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

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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 i n the Department of Chemistry  We accept this thesis as conforming t o the standard required from candidates f o r the degree of MASTER OF SCIENCE  Members of the Department of Chemistry THE UNIVERSITY CF BRITISH COLUMBIA October, 1955  ABSTRACT  The p o s s i b i l i t y of using a calomel electrode as a reference standard i n the polarographic comparisons i n a c e t o n i t r i l e has been studied.  The r e s u l t s indicate that  the time required f o r a t t a i n i n g a constant p o t e n t i a l i s too long to be convenient  f o r such use.  The behaviour of some organic nitrocompounds at the dropping mercury electrode i n a c e t o n i t r i l e containing 0.1 molar tetrabutylammonium iodide has been studied.  The n i t r o a n i l i n e s and the nitrophenols give  d i s t i n c t double waves on polarographic reduction.  There  i s no formation of any maxima i n the case of the n i t r o a n i l i n e s , whereas, maxima are found to develop with time i n the case of the nitrophenols.  The maxima are found  to be non-suppressible. The ease of reduction of the nitrophenols and the n i t r o a n i l i n e s , as given by t h e i r half-wave p o t e n t i a l s , i s i n agreement with the order expected on the basis of the previously established theories of Shikata and A s t l e . A possible mechanism of the reduction i s given with the i r r e v e r s i b i l i t y of the reduction i n view.  ACKNOWLEDGMENT  The author wishes to express h i s gratitude to Dr. H.M. Daggett, J r . f o r h i s invaluable assistance and encouragement throughout the work and to the National Research Council of Canada f o r f i n a n c i a l assistance and equipment.  The author also wishes to thank the Standard  O i l Company of B r i t i s h Columbia f o r 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 r e s u l t s  9  DISCUSSION PART I I .  13 THE POLAROGRAPHIC BEHAVIOUR OF SOME ORGANIC COMPOUNDS  INTRODUCTION  15  General  15  Theoretical - General Concepts  17  Ilkovic Equation  19  Factors governing d i f f u s i o n current  21  Residual current  25  Maxima and suppression  28  Historical  32  EXPERIMENTAL  35  Apparatus and Materials  35  Procedure - ( i ) Characterisation of c a p i l l a r y . .  41  ( i i ) E l e c t r o c a p i l l a r y Curve  42  ( i i i ) Measurement of polarographic wave  43  Page RESULTS ( i ) Characterisation of c a p i l l a r y ( i i ) E l e c t r o c a p i l l a r y Curve ( i i i ) Residual current  48 48 48 50  ( i v ) Sodium Iodide  51  (v) o-nitrophenol  55  ( v i ) m-nitrophenol  62  ( v i i ) 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. II.  Variation of potential with time  11  Variation of potentials i n a solution saturated with calomel  12  PART I I . Table I. II. III. IV. V. VI. VII. VIII. IX.  Characterisation of the c a p i l l a r y  49  E f f e c t of potential on m ^ t /  50  1  6  Residual current  51  Measurements with sodium iodide solution . .  53  Reduction of o-nitrophenol  57  o-nitrophenol (after 36 hours' standing) . .  59  Reduction of m-nitrophenol solution  65  . . . .  m-nitrophenol (after standing 24 hours)  . .  66  Reduction of p-nitrophenol  69  p-nitrophenol (after 24 hours ' standing) . .  71  Reduction of o-nitroaniline  78  XII.  Reduction of p-nitroaniline  79  XIII.  Reduction of m-nitroaniline  84  X. XI.  ILLUSTRATIONS Figure 1.  Page E l e c t r o c a p i l l a r y curve of mercury i n acetonitrile  44  2.  Residual current curve  46  3.  Polarographic reduction wave of sodium ion (2.974 millimolar) i n a c e t o n i t r i l e . . . .  4.  Logarithmic plot of V i ^ - i vs. potential f o r the sodium ion  5.  52  54  Polarographic wave of o-nitrophenol (1.018 millimolar) i n a c e t o n i t r i l e  immediately  after solution  56  6.  Logarithmic plot corresponding to Figure 5.  7.  Polarographic wave of o-nitrophenol (1.018  58  millimolar) i n a c e t o n i t r i l e a f t e r standing f o r 36 hours  60  8.  