t of m. By f a r the most important i s the temperature c o e f f i c i e n t of the d i f f u s i o n c o e f f i c i e n t . The magnitude of this v a r i a t i o n i s found to he about 2% per degree f o r most of the ions. The other two variations are found to be so small that they could be neg-lected (17 p. 92 ). Residual current The measured d i f f u s i o n current i s usually greater than the actual current because i t includes the residual cur-rent. Residual current i s the current that flows when no electroreducible substance i s present i n the solution. I t i s a c t u a l l y the sum of: 1) a non-Faradayic condenser cur-rent, a r i s i n g as a r e s u l t :of the mercury being charged on either side of the e l e c t r o c a p i l l a r y zero. I t i s p o s i t i v e l y charged at potentials less negative than the e l e c t r o c a p i l l a r y zero and negative at more negative potentials. 2) a Faradayic current due to the reduction of traces of reducible impur-i t i e s i n the solution, l i k e oxygen, etc. (17, p. 151). The measured d i f f u s i o n currents must be corrected for the r e s i d -ual current. - 26 -Potential of tbe dropping electrode: The potential of the dropping electrode i s given by, p - p 0-0591 i n p i Ed.e " B * ~ l0% Tprj where i = current at any instant i n microamperes, i ^ = l i m i t i n g current i n microamperes, n = number of electrons involved i n the reduction, and Ex = half wave po t e n t i a l or the potential i n volts 2 when i = xd/2. This fundamental equation of the polarographic wave shows that when the logarithm of j - ^ — * - i s plotted against x d " 1 the potential of the dropping electrode, the slope of the l i n e 0 0591 would give the value of • * ^ . Prom the slope of the log-arithmic p l o t , therefore, one can get the value of n, the number of electrons involved i n the reduction. Since, at the half wave p o t e n t i a l , i = x&/2,log - — T = 0 so that the potent i a l corresponding to the point on the l i n e where log -.—1—v = 0, gives the half wave p o t e n t i a l . The half wave d " 1 potential i s constant for any p a r t i c u l a r ion and independ-ent of the concentration i n reversible reductions. The half-wave potential has a d e f i n i t e thermo-dynamic significance (17, page 199, 26, 4 6 ) . E i i s given by E i = E ° a + RT/nFy In a ^ - RT/nF y In ^ where f i s the a c t i v i t y c o e f f i c i e n t of the amalgam formed a f t e r reduction on the surface of the mercury drops and f i s the a c t i v i t y c o e f f i c i e n t of the reducible ion at the - 27 -surface of the mercury drops, and the a c t i v i t y of mer-cury i n amalgam. i n the amalgam and E° , the standard potential of the a Consider the standard pot e n t i a l of the c e l l , K (s) M (Hg) saturated. Let a s a ^ ^ be the a c t i v i t y of the metal i n the saturated amalgam. The e.m.f. of this c e l l , E , i s independent of the concentration of the metal ions i n solution E s = E ° a " E ° M " RT/nFy In !§§*L a Hg where E 0 ^ = standard potential of the pure metal and a!! = a c t i v i t y of mercury i n the saturated amalgam r e l a t i v e to pure mercury E ° a = E s + E ° M - R T / n F y l n C s a t d . f s a t d . - R T / n F y l n a*Hg substituting t h i s back again i n the expression f o r E^, we _ „o• , _ , 0.0591 - n „ f 0.0591 obtain = E M + E s + — log C s a t d < f s M ^ - — 1 „ p * * * * The l a s t factor i n most cases i s n e g l i g i b l e and so, E r E s + B ° K * 2 ^ s a i „ g c s a t d - f s a t d -Thus, the E i values may be calculated from standard pot e n t i a l of the metal, i t s a f f i n i t y f o r mercury and the s o l u b i l i t y of the metal i n mercury. Sati s f a c t o r y agreement i s obtained between observed and calculated values (17, p. 201). - 28 -In the case of irreversible reductions, E i may be 2 constant and independent of concentration only in a few cases but generally not so. But the slope of the logarithmic plot w i l l differ from the reversible value. Since the rates of oxidation and reduction are different, the two waves gener-al l y do not coincide with each other and polarisation ef-fects, in addition to concentration polarisation come into play. The equation of the wave then involves the rate constant of the slow step in the electrode reaction. The potential of the dropping electrode i s always reported negative. Corrections to the half-wave potential have to be made in the case of cells with high internal resistance. The apparent half-wave potential is always larger than the true value as a result of the IR-drop through the c e l l . In case the c e l l resistance exceeds 1000 ohms, the IR-drop should be calculated and subtracted from the apparent half wave potential (17, p.374). Maxima and suppression The appearance of the maxima in the current-voltage curves i s one of frequent occurrence. They are considered undesirable and wherever possible, they should be eliminated. This i s done by the use of substances commonly called •Suppressors'. They are usually capillary active substances, charged colloids or coloured dyes. The maxima are observed - 29 -to be sometimes acute and sometimes rounded but always re-producible. In the case of acute maxima, i t has been observed that the potential often remains constant from the beginning of the discharge until the maximum is reached. The slope of the line, then i s found to be the reciprocal of the c e l l resistance. The height is found to be dependent on the con-centration of the reducible ion. The maxima were believed to be due to the adsorption of the electro-reducible substance on the growing mercury drop, on account of the inhomogeneous electric f i e l d around the drop (17, p. 168). Two different theories were advanced— one by Heyrovsky and the other by Ilkovic to account for the inhomogeneous electric f i e l d . But the recent work of Ant-weiler has shown that there is a streaming of the liquid around the mercury drop on the part of the current-voltage curve showing the maximum. At potentials, where the limit -ing current was seen, there was no streaming and a well defined diffusion layer was seen. When suppressors were added, the streaming ceased, and a quietly developing d i f -fusion layer was seen. In general, downward streaming was observed in case of 'positive' maxima and sideward streaming, in case of 'negative' maxima—the positive and negative referring to whether the maxima was at positive potentials of the Electrocapillary Zero or on the side of negative potentials. - 30 -The streaming, according to Antweiler, i s an el e c t r o k i n e t i c phenomenon. The e l e c t r i c a l double layers can migrate under the influence of potential gradients. Since i t i s the l i q u i d - l i q u i d interface that vie are dealing with and since mercury i s a good conductor, the double layer can move very e a s i l y even due to a s l i g h t difference i n surface ten-sion between the top and bottom of the drops. In the case of the posit i v e maximum, the mercury pool i s p o s i t i v e l y charged and hence, by e l e c t r i c a l a t t r a c t -ion, there i s a large concentration of anions on the surface of the pool. Since there i s a downward streaming, the potential at the bottom of the mercury drop must be posit i v e and at the top, negative. This i s because the current den-s i t y at the bottom of the mercury drop i s greater than at the top as a r e s u l t of the bottom surface being free to the reducible ions, whereas the c a p i l l a r y t i p above exercises a screening e f f e c t at the top. In the case of the negative maximum, the mercury pool i s negatively charged and hence there accumulates a greater concentration of cations at i t