Logarithmic plot corresponding to Figure 7 .  9.  Polarographic wave of m-nitrophenol millimolar) i n a c e t o n i t r i l e ,  61  (1.503  immediately  after solution  63  10.  Logarithmic plot corresponding to Figure 9.  11.  Polarographic wave of m-nitrophenol  64  (1.503  millimolar) i n a c e t o n i t r i l e a f t e r standing for 24 hours 12.  Logarithmic plot corresponding to Figure 11  67 68  Figure 13.  Page Polarographic wave of p-nitrophenol (3.1'97 millimolar) i n a c e t o n i t r i l e , immediately after solution  14.  70  Polarographic wave of p-nitrophenol (3.197 millimolar) i n a c e t o n i t r i l e , a f t e r standing f o r 24 hours  72  15.  Logarithmic plot corresponding to Figure 14. . .74  16.  Polarographic wave of o-nitroaniline (1.084 millimolar) i n a c e t o n i t r i l e  76  17.  Logarithmic plot corresponding to Figure 16. . .77  18.  Polarographic wave of p-nitroaniline (2.1 millimolar) i n a c e t o n i t r i l e  80  19.  Logarithmic plot corresponding to Figure 18. . . 82  20.  Polarographic wave of m-nitroaniline (1.062 millimolar) i n a c e t o n i t r i l e  21.  83  Logarithmic plot corresponding to Figure 20. . . 8 5  •  •  • •  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 f o r a very long time.  The use  of the standard hydrogen electrode i s not always  convenient,  and hence several subsidiary reference electrodes have been tested and accepted  f o r comparison. There have been several  of them i n use; the most extensively used being the calomel electrode and the s i l v e r - s i l v e r chloride electrode. l a t t e r has been so widely used that i t has acquired  The consid-  erable i n d i v i d u a l 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 i n aqueous solutions. There has been comparatively  l i t t l e work done i n  developing reference electrodes f o r use i n non-aqueous s o l vents.  The use of the aqueous hydrogen gas electrode and  the other aqueous reference electrodes i s not f e a s i b l e i n comparing the potentials i n non-aqueous media because of the uncertain l i q u i d - l i q u i d junction potentials involved.  The  - 2evaluation of the l a t t e r  i s not easy.  Moreover, where the  system i s t o be absolutely anhydrous, i t i s not desirable to use any such electrodes.  The need f o r r e l i a b l e 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 f o r a good reference electrode are ( 1 ) i ) The electrode must a t t a i n equilibrium quickly and show a stable standard potential f o r an i n d e f i n i t e length , of time. i i ) The p o t e n t i a l must not be altered by the passage of small currents of the order of 50-60 pa. f o r 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. i v ) The electrode must be non-polarisable.  The  r e l a t i o n between current and applied voltage must be a straight l i n e function over a range of zero to two v o l t s , i f i t i s -to be of use i n polarography. The p o t e n t i a l range i n the l a s t condition i s the normal polarographic range and f o r polarographic  comparisons,  i t i s necessary that the electrodes be non-polarisable i n that range of potentials.  The need f o r electrodes of this kind  i n polarography i s great.  The mercury pool reference that  i s often used i n the absence of r e l i a b l e reference  electrodes  has been found unsatisfactory i n several solvents (1,17,21).  :  - 3 The present attempt i s to investigate the possib i l i t y of using a calomel electrode i n a c e t o n i t r i l e solutions f o r polarographic  work.  - 4 Hist o r i c a l  Electrolytic been rather l i m i t e d .  studies i n non-aqueous media have With the development of organic  polarography, the investigations i n pure non-aqueous media have increased considerably.  But,  the work on the stab-  i l i t y of electrodes i n non-aqueous media i t s e l f has not received too much attention. A survey of the l i t e r a t u r e shows that the use of the s i l 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),  found that the electrode exhibited a stable and potential.  who  reproducible  Woolcock and Hartley (48) showed that the same  electrode exhibited good s t a b i l i t y and r e p r o d u c i b i l i t y i n anhydrous ethanol also.  Yoshida (49) showed that the  e.m.f.»s of the c e l l , Cd (amalgam) had a constant  Cdl  9 z  (Solid & Saturated) Solution )  Hg  Hg2"2 I 0  value i n methanol, ethanol, propyl alcohol,  and acetone and were almost independent of the solvent. Measurements made i n l i q u i d sulphur dioxide by Cruse demonstrated the r e l i a b i l i t y  (6)  of s i l v e r - s i l v e r chloride  electrodes as reference electrodes i n that solvent;  but,  he found that the e.m.f. of the mercury-mercurous chloride electrode d r i f t e d with time.  Scherer and Newton (40) i n -  vestigated the use of the magnesium electrode i n a saturated solution of magnesium bromide i n ether and found that there  -  was  5  -  a gradual increase or decrease in the potential while  the equilibrium was being attained, followed by a period of constancy l a s t i n g from 2 - 7 showed some i r r e g u l a r i t i e s .  days; thereafter, i t again  E l l i o t t and Yost (9) measured  the c e l l , T l (amalgam)  T1C1  ZnCl .10 NH (s) 2  3  Zn (amalgam)  i n l i q u i d ammonia and showed that either h a l f - c e l l could be used as a reference standard  i n that solvent.  The  chloranil  electrode has also been used as a reference standard e r a l workers.  by sev-  Heston and H a l l (13) have used i t i n g l a c i a l  acetic acid i n the course of the determination  of the act-  i v i t y of hydrogen chloride i n g l a c i a l acetic acid.  They  observed some i r r e g u l a r i t i e s in that medium, which they blamed  orr possible side reactions.  But, several l a t e r  workers, p r i n c i p a l l y , Swan and Edelman (43), Conant and co-workers (5), and Bergman and James (4), have found i t quite s a t i s f a c t o r y i n the same solvent.  More recently,  Arthur and Lyons (1) have studied two reference  electrodes  i n acetone—the acetone calomel electrode (A.C.E. ) and acetone-saturated polarographic  the  calomel electrode (A.S.C.E.) for use i n  comparisons.  They found that both of them  were stable, reproducible and  non-polarisable.  In a c e t o n i t r i l e i t s e l f , studies of several d i f f e r ent c e l l s have been made by Uhlich and Spiegel (44), the p r i n c i p a l ones being the s i l v e r - s i l v e r halide c e l l s and mercury-mercurous halide c e l l s .  the  They found that, even though  the s o l u b i l i t y of s i l v e r chloride and mercurous chloride i n  6 pure a c e t o n i t r i l e was n e g l i g i b l e , this was not the case i n a solution of lithium chloride i n a c e t o n i t r i l e .  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 complex formation alone was not s u f f i c i e n t reason f o r the f a i l u r e of the electrode, and that the s t a b i l i t y of the complex determines the s t a b i l i t y of the electrode.  This was  shown by the improvement of the potentials i n 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 i n agreement with the observed values, thus i n dicating p a r t i a l solvation e f f e c t s .  The l a t t e r , however,  x^ere found to be l e a s t i n the case of the calomel h a l f - c e l l . I t i s our purpose to see i f the calomel half  cell  shows a f a i r l y stable p o t e n t i a l , even though i t may be d i f ferent from the true thermodynamic potential and hence ascertain i f i t may not be suitable f o r use as a standard polarographic reference electrode.  - 7-  Experimental  Materials The solvent used was a c e t o n i t r i l e supplied by the Chemical D i v i s i o n of the United States Vanadium Corporation. It was p u r i f i e d before use by the method of Wawzonek and Runner (47), s l i g h t l y modified. stoppered  The solvent was kept i n a  container with anhydrous potassium carbonate  (B.D.H. chemical) f o r several hours.  I t was f i 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 f l a s k and a few crystals of s i l v e r n i t r a t e were added to i t .  