s surface. Since the bottom of the mercury drop i s positive with respect to the top, there i s a migration of the double layer from the bot-tom to the top ( i . e . , from high.cation concentration to low cation concentration). There i s thus a sideward streaming. - 31 -The suppression of the maxima depends on the charge of the mercury. In the case of non-capillary active ions, the suppression depends on their charge and valence. The positive maxima are suppressed more effectively by anions and the negative maxima, more effectively by cations. Again, the bivalent ones are more effective than the univalent ones and so on. The effect of capillary active ions like acid dyes and negative colloids is to suppress the positive maxima while the basic dyes and positive colloids are effective on negative maxima. However, such i s not always the case, and exceptions are known (17, p. 164). The polarizability of the ions, their specific a b i l i t y to be adsorbed on mercury at various potentials and their effect of shifting the electrocapillary zero are factors to be reckoned. - 32 -Historical The polarographic studies in non-aqueous solvents may be said to have started with Shikata (41) in 1923, when he investigated sodium ethoxide: in alcohol. The earlier studies were made mostly with alcohols because they were similar to water in many respects in addition to being good solvents for a number of organic and inorganic compounds. Gosman and Heyrovsky (10) employed a methyl alcohol medium for determin-ing the capillary-active substances in petroleum. Acetic acid was for the f i r s t time tried as a polarographic medium by MacGillavry (29) with relatively l i t t l e success. However, more successful results were obtained in that solvent by Bachman and Astle (3). Later, Bergman and James (4) studied a number of organic compounds in that solvent to investigate the effect of substituents on the reduction potentials of substances. The use of mixtures of solvents was made by several different workers using several different mixtures. Thus, mixtures of methanol and benzene, dioxane and glycerine, and glycol and glycerine were made use of for studying nitro compounds by Radin and De Vries (36). The nitro compounds were also studied in isobutyl alcohol and methanol by the same workers. Lewis, Quackenbush and De Vries (25) employed methanol-benzene mixtures for investigating oxygen-containing com-pounds like ketones, aldehydes and peroxides. Runner and Wagner (37) investigated the polarographic reduction of - 33 -ortho-, meta-, and para- a c e t a n i l i d e s and n i t r o a n i l i n e s i n absolute alcohol. To determine t e t r a e t h y l lead i n gaso-l i n e , Parks and Hansen (35) employed ethylene g l y c o l and gl y c o l ethers. H a l l (11) carried out a polarographic det-ermination of sulphur i n petroleum. He also determined the amounts of dissolved oxygen in the petroleum f r a c t i o n s . The determination of several inorganic cations was made by Sanko and Manussova (38) i n several organic solvents l i k e meth-anol, ethanol, glycerine and formamide. Thus, most organic solvents that had been used t i l l then were a l l OH -"-containing solvents, which set up a serious l i m i t a t i o n . Among the s o l -vents studied which do not contain any 0H~ groups are form-amide, which was f i r s t employed by Sanko and Manussova (38). I t has been more recently used by Letaw and Gropp (24) f o r studying several inorganic cations and some organic alde-hydes and ketones. Liquid ammonia i s another solvent which has been employed with success f o r the determination of a l k a l i metals by Laitinen and Nyman (20). Laitenen and Shoemaker (22) used the same solvent i n the polarography of thallium, copper, and ammonium ions and molecular oxygen. Anhydrous acetone has been studied by Arthur and Lyons (1) and concen-trated sulphuric acid by Vlcek (45) for the polarography of some inorganic cations. Sargent, C l i f f o r d and Lemmon (39) have used hydrogen flu o r i d e as polarographic solvent making use of a ro t a t i n g platinum electrode. 34 -In a c e t o n i t r i l e i t s e l f there has been very l i t t l e work done. The only work reported in l i t e r a t u r e using this solvent has been that of Wawzonek and Runner (47) who i n -vestigated the polarographic reduction of some inorganic cations. They reported the half wave potentials of some inorganic cations and the d i f f u s i o n currents for a millimolar solution of each. They employed tetrabutyl ammonium iodide and tetrabutylammonium perchlorate separately as supporting e l e c t r o l y t e s and found that both of them were s a t i s f a c t o r y i n a c e t o n i t r i l e . They referred the potentials measured to the potential of the mercury pool and found that the maxima observed in some cases could not be suppressed by any of the commonly used suppressors. No organic compounds have been reported studied i n the l i t e r a t u r e t i l l now. The scope of the present research has been to study the polarographic behaviour of solutions i n a c e t o n i t r i l e and to investigate the cathodic reduction of some organic nitro-compounds i n that solvent. 35 -Experimental Apparatus and materials The polarographic c e l l consisted of an H-shaped vessel, the two limbs of which were connected to each other about a centimetre from the base. Both the limbs were c y l i n d r i c a l and were about 15 cms. high; but t h e i r diameters d i f f e r e d — o n e was about 5.5 cms. and the other about 2 cms. The two were provided with ground glass j o i n t s of size $ 50/50 and $ 19/38 respectively. The smaller limb was designed to accommodate the reference calomel electrode des-cribed in section 1, i f necessary. The larger limb was f i l l e d with a ground glass top i n the centre of which was an open glass tube just wide enough for the c a p i l l a r y end of the dropping mercury electrode t o be inserted. The top also carried a side tube with a ground glass j o i n t of size $ 10/30 and stopper, through which mercury could be poured i n t o the vessel. A small gas exit tube, with a narrow bore, was also provided i n the top, f o r the nitrogen to escape.,;;. Joined to the larger limb almost at the base, and on op-posite sides of i t , were two narrow tubes bent at r i g h t angles to the vessel. One of them was to pass nitrogen through and i t carried a three way stop-cock by means of which nitrogen could either be bubbled through the solution or streamed into the vessel over the solution. Through the other tube, a long piece of platinum wire could be inserted - 36 -so as to make contact with the mercury pool at the bottom of the vessel. The dropping mercury electrode consisted of a long tube, about 80 cms. i n length, connected to a shorter and nar-rower glass tubing at the end of which there was a c a p i l l a r y tube, 4.7 cms. i n length, of very narrow bore. A side tube provided at the bottom of the glass tube was connected to a reservoir of mercury by means of a p l a s t i c 'Tygon' tubing. The reservoir was a glass bulb standing on a r i n g stand, whose height could be adjusted. A platinum wire, fused i n t o a glass tube dipped under the mercury surface and made the e l e c t r i c a l contact. The voltage which was applied to the polarographic c e l l could be regulated by means of a rheostat and was approximately indicated by a voltmeter i n the c i r c u i t . Any f r a c t i o n of the t o t a l p o t e n t i a l could be applied by adjusting two resistance c o i l s in s e r i e s — o n e f o r coarse adjustments and the other f o r fine adjustments. The ap-p l i e d p o t e n t i a l , however, could be accurately measured by means of a potentiometer