I t was d i s -  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 b o i l i n g at 81.6°C was collected d i r e c t l y i n a solvent b o t t l e . A drying tube containing D r i e r i t e was always used to keep the d i s t i l l a t e  out of contact with external moisture.  The l i t h i u m chloride used was the Baker and Adamson reagent grade. 0.01  The r e q u i s i t e quantity of i t t o make a  molar solution i n a c e t o n i t r i l e was weighed out i n t o a  bottle containing the p u r i f i e d solvent, t i g h t l y  stoppered  and shaken i n a mechanical agitator f o r several hours. This 0.01  molar solution was used as the e l e c t r o l y t e s o l u t i o n . The calomel used was of reagent grade supplied by  the  Merck and Co.  used.  Throughout the study the same sample was  A paste of the calomel was made with t r i p l y  distilled  mercury i n an agate mortar and pestle together with a small  - 8quantity of the pure a c e t o n i t r i l e .  A tiny spoon made of  platinum was used to handle t h i s paste i n preparing the calomel electrodes. Apparatus The electrode vessel consisted of a 500 ml. capacity f l a t bottomed f l a s k provided with a central neck with a ground glass j o i n t of size  ^ 24/40 and s i x other  necks with ground glass j o i n t s , each of size arranged at the periphery.  $ 19/38  The central neck was f i t t e d  with a sintered glass bubbler projecting i n t o the centre of the vessel, through which nitrogen gas could be bubbled. The  other necks at the periphery were designed f o r the  electrodes. The electrodes were c y l i n d r i c a l glass tubes, about 18 cms. i n length f i t t e d with ground glass j o i n t s to f i t i n t o the necks of the electrode vessel.  A short piece  of platinum wire about 3 cms. i n length was fused i n t o the other end, so as to project p a r t l y out of the tube i n t o a small cup attached at that end.  The cup carried a small  hole i n i t to bring i t i n t o contact with the external solution.  E l e c t r i c a l connections  were made by means of  mercury i n contact with the platinum wire fused i n t o the tube. The p o t e n t i a l 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 s e n s i t i v i t y of 0.025 microampere per mm. was  used as the n u l l point detector.  division  Comparisons, could be  made between the different electrodes by a switching arrangement. 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 i n the electrode cups  and covering i t with a paste of mercurous chloride with mercury prepared as described before.  The paste was  by means of a tiny platinum spoon throughout. was  The  handled  electrode  then introduced i n t o the electrode vessel containing the  e l e c t r o l y t e solution. The e l e c t r o l y t e solution was a 0.01 of lithium chloride i n a c e t o n i t r i l e .  molar solution  The electrode vessel  containing this solution was kept immersed i n 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 i n t o i t . ing  No special precautions f o r p u r i f y -  the nitrogen were taken. A f t e r the gas has been passed  through the solution f o r about half an hour, some t r i p l y d i s t i l l e d mercury was poured i n t o the vessel to form a pool at the bottom.  The pool could be used as an electrode by  dipping i n t o i t a platinum electrode, s i m i l a r i n form to the other electrodes.  - 10 -  Measurements were made with electrodes ( T ) (2) prepared as above.  and  A straight platinum electrode (4)  was taken to serve as a reference electrode.  I t consisted  of a c y l i n d r i c a l glass tube, about the same length as the calomel electrodes, f i t t e d with a ground glass j o i n t at one end and a short length of platinum wire fused i n t o the other.  A platinum f o i l , about 4.5 sq. cms. i n area, was  attached to the platinum wire.  The platinum wire  was  previously cleaned thoroughly by e l e c t r o l y s i n g i n 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 ) d i f f e r e n t intervals of time.  and @  at  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 m i l l i v o l t s respect to the calomel electrodes.  m o r e ;  negative with  There was found to be  a rather haphazard variation with time but the calomels showed a tendency to a t t a i n the same p o t e n t i a l . end of 24 hours, ( T ) and (2) d i f f e r e d by 0.2 A new  calomel electrode (3) was  millivolt.  then prepared i n  the same manner as before and i t s potential was with the potentials of the two aged ones.  At the  compared  The new  one  found to be s l i g h t l y more negative than the aged ones. Measurements made at d i f f e r e n t intervals of time showed again an i r r e g u l a r variation of the p o t e n t i a l .  At the  was  - 11 -  end of 40 hours they were found to s e t t l e down to a constant potential and were uniformly 1.4 m i l l i v o l t s more positive than the platinum electrode.  There was observed no change i n the  potential f o r several hours a f t e r this was reached.  The  results are shown i n Table I.  (a)  (Electrodes prepared at the same time)  Time (Hours)  Measured potentials (volts) ©  vs.  +  ®  ©  +  vs. ©  ©  +  vs.  0.5 1.0 2.0 3.5 4.5 5.5 6.5  0.0009 0.0006 0.0012 0.0010 0.0007 0.0003 0.0002  0.0175 0.0046 0.0043 0.0042 0.0046 0.0042 0.0042  0.0166 0.0040 0.0031 0.0032 0.0039 0.0039 0.0040  24.0  0.0002  0.0026  0.0024  (b)  A f t e r the introduction of the newly prepared  calomel (3), the potentials were as follows: Measured potentials ( v o l t s )  Time (Hours)  %?• ®dr ®dr % vs  1.0 2.0 10.0 15.0 16.0 33.0 34.0 35.0 Note;  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  + sign indicates the electrode found positive to the other.  - 12 A set of new calomel electrodes was prepared i n the same manner as described  above to examine t h e i r behaviour i n  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  platinum wire fused'iinto i t was thrust into t h i s larger electrode and measurements of the potentials of the small (2) and (3) against the larger calomel electrode (6J were made as before.  The r e s u l t s showed that there was  a more systematic variation of the potential with a tendency f o r equalisation.  There was, at f i r s t , a f a s t decay i n the  potentials and l a t e r a slower decay.  After about 20 hours,  the potentials were a l l within a m i l l i v o l t of each other. The results i n Table II show the tendency of v a r i a t i o n . Table I I . Variation of potentials i n a solution saturated with calomel Measured potentials ( v o l t s )  Time(Hours)  7.0 8.5 17.5 18.5 20.0 22.5  0.0011 0.0009 0.0003 0.0001 0.0001 0.0001  0.0013 0.0014 0.0002 0.0000 0.0001 0.0001  0.0037 0.0030 0.0009 0.0007 0.0007 0.0006  0.0002 0.0005 0.0001 0.0000 0.0000 0.0000  0.0026 0.0021 0.0006 0.0006 0.0006 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 a c e t o n i t rile,  complex f o r m a t i o n i s the most important f a c t o r .  It is  a l s o c l e a r that s o l v a t i o n does have some e f f e c t , however small in  comparison.  effect  The  two f a c t o r s  operate s i d e by s i d e and  the  of the complex f o r m a t i o n i s t o a g r e a t e r or s m a l l e r  extent counteracted by the e f f e c t The  of s o l v a t i o n .  complex f o r m a t i o n tends t o d i s t u r b the d i s -  s o c i a t i o n e q u i l i b r i u m of mercurous c h l o r i d e i n the d i r e c t i o n of ing  the complex forming substance as i n d i c a t e d by the f o l l o w equations: HgCl  + CI" — •  HgCl " 2  ^  HgCl " 2  Hg  +  (complex  formation step)  + 2 CI""  T h i s goes on u n t i l the s a t u r a t i o n c o n c e n t r a t i o n of the comp l e x forming substance i s reached. of  There i s thus a v a r i a t i o n  the mercurous i o n c o n c e n t r a t i o n c a u s i n g the v a r i a t i o n of  p o t e n t i a l u n t i l the s a t u r a t i o n c o n c e n t r a t i o n of the complex is ion  reached, when there i s an e q u i l i b r i u m i n the mercurous c o n c e n t r a t i o n ; the e l e c t r o d e then shows a constant poten-  tial. The i n i t i a l ate  variation  of the p o t e n t i a l and  ultim-  e q u a l i s a t i o n observed i n the r e s u l t s i s , t h e r e f o r e , not  - 14 surprising. not  The  constant  value  of the p o t e n t i a l i s  the r i g h t thermodynamic p o t e n t i a l c a l c u l a t e d on  basis  of complex formation  alone,  still the  as a r e s u l t of the accom-  panying s o l v a t i o n e f f e c t . In c o n s i d e r i n g the f e a s i b i l i t y calomel e l e c t r o d e as a r e f e r e n c e on the b a s i s  of the  electrode  in a c e t o n i t r i l e ,  concluded t h a t the use  has  As a consequence, i t  of the  calomel e l e c t r o d e  a c e t o n i t r i l e i s f a r from s a t i s f a c t o r y .  