assembly. The potentiometer used was a Leeds and Northrup student's type which had a normal range of 0 to 1.6 v o l t s . By standardising against a Weston standard c e l l , a f t e r s e t t i n g the potentiometer at half the potent i a l of the standard c e l l , the range could be extended from 0 to 3.2 v o l t s . The n u l l point detector i n the potentio-metric setup was a galvanometer supplied by the Rubicon Co. - 37 (Catalogue number 3002-SYS) with a s e n s i t i v i t y of 1 micro-ampere per mm. d i v i s i o n . The current measuring device was a cal i b r a t e d galvanometer provided with an Ayrton shunt made of two pre-c i s i o n decade resistance boxes supplied by the General Radio Company. The galvanometer was supplied by the Rubicon Com-pany, Philadelphia, and had a s e n s i t i v i t y of 0.0014 mm/ (aa. and a period of 4.9 seconds. By adjusting the Ayrton shunt, the s e n s i t i v i t y could be changed by a known amount. A second shunt served to adjust the maximum s e n s i t i v i t y to an i n t e g r a l value and also to provide the proper damping resistance. The method of c a l i b r a t i o n was that given by Kolthoff and Lingane (17, p. 301). There were provisions made in the apparatus to reverse the p o l a r i t y of the potential applied, and to re-verse the current flow i n the galvanometer. An a l t e r -native method of measuring the current was afforded by hav-ing standard resistances of 1000 ohms, 10,000 ohms or any desired external resistance, i n place of the galvanometer and measuring the IR-drop by means of the potentiometer. By appropriate switches these resistances could be brought into the c i r c u i t . Materials The solvent employed was a c e t o n i t r i l e which was p u r i f i e d as described in section 1. 38 -The supporting e l e c t r o l y t e employed throughout the work was tetrabutylammonium iodide which had been used previously i n the same solvent by Wawzonek and Runner (47) and reported suitable. It was prepared by a method given by Laitinen and Wawzonek (23), s l i g h t l y modified. A mix-ture of equal volumes of n-butyl iodide and tributylamine were taken i n a bottle and the a i r inside displaced by dry nitrogen. I t was t i g h t l y stoppered and kept i n a hot oven f o r about 3 days. The s o l i d that separated was washed with a l i t t l e ethyl acetate and dissolved in the least amount of cold ethanol. It was f i l t e r e d and the f i l t r a t e par-t i a l l y d i s t i l l e d under reduced pressure. The s o l i d that separated on cooling was r e c r y s t a l l i s e d from ethyl acetate. The r e c r y s t a l l i s a t i o n was repeated three times. The crys-t a l s were then dried in a vacuum desiccator. The melting point of the c r y s t a l s was found to be 144-45°C. The s o l i d was preserved i n a well-stoppered brown bottle. The substances that were investigated polaro-graphically were sodium iodide, ortho-, meta-, and para-nitrophenols and ortho-, meta-, and para- n i t r o a n i l i n e s . Sodium iodide used was the a n a l y t i c a l reagent grade. I t was used without further p u r i f i c a t i o n . Ortho-nitrophenol and meta-nitrophenol were both products of the Eastman Kodak Company and both were used without further p u r i f i c a t i o n . 39 -para-nitrophenol used was the Eastman Kodak Co. product. Since the marketed product i s supposed to con-t a i n a mixture of two forms (12) i t was necessary to p u r i f y the substance. I t was r e c r y s t a l l i s e d from toluene below 63°C to obtain the yellow needles of p-nitrophenol, stable at room temperature and somewhat sensitive to l i g h t . They were preserved in a bottle wrapped round with aluminium f o i l i n dark u n t i l they were used. ortho- and para- n i t r o a n i l i n e s were both products of the B r i t i s h Drug House and were used without further p u r i f i c a t i on. meta-nitroaniline was an Eastman Kodak Co. product. It was r e c r y s t a l l i s e d from hot water to obtain orange-red flakes of m-nitroaniline. The nitrogen used f o r degassing the e l e c t r o l y t e solution was of premium grade. I t was p u r i f i e d by the method of Meites and Meites (31). An 0.1 molar solution of vanadyl sulphate was prepared i n d i l u t e sulphuric acid and about 50 m i l l i l i t r e s of the solution was placed in each of two gas washing bottles, each containing 100 gms. of l i g h t l y amalgamated zinc. The nitrogen gas from the cylinder was f i r s t bubbled through these and then through a wash bottle containing d i s t i l l e d water. It was then dried by passing through a drying tower packed with ' D r i e r i t e 1 . I t was next passed through an empty bottle to deposit any dust and - 40 -f i n a l l y i n t o another wash bottle through a sintered glass tube dipping into pure a c e t o n i t r i l e . This l a s t wash bo t t l e was designed to eliminate concentration changes i n the polarographic vessel due to the vapours of the solvent being carried away by nitrogen gas. To prevent the uneven vapourisation of the solvent due to changes i n temperature,,, this wash bottle was maintained at the same temperature as the polarographic c e l l . The solution of the supporting e l e c t r o l y t e employed throughout the work was an 0.1 molar solution of t e t r a -butylammonium iodide in a c e t o n i t r i l e . Only 250 ml. of the solution was prepared each time and the solution was kept in a well stoppered b o t t l e . The mercury used for both the dropping mercury electrode and the mercury pool i n each experiment was spec-i a l l y p u r i f i e d as follows. The commercial mercury was f i r s t oxidised using 4N n i t r i c acid. It was thoroughly agitated i n contact with n i t r i c acid by bubbling a i r through i t . It was f i l t e r e d through a porous wooden funnel under pres-sure and washed thoroughly i n running water. I t was then d i s t i l l e d under reduced pressure. The d i s t i l l a t i o n was re-peated thrice and the t r i p l e - d i s t i l l e d mercury was used i n a l l the experiments. - 41 -Procedure i ) Characterisation of the c a p i l l a r y : As already described in the introduction, the 'Capillary Constant 1 serves to characterise a l l kinds of c a p i l l a r i e s . It represents the pressure required to force 1 milligram of mercury through the c a p i l l a r y per second. The determination of the c a p i l l a r y constant was made by the method of O.H. Muller (32,33). The apparatus used was very s i m i l a r to the polaro-graphic c e l l already described, except that i t had a side tube attached to the larger limb, at an angle of about 45° to i t . This side tube was provided with a ground glass j o i n t of size ip 24/40. A ground glass cap f i t t e d i n t o i t and through the centre of the cap, a glass rod having a tiny glass spoon at i t s end could be moved up and down, so that i t was exactly below the dropping mercury or outside i t . F i r s t l y , the top of the polarographic vessel was removed and the mercury allowed to drop f r e e l y i n a i r . A d e f i n i t e number of drops was collected i n the spoon and transferred to a beaker f o r determining the weight. The time required f o r the drops to form was determined by a precision stop watch reading to 0.1 second. This was re-peated with d i f f e r e n t heights of mercury in the reservoir. The r e s u l t s are tabulated i n Table I. - 42 Next an 0.1 molar solution of potassium chloride i n water was placed in the polarographic vessel and the c a p i l l a r y dipped into i t . With the c a p i l l a r y dropping i n the solution, the same procedure was repeated again at d i f -ferent heights of the mercury res e r v o i r . The r e s u l t s are shown in Table I. [ i t was noticed during t h i s experiment that i f the c a p i l l a r y i s allowed to stand above a c e t o n i t r i l e , the drop time observed w i l l not correspond to that in a i r any more. The a c e t o n i t r i l e vapour seems to aff e c t the surface tension of mercury so that the drop times are considerably smaller. ] i i ) E l e c t r o c a p i l l a r y Curve: For determining the variation of drop time with applied p o t e n t i a l , the apparatus used was the s p e c i a l polarographic c e l l described in section i ) on the charact-e r i s a t i o n of the c a p i l l a r y . A 0.1 molar solution of the supporting e l e c t r o l y t e was placed in the polarographic vessel over some mercury serving as the pool anode. The dropping mercury electrode was lowered into i t and the height of the mercury reservoir adjusted to a convenient height. The mercury pool anode and the dropping mercury cathode were then shorted. A d e f i n i t e number of drops of mercury from the c a p i l l a r y was collected in the spoon and transferred to beaker, washed, dried and weighed. The time - 43 -required f o r the formation of the drops was accurately-determined by means of a precision stop watch. The drop time and the number of milligrams of mercury passing through the c a p i l l a r y per second (m) were calculated. Increasing negative potentials were applied to the dropping mercury electrode and the corresponding drop times and the m valves were determined. The values of m2^3 t 1 / 6 were determined at each applied p o t e n t i a l . The r e s u l t s of the determinat-ions are shown in Table I I . The values of the drop times and m2/3 t 1 / 6 have been plotted against pot e n t i a l i n F i g . 1. The curve represents the ' E l e c t r o c a p i l l a r y Curve' of mer-cury i n the p a r t i c u l a r solution used and the maximum in the curve corresponds to the ' E l e c t r o c a p i l l a r y Zero'. i i i ) Measurement of the polarographic wave: A known volume of the supporting e l e c t r o l y t e solution, about 50 or 60 ml. was accurately measured out in t o the polarographic c e l l and the vessel introduced into the constant temperature bath maintained at 25 - 0.2°C. The nitrogen gas, p u r i f i e d as described, was bubbled i n t o the e l e c t r o l y t e at a steady rate for about half an hour before every determination. As a r u l e , the addition of the t r i p l y d i s t i l l e d mercury into the c e l l for the pool anode was made after degassing f o r at least half an hour to guard against the p o s s i b i l i t y of complex formation of mercury with the ions and oxygen suggested by Arthur and Lyons (1). Nitrogen was again bubbled f o r - 44 -- 45 -another half an hour to secure v i r t u a l removal of a l l d i s -solved oxygen from the e l e c t r o l y t e solution. The stream of nitrogen was then directed over solution by means of the two-way stopcock, before the measurements were begun. The dropping mercury electrode was then lowered into the polarographic vessel so that the t i p of the c a p i l l a r y was as near the pool of mercury as possible. The reservoir of mercury was then adjusted so that the height of mercury was exactly 30 oms. from the t i p of the c a p i l l a r y . The potentials applied were referred to the mer-cury pool. The residual current was measured by means of the galvanometer at gradually increasing p o t e n t i a l s . The s e n s i t i v i t y of the galvanometer was adjusted by means of the Ayrton shunt to a d e f i n i t e value and the currents at each applied p o t e n t i a l were read off on the galvanometer scale. The applied potentials were accurately measured by the potentiometric setup described already. The current was then plotted as a function of p o t e n t i a l . F i g . 2 shows the residual current curve of the e l e c t r o l y t e solution used. In the determination of the polarographic re-duction waves of any substance, the substance under i n -vestigation was taken i n a small weighing piggy and weighed correct to 0.1 mg. I t was c a r e f u l l y tipped into the known volume of e l e c t r o l y t e solution previously degassed as described above. The amount of substance thus transferred Figure 2. Residual current curve - 47 -was such that an approximately millimolar solution was thus obtained. Nitrogen gas was again bubbled for about 10 minutes to ensure thorough mixing. Measurements were again made of the current with gradually increasing potentials. The potential increase was done in steps of 0.1 volt except in the region of sudden increase in the current. In this region of rising current, the potentials were increased 0.02 v or 0.05 volt at each step and the corresponding increase in the current measured. Experiments were repeated with at least three dif-ferent concentrations in each case. Typical measurements have been shown for every compound studied. The half-wave potentials of the solutions at different concentration were within - 0.01 volt of each other. The values of the slopes of the logarithmic plot remained constant within - 0.005 v. 48 Results The polarographic c a p i l l a r y made use of through-out was 4.7 cms. long and had a drop time of 4.24 seconds with the mercury pool and the dropping mercury electrode shorted. The height of the mercury was always maintained at 30 cms. throughout the measurements. i ) The re s u l t s of the characterisation of the c a p i l l a r y were as shown i n Table I. The "C a p i l l a r y Constant" could be taken as the mean of these two average values. K = 22.79 According to K u l l e r , < = 2.1567 x 10~ 1 0 * l / r 0 4 . Hence, the radius of the c a p i l l a r y could be given by 4 _ 2.1567 X 1 0 " 1 0 x 4.7 r c ~ 22.79 Whence r = 0.002583 cm. c i i ) The experimental r e s u l t s obtained i n studying the variation of m2^3 t l y ^ 6 with potential were as shown in Table I I . Figure 1 shows the 'E l e c t r o c a p i l l a r y Curve' of mercury i n 0.1 molar tetrabutylammonium iodide solution in a c e t o n i t r i l e . The maximum value of m2^3 t 1 ^ 6 occurs at a potential of -0.18 v when the drop time i s longest. The - 49 -Table I. Characterisation of the c a p i l l a r y a) C a p i l l a r y dropping i n p t W m (mg/ app (cm) (sec) (mg) sec) 40.0 5.15 90.26 1. 753 50.0 41.6 90.98 2. 187 60.0 34.8 91.04 2. 616 70.0 30.0 91.00 3. 033 Average a i r : app w l / 3 Pbaclc P 22.82 4.485 0.5 39.5 22. 54 22.87 4.498 0.5 49.5 22. 63 22.94 4.499 0.5 59.5 22. 75 23.08 4.498 0.5 69.5 22. 92 = 22.71 b) Capill a r y dropping i n 0.1 molar potassium chloride solution: P app (cm) t (sec) W (mg) m (mg/ sec) app w l / 3 Pback P 30.0 40.0 50.0 60.0 70.0 4.50 -::-4.05 3.50 •::-3.25 2.90 -::-2.72 2.45 -:t-2.40 2.12 -::-2.05 5.77 5.08 5.95 5.54 6.22 5.85 6.29 6.16 6.31 6.12 1.282 1.283 1.670 1.704 2.145 2.151 2.567 2.566 2.977 2.985 23.40 23.43 23.54 23.47 23.31 23.31 23.37 23.36 23.52 23.45 1.793 1.719 1.813 1.769 1.839 1.802 1.846 1.833 1.848 1.829 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 28.7 38.7 48.7 58.7 68.7 22.39 22.37 23.17 22.72 22.70 22.64 22.81 22.88 23.08 23.02 Observations made during the lowering of mercury Average K =22.87 — 50 Table I I . E f f e c t of potential on m2/3 t 1 / 6 E t W log m 2/3 log 1/6 log (volts) (seconds) (rag) m t m2/3 ? l / 6 5 drops 1 drop 0.000 21.2 4. 24 29.7 0.1465 0.0976 0.1046 1. 593 0.100 21.7 4. 34 30.3 0.1449 0.0966 0.1062 1. 595 0.150 21.8 4. 36 30.5 0.1458 0.0972 0.1066 1. 599 0.180 21.9 4. 38 30.7 0.1467 0.0978 0.1069 1. 602 0.200 21.7 4. 34 30.3 0.1449 0.0968 0.1062 1. 595 0.400 21.4 4. 28 29.6 0.1409 0.0939 0.1052 1. 581 0.500 20.8 4. 16 28.8 0.1413 0.0942 0.1021 1. 571 0.700 19.6 3. 