the  the  that upwards of 50 hours are  r e q u i r e d f o r a t t a i n i n g that s t a t e .  polarographic  of  constancy i t a t t a i n s with time, one  t o take i n t o account the f a c t  must be  of the use  studies that followed,  Accordingly,  i t was  decided  in  i n the to  use  mercury p o o l 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 f o r a long time were limited to substances which are soluble i n 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 i n 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 investigations. Several factors deterred the early investigators from using non-aqueous solvents as polarographic media. In general, most of the non-aqueous solvents show f a r greater c e l l resistance than the aqueous solutions, so that the corrections f o r the IR-drop become imperative.  The d i f f u s i o n  c o e f f i c i e n t s of the ions i n most non-aqueous solvents are considerably lower than those i n aqueous solutions which makes the d i f f u s i o n currents f a r smaller compared to those i n aqueous solutions.  The non-aqueous solvents are generally  non-polar and i n most cases, only a limited number of substances could be employed as supporting e l e c t r o l y t e s , i . e . substances that are added to conduct the current.  Moreover,  - 16 the s o l u b i l i t y of oxygen i s considerably greater i n non-aqueous than i n aqueous medium which results i n undesirable i n t e r f e r i n g waves.  A l l these factors make the a n a l y t i c a l conditions not  too ideal and f o r a long time polarography  i n non-aqueous s o l -  vents suffered neglect. The advantages of developing non-aqueous solvents suitable f o r use i n polarography early.  was, however, r e a l i s e d quite  The foremost advantage i s , of course, the p o s s i b i l i t y  of studying a large number of organic reactions which could not be studied i n 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 nonaqueous solvents i n recent years.  Some of the solvents r e -  ported suitable i n l i t e r a t u r e , are methanol, ethanol, ethylene g l y c o l , a c e t i c acid, l i q u i d ammonia, acetone and formamide. These have been considered  'well behaved whereas other s o l 1  vents such as benzene, toluene, and a n i l i n e have been considered unsatisfactory because most of the inorganic e l e c t r o l y t e s are insoluble i n them while most organic e l e c t r o l y t e s have r e l a t i v e l y large resistances.  - 17 Polarography  General Concepts 'Polarography' i s e s s e n t i a l l y a study of the r e l a t i o n ship between the current  and applied voltage, when the r e l e -  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 p o t e n t i a l and i s characterised by a transfer of electrons, which produce a flow of current.  The  current then i s the r e s u l t of the electrode reaction and not i t s cause. When no p o l a r i s a t i o n i s taking place, the currentvoltage curve i s l i n e a r . In the present work, only polarographic processes  reduction  have been studied at the dropping mercury electrode.  However, oxidation reactions can a l s o be studied i n a s i m i l a r way.  In the following discussion, reference w i l l be made only  to polarographic  reductions.  In polarography, a r e l a t i v e l y large concentration of a non-reducible  e l e c t r o l y t e i s usually added to carry the  greater part of the current and prevent the e l e c t r i c a l migration of the reducible ions, which are present i n a very low concentration.  