92 27.0 0.1391 0.0927 0.0989 1. 555 0.900 18.0 3. 60 24.9 0.1409 0.0939 0.0927 1. 537 1.100 16.4 3. 28 22.75 0.1422 0.0948 0.0860 1. 516 1.300 14.8 2. 96 20.60 0.1436 0.0957 0.0785 1. 493 1.500 13.0 2. 60 18.25 0.1474 0.0983 0.0692 1. 471 e l e c t r o c a p i l l a r y zero for the system therefore corresponds to -0.18 volt. This i s i n agreement with the reported value of Wawzonek and Runner (47). i i i ) Residual current: The residual current at gradually increasing potentials measured in a i r - f r e e solution of the supporting e l e c t r o l y t e solution were as shown i n Table I I I . Figure 2 shows the residual current curve. The small wave occurring i n the residual current curve could not be eliminated i n spite of repeated r e c r y s t a l l i s a t i o n s of the supporting e l e c t r o l y t e . Hence i n the following measurements, corrections have been made in the d i f f u s i o n currents accordingly. - 51 -Table I I I . Residual current - Potential (volts) Current (microamperes) 0.000 -0.09 0.100 0.32 0.200 0.78 0.300 0.80 0.400 0.80 0.500 0.80 0.600 0.80 0.700 0.80 0.800 0.80 0.900 0.80 1.000 0.90 1.100 1.05 1.200 1.15 1.300 1.30 1.400 1.40 1.500 1.50 1.600 1.60 1.700 1.70 1.800 1.90 1.900 2.60 2.000 3.40 i v ) Sodium iodide: Preliminary to the study of the nitrocompounds, a study of the e l e c t r o reduction of the sodium ion i n a c e t o n i t r i l e was made. A t y p i c a l set of measurements has been given below. The concentration of the solution studied was 2.974 millimolar. The corrected values have been plotted against the potential in Figure 3. We f i n d there i s a well de-fined wave whose half-wave potential corresponds to -1.28 - 0.01 v o l t s . The resistance of the c e l l was measured by means of an audio-conductance bridge and was found to be of the - 1 2 8 Figure 3. Polarographic reduction wave of sodium ion (2.974 millimolar) i n a c e t o n i t r i l e - 53 -Table IV. Measurements with sodium iodide solution (2.974 millimolar) -E (volts) i (observed) (microamperes) i (corrected) micro-amperes 0.000 -0.09 0.00 0.100 0.32 0.00 0.200 0.78 0.00 0.300 0.80 0.00 0.400 0.80 0.00 0.500 0.80 0.00 0.600 0.80 0.00 0.700 0.80 0.00 0.800 0.80 0.00 0.900 0.80 0.00 1.000 0.90 0.00 1.100 1.05 0.00 1.200 1.58 0.43 1.250 3.30 2.10 1.280 5.50 4.30 1.300 7.0 5.7 1.320 8.3 7.0 1.350 9.3 8.0 1.400 10.0 8.6 1.500 10.1 8.6 1.600 10.2 8.6 1.700 10.2 8.5 order of 500 ohms. The IR-drop at the half wave, calculated on that basis comes out to be 0.002 v o l t . The magnitude of th i s correction i s very small and hence i t may be neglected. The constant 1 d / e m 2 / 3 t l / 6 was found to be 1.96 - 0.0% Figure 4 shows the logarithmic plot of 1/. against the potential applied. It gives a straight l i n e having a slope of 0.062 v. constant to within - 0.002 v. This indicates a one-electron reduction, as expected. • 0 - 8 0 .0-401-0*0 - 0 8 0 - 1 0 0 Figure 4. Logarithmic plot of ~ vs. potential —SAIIII II Tor sodium ion - 55 -Nitrophenols The nitrophenols show some abnormal behaviour on e l e c t r o l y t i c reduction; a l l of them show d e f i n i t e stepwise reduction--in some the waves are well separated and i n others, rather cl o s e l y spaced. In a l l cases, we f i n d that there i s formation of maxima, on allowing the solutions to stand i n contact with mercury fo r some hours. The solutions are also found to develop deep colours. The maxima are formed on the p o s i t i v e side of the e l e c t r o c a p i l l a r y zero i n a l l cases and cannot be suppressed by the commonly used suppressors, such as a- Naphthol, | 3 - Naphthol, fuchsine, and methyl red. v) o-nitrophenol: Table V shows the r e s u l t s of measurements made with a 1.018 millimolar solution of o-nitrophenol and represents the t y p i c a l behaviour of that Substance during e l e c t r o l y t i c reduction. Figure 5 shows the polarographic waves of o-nitrophenol. There are two d i s t i n c t waves corresponding to the two stages of reduction. There i s also observed a small wave about 1 microampere i n height preceding the two waves. The half wave potentials of the two waves are -0.38 - 0.01 volt and -1.38 - 0.01 volt respectively. The correction f o r the IR-drop i n t h i s and the following cases have been neglected because of t h e i r small magnitude. Figure 5. Polarographic wave of o-nitrophenol (1.018 millimolar)  in aceJfcVonitrile, immediately after solution - 57 -Table V. Reduction of o-nitrophenol ( 1 . 0 1 8 millimolar) -E (volts) Current (Observed) Current (corrected) (microamperes) 0.000 -0.09 0.00 0.100 1.22 0.90 0.200 1.78 1.00 0.300 2.10 1.30 0.340 2.70 1.90 0.380 3.90 3.10 0.400 4.60 3.80 0.440 5.20 4.40 0.480 5.40 4.60 0.500 5.60 4.80 0.540 5.70 4.90 0.580 5.70 4.90 0.600 5.70 4.90 0.700 5.70 4.90 0.800 5.70 4.90 0.900 5.70 4.90 1.000 5.70 4.80 1.100 5.85 4.80 1.200 6.95 5.80 1.240 8.55 7.30 1.280 10.5 9.2 1.300 11.5 10.2 1.340 14.2 12.8 1.380 16.7 15.3 1.400 17.7 16.3 1.440 20.8 19.3 1.480 22.8 21.3 1.500 23.6 22.1 1.540 24.9 23.3 1.600 25.8 24.2 1.640 26.6 24.9 1.700 26.6 24.9 1.800 26.6 24.7 Figure 6 shows the logarithmic plot of -. against the po t e n t i a l . The slope of the straight l i n e corresponding to the f i r s t wave i s found to be 0.086 ± 0.005 v and that corresponding to the second wave 0.154 * 0.005 v. The values of n calculated according to these slopes are 0.73 and 0.38 respectively. yjffure 6. Logarithmic plot corresponding to figure 5 - 59 In finding the diffusion current for the second wave, corrections were made for the change of drop time with potential by taking the. ratio of t i^Aj^ s b y t h e m e t n o d of Kolthoff and Orlemann (18). On allowing the solution to stand in contact with mercury for 36 hours, the solution changed colour from yellow to orange. Measurements were again made after 36 hours, after further degassing and the results are shown in Table VI. Table VI. o-nitrophenol (after 36 hours' standing) (1.018 millimolar) -E volts i (observed) i (corrected) Microamperes 0.020 14.5 14.4 0.040 30.8 30.6 0.060 40.4 40.2 0.080 47.6 47.3 0.100 51.9 51.6 0.140 44.5 44.0 0.180 5.35 4.70 0.200 5.40 4.60 0.300 5.50 4.70 0.400 5.60 4.80 0.500 5.60 4.80 0.600 5.60 4.80 0.700 5.60 4.80 0.800 5.60 4.80 0.900 5.60 4.80 1.000 5.60 4.70 1.100 6.05 5.00 1.200 8.45 7.30 1.300 12.7 11.4 1.400 18.6 17.2 1.500 24.9 23.4 1.600 28.7 27.1 1.700 29.5 27.8 1.800 29.7 27.8 -20 c n o Figure 7. Polarographic wave of o-nitrophenol  (1.018 millimolar) in acet^onitrile, after standing for 36 hours volts Figure 8. Logarithmic plot corresponding  to figure 7 - 62 -Figure 7 shows these values plotted. It was found that there was a distinct maximum which could not be suppressed by the commonly used suppressors like oc-Naphthol, |3-Naphthol, fuchsine, and methyl red. Most of the inorganic suppressors and gelatin were unsuitable because of insolubility in the electrolyte solution. It was noticed that the half wave poten-t i a l of the second wave was unaffected in spite of the disap-pearance of the f i r s t wave and the formation of the maximum. The logarithmic plot i s shown in Figure 8. The slope of this line was found to be 0.204 - 0.005 and the cor-responding value of n was 0.29. This was distinctly different from the original slope and the corresponding n. The results indicate an irreversible reduction (17, p. 266). vi) m-nitrophenol: Table VII contains a typical set of results obtained with a 1.503 millimolar solution of m-nitrophenol in the supporting electrolyte solution. Figure 9 shows the polarographic waves of m-nitrophenol. It is found that there are two closely-spaced waves preceded by a tiny wave about 0.4 |*a. high. The half wave potentials, as nearly as they could be found out from the graph, were 0.55 - 0.01 v and 0.84 - 0.01 v respectively. The constant 1d/ C m2/3 tl/6 for the f i r s t wave was 2 .82 - 0.05 and that for the second, after correcting for the o Figure 9. Polarographic wave of m-nitrophenol (1.503 millimolar) i n a c e t f o n i t r i l e , immediately aft e r solution"" Figure 10. Logarithmic plot corresponding to figure 9 - 65 -Table VII. Reduction of m-nitrophenol solution ( 1 . 5 0 3 milllmolarl -E (volts) i (observed) i (corrected) microamperes 0.000 -0.09 0.00 0.100 0.42 0.10 0.200 1.18 0.40 0.300 1.20 0.40 0.400 1.30 0.50 0.500 2.60 1.80 0.550 3.90 3.10 0.600 5.50 4.70 0.650 6.60 5.80 0.700 7.10 6.30 0.750 7.40 6.60 0.800 8.50 7.70 0.850 9.40 8.60 0.900 11.2 10.4 1.000 11.3 10.4 1.100 11.5 10.45 1.200 11.6 10.45 1.300 11.7 10.4 1.400 11.7 10.3 change in drop time with potential was 1.76 - 0.05. Figure 10 shows the logarithmic plot of against potential. According to the graph, the slope of the f i r s t was 0.108 i 0.005 v and that of the second 0.096 - 0.005. The corresponding n values were 0.55 and 0.61 respectively, again indicating irreversible reductions. On allowing the solution to stand overnight in contact with the mercury pool, the solution turned orange in colour. Measurements were again made after a period of 24 hours. They revealed the formation of a maximum and the separation of the waves. The half wave potentials had moved to more negative values. Table VIII shows the results - 66 -of these measurements. Table VIII. m-nitrophenol (after standing 24 hours) (1.503 millimolar) -E (volts) i (observed) i (corrected) microamperes 0.025 9.3 9.2 0.050 18.7 18.5 0.075 25.1 24.8 0.100 22.1 21.8 0.150 5.10 4.60 0.200 4.50 3.70 0.300 4.60 3.80 0.400 4.60 3.80 0.500 4.60 3.80 0.600 4.90 4.10 0.620 5.20 4.40 0.650 5.60 4.80 0.680 5.70 4.90 0.700 5.80 5.00 0.800 5.90 5.10 0.850 6.70 5.90 0.900 8.70 7.90 0.950 10.4 9.5 1.000 10.4 9.5 1.100 10.5 9.45 1.200 10.6 9.45 1.300 10.5 9.2 Figure 11 shows that there i s the formation of a maximum on the positive side of the electrocapillary zero. The waves are now observed to be well separated and distinct with their half wave potentials showing a shift towards more negative values. They were found to be -0.63 v and -0.89 v respectively. The maximum was again found non-suppressible. Figure 12 shows the logarithmic plot of "Vi^-i against potential. The slope of the line corresponding to Figure 11. Polarographic wave of m-nitrophenol (1.503 millimolar)  in a c e t o n i t r i l e , a f t e r standing for 24 hours F i g u r e 12. Logarithmic plot corresponding to figure 11 - 69 -the f i r s t wave was found to be 0.057 and that corresponding to the second was 0.069. Both the slopes indicate one electron reductions. v i i ) p-nitrophenol The results of the polarographic study of p-nitrophenol are given in Table IX. Measurements were made with a 3.197 millimolar solution. Table IX. Reduction of p-nitrophenol (3.197 millimolar) •E (volts) i (observed) i (corrected) (Microamperes) 0.000 -0.09 0.00 0.100 0.84 0.52 0.200 2.28 1.50 0.300 2.40 1.60 0.400 2.70 1.90 0.500 5.20 4.40 0.600 8.40 7.60 0.700 11.5 10.7 0.800 13.3 12.5 0.900 15.3 14.5 1.000 17.4 16.5 1.100 21.2 20.1 1.200 28.3 27.1 1.300 29.4 28.1 1.400 29.4 28.0 1.500 29.6 28.1 1.600 29.9 28.3 The results have been plotted in Figure 13. The waves were found to be rather closely spaced and the limiting current hardly distinguishable. There was again visible a small wave preceding the two reduction waves. The waves were rather i l l defined for the half-wave potentials to be 0 0 -0-2 -0-4 -0-6 -0-8 - 1 0 H - 2 -1-4 -1-6 volts Figure 13. Polarographic wave of p-nitrophenol (3.3.97 millimolar)  in a c e t o n i t r i l e . immediately a f t e r solution - 71 -determined with any accuracy. Even the graphical method of Zimmermann and Gropp (50) was not quite satisfactory. They were however found to be in the region of -0.62 volt and -1.12 volt respectively. The f i n a l limiting current was observed at a potential of -1.30 volt. The constant 2/3 l/6 ' t a k : J L n K as equal to this f i n a l diffusion current was found to be £.98 - 0.0£. On allowing the solution to stand in contact with the mercury pool for several hours, i t was found that the colour of the solution deepened. Measurements of the current were again made after 24 hours at gradually increasing negative potentials. The results are shown in Table X and plotted in Figure 14. Table X. p-nitrophenol (after 24 hours' standing) 7 *3.197 m i l l i m o l a r ) -B (volts) i (observed) i (corrected) (microamperes) 0.050 34.0 33.8 0.100 43.3 43.0 0.150 11.0 10.5 0.200 6.8 6.0 0.300 6.8 6.0 0.400 6.8 6.0 0.500 7.3 6.5 0.600 10.2 9.4 0.700 13.3 12.5 0.800 15.3 14.5 0.900 16.1 15.3 1.000 16.4 15.5 1.100 19.8 18.7 1.200 28.2 27.0 1.300 30.4 29.1 1.400 30.5 29.1 1.500 30.6 29.1 - 72 --—V $ 1 <a rH O S • H 73 -Figure 14 shows the formation of the maximum and also the separation of two distinct waves. It was noticed that the maximum obtained in this case was non-suppressible in the same way as the others. The half-wave potentials of the two waves were -0.64 - 0.01 volt and -1.14 - 0.01 volt respectively. Corrections for the IR-drop were again neglected. Figure 15 shows the logarithmic plot of *-x d " x against the potential. The slopes corresponding to the two waves were 0.191 - 0.005 and 0.092 - 0.005 respectively. The corresponding n values would be 0.309 and 0.641 res-pectively. These values indicate that the reductions tak-ing place are irreversible processes. Figure 15. Logarithmic plot corresponding to figure 14 - 75 -Ni t r o a n i l i n e s Unlike the nitrophenols, the n i t r o a n i l i n e s were polarographically well behaved. They showed no abnormal behaviour. They a l l produced well defined waves and there was no indicati o n of any maxima. No addition of any maxi-mum suppressor was therefore necessary. No prewaves of any kind were observed but only two d i s t i n c t waves corresponding to the stepwise reduction of the nitrocompound. Their slopes were in d i c a t i v e of the i r r e v e r s i b i l i t y of the electrode processes. v i i i ) ortho-nitroaniline The r e s u l t s of measurements made with a 1.084 millimolar solution of o-nitroaniline i n the supporting e l e c t r o l y t e have been given i n Table "EL and they have been plotted i n Figure 16. Figure 16 shows that there are two d i s t i n c t waves corresponding to half-wave potentials of -0.67 - 0.01 v and -1.59 - 0.01 v respectively. The constants / Q ^ / ^ I 1 / 6 f or t h e t w o waves were found to be 4.87- 0.05 and 11 . 8 0 - 0.15 respectively. The logarithmic plo t i s shown i n Figure 17. The slopes of the l i n e s corresponding to the two waves were 0.078 - 0.005 and 0.125 - 0.005. The corresponding n values were 0.756 and 0.472 respectively. The electrode processes Figure 16. Polarographic wave of o-nitroaniline (1.084 milliinolar) in a c e t r o n i t r i l e Figure 17. Logarithmic plot corresponding to figure 16 - 78 -may thus be regarded as i r r e v e r s i b l e . Table XI. Reduction of o-nitroaniline (1.081j. millimolar) -B (vo l t s ) i (observed) i (corrected) (microamperes) 0.000 -0.09 0.00 0.100 0.32 0.00 0.200 0.78 0.00 0.300 0.80 0.00 0.400 0.80 0.00 0.500 0.80 0.00 0.600 1.70 0.90 0.650 3.80 3.00 0.700 6.40 5.60 0.750 8.30 7.50 0.800 9.00 8.20 0.850 9.00 8.20 0.900 9.00 8.20 0.950 9.00 8.20 1.000 9.00 8.10 1.100 9.10 8.05 1.200 9.10 7.95 1.300 9.20 7.90 1.400 9.60 8.20 1.450 10.4 9.00 1.500 12.3 10.8 1.550 15.5 14.0 1.600 19.2 17.6 1.650 22.3 20.7 1.700 25.2 23.5 1.750 26.2 24.4 1.800 27.5 25.6 1.900 28.3 25.7 2.000 28.8 25.4 i x ) para-nitroaniline The r e s u l t s obtained with a 2.1 millimolar s o l u t i of p - n i t r o a n i l i n e have been given i n Table XII. - 79 -Table XII. Reduction of p- n i t r o a n i l i n e (2.1 millimolar) -E (volts) i (observed) i (corrected) (microamperes) 0.000 -0.09 0.00 0.100 0.32 0.00 0.200 0.78 0.00 0.300 0.80 0.00 0.400 0.80 0.00 0.500 0.80 0.00 0.600 1.00 0.20 0.650 2.10 1.30 0.700 4.45 3.65 0.750 7.00 6.20 0.800 10.8 10.0 0.850 12.3 11.5 0.900 12.8 12.0 1.000 12.8 11.9 1.100 12.8 11.8 1.200 12.9 11.7 1.300 13.5 12.2 1.400 14.1 12.7 1.450 15.9 14.5 1.500 19.4 17.9 1.550 23.5 22.0 1.600 29.5 27.9 1.650 33.5 31.9 1.700 37.0 35.3 1.750 41.0 39.2 1.800 41.6 39.7 1.850 42.0 39.7 1.900 42^3 39.7 2.000 42.8 39.4 Figure 18 shows that there are two d i s t i n c t l y sep-arated reduction waves corresponding to half wave potentials of -0.74 - 0.01 v and -1.58 ± 0.01 v respectively. The constants f o r the two waves were found , i : a f t e r correcting the d i f f u s i o n currents f o r the change i n drop time with p o t e n t i a l . The constants thus obtained were 3.71 - 0.05 and 9.64 - 0.05 respectively. Figure 18, Polarographic wave of p - n i t r o a n i l i n e (2.1 millimolar) in a c e t o n i t r i l e - 81 -The logarithmic plot of i / i ^ - i against potential has been given i n Figure 19. The slopes corresponding to the two waves were found to be 0.092 - 0.005 v and 0.151 - 0.005 v respectively. I r r e v e r s i b l e reductions were thus indicated. x) meta-nitroaniline Table XIII gives the res u l t s of a t y p i c a l set of measurements i n the polarographic reduction of m-nitroaniline. The concentration of the solution employed was 1.062 m i l l i -molar,.. Figure 20 shows the polarographic waves of m-n i t r o a n i l i n e . Two well defined waves corresponding to half wave potentials of -0.61 - 0.01 v and -1.37 - 0.01 v were found. There was found no trace of any maximum. The constants 1d/ e n i2/3 tl/6 were determined as before f o r both waves, correction being made i n the d i f f u s i o n current for the change i n drop time with p o t e n t i a l . They were 3.28 - 0.05 and 7.69 - 0.05 respectively. The logarithmic p l o t , shown i n Figure 21, gave the slopes of the l i n e s corresponding to the two waves as 0.071 - 0.005 v and 0.155 - 0.005 v respectively. Figure 19. Logarithmic plot corresponding to figure 18 18 0 I-F i g u r e 20. P o l a r o g r a p h i c wave of m - n i t r o a n i l i n e (1.062 m i l l i m o l a r ) i n a c e t i f ' o n i t r i l e - 84 -Table XIII. Reduction of m-nitroaniline (1.062 millimolar) S (volts) i (observed) i (correc 0.000 -0.09 0.00 0.100 0.32 0.00 0.200 0.78 0.00 0.300 0.80 0.00 0.400 0.80 0.00 0.500 0.80 0.00 0.550 1.40 0.60 0.600 2.90 2.10 0.620 3.60 2.80 0.640 4.50 3.70 0.660 5.20 4.40 0.680 5.80 5.00 0.700 5.90 5.10 0.720 6.20 5.40 0.740 6.20 5.40 0.780 6.20 5.40 0.800 6.15 5.35 0.900 6.10 5.30 1.000 6.20 5.30 1.100 6.30 5.25 1.200 7.00 5.85 1.225 7.50 6.30 1.250 7.90 6.70 1.275 8.60 7.30 1.300 9.20 7.90 1.325 10.1 8.8 1.350 11.0 9.7 1.375 11.8 10.4 1.400 13.1 11.7 1.425 14.2 12.8 1.450 15.1 13.7 1.475 16.0 14.5 1.500 16.4 14.9 1.550 17.4 15.8 1.600 18.3 16.7 1.700 18.4 16.7 1.800 18.5 16.6 1.900 19.2 16.6 2.000 20.0 16.6 - 86 -Discussion The e f f e c t of the substituent on the reduction potentials has been given by Shikata (42) i n the 'Electro-negativity Rule'. According to t h i s r u l e , organic compounds are more e a s i l y reduced as more electronegative groups are substituted in the same molecule. The OH group i s known to be more electronegative than the NH2 group (15, p. 10). That requires that the nitrophenols must be more e a s i l y reducible than the n i t r o a n i l i n e s or that the half wave pot-en t i a l s of the n i t r o a n i l i n e s must be more negative than those of the corresponding nitrophenols. The r e s u l t s i n -dicate that Shikata's electronegativity rule i s obeyed. Astle (8) has extended this theory by postulating that where substitution occurs causing the N of the n i t r o -group to be l e f t more posit i v e or at a lower electron den-s i t y than the normal nitrogroup, then, as a r e s u l t , resonance within the group i s decreased and hence the compound i s more e a s i l y reduced. In addition to the negative inductive ef-fects of the OH and NH2 groups, they also exercise a + T e f f e c t . It i s also known that the NH2 has a greater + T e f f e c t than the OH group (15, p. 77). Hence, the normal nitrogroups exhibit resonance as shown below: - 87 -As a re s u l t of the electromeric s h i f t s , we f i n d that the N of the nitrogroup becomes more pos i t i v e than i n the normal nitrogroup; t h i s e f f e c t i s more in the case of the NHg group which has a stronger + T e f f e c t . As a res u l t of t h i s , the N of the nitrogroup becomes more posi -t i v e with respect to the normal nitrogroup but less p o s i -tive than the N i n the nitrophenols. Hence we f i n d that the nitrophenols are more e a s i l y reduced than the n i t r o -a n i l i n e s . An examination of the res u l t s shows that i n the case of both the nitrophenols and the n i t r o a n i l i n e s , the ease of reduction i s i n the order meta > ortho ) para. In other words, the half-wave potentials are increasingly negative i n the order meta )> ortho ) para. An explanation of t h i s could be given on the basis of the + T e f f e c t s of NH2 and OH groups. As already shown, i n the case of both the ortho-and the para- compounds, resonance leaves the N more p o s i t i v e . 88 -Hence, both compounds should be expected to be more e a s i l y reducible than nitrobenzene. But i n the case of the ortho-compounds, there i s another additional f e a t u r e — t h e formation of the H-bonds as shown below: The p o s s i b i l i t y of H-bonding does not e x i s t i n the case of the para-compounds on account of s t e r i c f a c t o r s . These H-bonds hinder the resonance of the normal nitrogroup and hence we f i n d tha^t the ortho-compounds are more e a s i l y reduced than the corresponding para-compounds. In the case of the meta-compounds, however, there i s no resonance pos-s i b l e and hence they are most e a s i l y reduced. Thus, the ease of reduction i n the f i r s t stage of reduction should be meta } ortho ) para, and the results show that t h i s i s true. This i s to be expected i n a l l media, - a c i d i c , neutral or a l k a l i n e . Experiments of Bergman and James (4) i n acetic acid and of Astle and co-workers (8) i n aqueous buffers bear out the truth of this assertion. The re s u l t s of the experiments show that i n the case of a l l three nitrocompounds studied, there are two reduction waves seen, i n d i c a t i n g that the nitrocompounds undergo a two-step reduction. A si m i l a r observation was made by Bergman " - 89 -and James (4) during the polarographic reduction of some nitrocompounds i n a c e t i c a c i d . But they dismissed the second wave without any explanation on the ground that they were not well formed i n most cases. The second waves, obtained i n the present study, however, are well formed and d i s t i n c t and merit further consideration. Haber, i n 1900, has shown1, that the mechanism of electro-reduction of the nitrocompounds i n acid medium was as follows: N0 2 N = 0 NHOH NH 8 0 * 0 TT 0 -ff 0 According to Haber, the reduction i n neutral or alk a l i n e medium i s given as follows: Presumably, the f i r s t stage of reduction i s a four-electron step giving the intermediate product,, azoxybenzene. It i s possible that the f i n a l product i s a n i l i n e a f t e r a s i x -electron reduction. As the reduction mechanism indicates, - 90 -some of the intermediate steps might be rate controlled, making the overall process i r r e v e r s i b l e . The 'n' of f r a c t -ional order, which i s p e r s i s t e n t l y seen i n a l l reductions can thus be explained. The intermediate compound i n the reduction of the nitrophenols might presumably be, In the case of ( i ) , the intermediate corresponding to the ortho-compound there i s the p o s s i b i l i t y of H-bond formation tending to hinder the removal of the oxygen, whereas, i n ( i i i ) , the para-compound the resonance makes the N almost neutral or even s l i g h t l y negative so that the oxygen could be very read i l y removed. Thus we f i n d that the para- i s more e a s i l y reduced than the ortho-compound. The i n t e r -mediate, corresponding to meta, ( i i ) i s of course the most e a s i l y reduced because i t has l i t t l e resonance so that N i s very p o s i t i v e . Hence, the ease of reduction of the n i t r o -compounds on the basis of the' above mechanism must be meta ) para ) ortho. We f i n d that the r e s u l t s bear out t h i s f a c t . The half-wave potentials corresponding to the o (iii) - 91 second stage of reduction are increasingly more negative i n the order meta )> para >^ ortho. This order, we f i n d i s d i f f e r e n t from the order ob-served i n the case of the aqueous buffers. Prom the re s u l t s of Astle and his co-workers (8) we f i n d that the order i s exactly opposite to that observed i n the f i r s t stage of reduction, i . e . , meta y ortho y para i n the case of reduct-ion. This could be explained on the basis of the formation of phenylhydroxylamine at the intermediate stage, as Haber's mechanism i n acid medium shows. Another i n t e r e s t i n g fact that emerges from a study of the reduction of nitrophenols i s the gradually developing maxima that are observed i n a l l cases. The height of the maximum increases with time and i t i s a l s o found that when the maxima have been formed, the half-wave potentials have a l l s h i f t e d to more negative values. Only i n the case of ortho-nitrophenol do we f i n d an exception—the half wave potential of the second wave remains the same whereas the f i r s t wave disappears almost completely giving place to a maximum. I t i s well known that i n the case of complex form-ati o n , the half-wave potentials generally s h i f t towards more negative values (17, p. 214 ). In a l l p r o b a b i l i t y , there i s complex formation between mercury and the reduction products i n the case of para- and meta- nitrophenols. It i s also well known that the height of a maximum depends on the concentration of the solution and increases with increasing concentration - 92 -(17, p.159). The f a c t that the height of the maxima i n the case of para- and meta- nitrophenols increases with time indicates that the complex formation must be a slow process. The growing height of the maxima, most probably re s u l t s from the gradual increase of concentration. I t i s most l i k e l y that the small waves preceding the reduction waves i n Figures 5, 9 and 13, are parts of the maxima, which are just beginning to form. In the case of ortho-nitrophenol, however, the h a l f -wave potential of the second wave i s not altered i n d i c a t i n g that i n a l l p r o b a b i l i t y , there i s no complex formation between the intermediate reduction product and mercury. However, i t i s possible that complex formation does occur i n the f i r s t stage; the change i n the hal f wave potential of the f i r s t wave i s rather obscured by the presence of the maximum. In conclusion, the r e s u l t s of this work are i n har-mony with previously established observations regarding the e f f e c t of the substituents on the ease of reduction of or-ganic compounds. The n i t r o a n i l i n e s show well defined double waves i n d i c a t i n g a two-step reduction and present no unusual features, whereas, the observations made with nitrophenols are compatible with complex formation of the reduction products with mercury. - 93 -BIBLIOGRAPHY 1. Arthur, P and Lyons, H. Anal. Chem. 24, 1422 (1952). 2. A s t l e , M.J. and McConnell, W.V. J. Am. Chem. Soc. 65, 35 (1943). 3. Bachman, G.B. and Astle , M.J. J. Am. Chem. Soc. 64, 1303 (1942). ~" 4. Bergman, I. and James, J.C. Trans. Paraday Soc. 48, 956 (1952). ~~ 5. Conant, J.B., Small, L.F., and Taylor, B.S. J. Am. Chem. Soc. 47,1959 (1945). 6. Cruse, K. Z. Electrochem. 46, 571, (1940. 7. Cruse, K., Goertz, E.P., and Petermuller, H. Z. Electrochem. 55, 5 (1951). 8. Dennis, S.F., Powell, A.S., and A s t l e , M.J. J. Am. Chem. Soc. 71, 1484, (1949). 9. E l l i o t t , N., and Yost, D.M. J. Am. Chem. Soc. 56, 1057, (1934). 10. Gosman, B. and Heyrovsky, J . Trans. Electrochem. Soc. 59, 249 (1931). 1 1 $ 11. H a l l , M.E. Anal. Chem. 22, 1137 (1950). 12. Heilbron, I. and Bunbury, H.M.,'Dictionary of Organic Compounds', Eyre and Spottiswoode, London (1946). 13. Heston, B.O., and H a l l , N.F. , J.Am. Chem. Soc. 56, 1462 (1934). 14. I l k o v i c , D. Col l e c t i o n Czechoslov. Chem. Communs. 6, 498, (1934) J . Chim. phys. 35, 129, (1938). 15. Ingold, C.K. 'Structure and Mechanism i n Organic Chem-i s t r y ' Cornell University Press, Ithaca, N.Y. (1953). 16. Janz, G.J., and Taniguchi, H. Chem. Revs. 53, 397 (1953). 17. Kolthoff, I.M., and Lingane, J . J . 'Polarography' Interscience Publishers, New York, London (1952). 94 -18. KMthoff, I.M., and Orlemann, E.F. J. Am. Chem. Soc. 63, 2085 (1941). 19. Kucera, G. Ann. Bhysik, 11, 529 (1903). 20. Laitinen, H.A. and Nyman, C.J. J. Am. Chem. Soc. 70, 2241, (1948). 21. Laitinen, H.A. and Shoemaker, C.E. J . Am. Chem. Soc. 72, 663, (1950). 22. Laitinen, H.A. and Shoemaker, C.E. J . Am. Chem. Soc. 72, 4975 (1950). 23. Laitinen, H.A. and Wawzonek, S. J . Am. Chem. Soc. 64, 1765 (1942). 24. Letaw, H. and Gropp, A.H. J. Phys. Chem. 57, 964 (1953). 25. Lewis, W.R., Quackenbush, F.M. and De V r i e s , T. Anal. Chem. 21, 762 (1949). 26. Lingane, J . J . J . Am. Chem. Soc. 61, 2099 (1939). 27. Lingane, J . J . and Loveridge, B.A. J. Am. Chem. Soc. 72, 438 (1950). 28. Lippmann, G. Pogg. Ann. 149, 547 (1873). 29. MacGillavry, D. Trans. Faraday Soc. 32, 1447 (1936). 30. MacGillavry, D. and Rideal, E.K. Rec. trav. chim. 56, 1013 (1937). 31. Meites, L. and Meites, T. Anal. Chem. 20, 984 (1948). 32. Mflller, O.H. J . Chem. Eds. 18, 172 (1941). 33. Mflller, O.H. J . Am. Chem. Soc. 66, 1019 (1944). 34. Nonhebel, G. and Hartley, G.S. P h i l . Mag (6) 50, 729 (1925) 35. Parks, T.D. and Hansen, K.A. Anal. Chem. 22, 1268 (1950). 36. Radin, N. and De Vries , T. Anal. Chem. 24, 971 (1952). 37. Runner, M.E. and Wagner, E.G. J . Am. Chem. Soc. 74, 2529 (1952). - 95 -38. Sanko, A.M. and Manussova, F.A. J. Gen. Chem. (U.S.S.R.) 10, 1171 (1946). 39. Sargent, J.W,, C l i f f o r d , A.F. and Lemmon, W.R. , Anal. Chem. 25, 1727 (1953). 40. Scherer, G.A. and Newton, R.F. J . Am. Chem. Soc. 56, 18 (1934). 41. Shikata, M. Trans. Faraday Soc. 19, 721 (1924). 42. Shikata, M. and Tachi, I. J . Chem. Soc. Japan 53, 834 (1932). C o l l e c t i o n Czechoslov. Chem. Communs. 6, 498 (1934). 43. Swan, S. and Bdelman, E.O. Trans. Am. Electrochem. Soc. 58, 179 (1930). 44. Uhlich, H. and Spiegel, G. Z. Physik. Chem. 177, 103 (1936). 45. Vlcek, A.A. Coll e c t i o n Czechoslov. Chem. Communs. Vol. 16, 2 3 0 - 3 8 , ( 1 9 5 1 ) . 46. Von Stackellsrerg, M. Z. E l e c t r ochem. 45, 466 (1939). 47. Wawzonek, S. and Runner, M.E. J. Blectrochem. Soc. 99, No. 11, •l±57-.9, (1952) . 48. Woolcock, J.W. and Hartley, H. P h i l . Mag. (7) 5, 1133 (1928). 49. Yoshida, T. J. Chem. Soc. (Japan) 48, 435-41 (1927). 50. Zimmermann, H.K. J r . and Gropp, A.H. J . Phy. Chem. 54, 764 (1950). 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
https://iiif.library.ubc.ca/presentation/dsp.831.1-0062449/manifest

Comment

Related Items