This i n d i f f e r e n t added e l e c t r o l y t e i s c a l l e d  the 'supporting e l e c t r o l y t e ' .  The transfer of the reducible  ions to the electrode when a large concentration  of ions of  the supporting e l e c t r o l y t e are present, takes place solely through d i f f u s i o n .  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 A g  +  ions i n an excess of  the ions of the supporting e l e c t r o l y t e , 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 i n the bulk of the solution.  This sets up  a concentration gradient and hence brings about a d i f f u s i o n of that ion from the bulk of the solution to the electrode. The d i f f u s i o n current that flows, arises as a r e s u l t of concentration p o l a r i s a t i o n .  It i s f a r greater i n magnitude than  the residual current, which i s the current that flows when no electro-reducible substance i s present i n s o l u t i o n .  The  process, therefore, becomes mainly d i f f u s i o n c o n t r o l l e d . If we consider the variation of the current with i n creasing potential we f i n d that, at f i r s t ,  ( i . e . , before  the  electrode attains the reduction potential of the reducible ion) the current i s wholly due to the e l e c t r i c a l p o t e n t i a l gradient; on increasing the p o t e n t i a l beyond the reduction p o t e n t i a l , the depletion of the concentration of the ions at the electrode surface starts and hence d i f f u s i o n of the ions takes place towards the electrode surface. the electrode surface.  The ions get reduced on reaching  The d i f f u s i o n current, then, i s d i r -  e c t l y proportional to the concentration gradient between the bulk of the solution and the electrode surface.  As the poten-  t i a l i s made more negative, reduction takes place at a f a s t e r rate, causing an increasing concentration gradient and hence,  - 19 -  the d i f f u s i o n current i s proportionately increased.  This  takes place u n t i l , at a p a r t i c u l a r p o t e n t i a l , the concentration of the ions at the electrode surface i s n e g l i g i b l y small, or v i r t u a l l y zero.  A further increase i n p o t e n t i a l does not  produce any further decrease i n the concentration and hence, the d i f f u s i o n current i s then proportional to the concentration in the bulk of the solution only. current r e s u l t s .  Thus, a constant  limiting  This represents v i r t u a l l y a state of complete  concentration p o l a r i s a t i o n .  The fact that the l i m i t i n g cur-  rent i s d i r e c t l y proportional to the concentration forms the basis of quantitative determinations  from current-voltage  curves. Polarography i s based on t h i s phenomenon of concentration p o l a r i s a t i o n .  A polarographic d i f f u s i o n current i s ,  as shown above, mainly d i f f u s i o n controlled and hence, the theoretical treatment of the d i f f u s i o n current i s based on the theories of d i f f u s i o n .  Since the electrode surface i n t h i s  case i s not stationary, but shows a periodic growth, suitable modifications have to be made i n deriving the d i f f u s i o n equation. Ilkovic equation The derivation of an expression f o r the d i f f u s i o n current was f i r s t made by I l k o v i c (14).  I t was l a t e r r e -  derived by MacGillavry and Rideal (30).  Ilkovic derived the  equation f o r the d i f f u s i o n current by employing the fundamental equation f o r a symmetrical  spherical d i f f u s i o n up to a  - 20 -  stationary surface: » C /  The ing  =  M  fy 2  D  •  T  2/r  «  /  J  s t a t i o n a r y co-ordinate r was r e p l a c e d by a mov-  co-ordinate  p  4/3 TT r where r r e p r e s e n t s  3  d e f i n e d by - 4/3 TT r  =  3 Q  4/3  TT p  3  the r a d i a l d i s t a n c e from a p o i n t i n the  s o l u t i o n t o the centre  of the drop and r  the r a d i u s of the o  drop a t any i n s t a n t .  The e x p r e s s i o n d e r i v e d thus f o r the  d i f f u s i o n c u r r e n t a t any i n s t a n t t i n the l i f e  of the drop i s  given by: i  +  = 0.732 nP  where F D  D* Cm / t 2  3  1  /  6  = Faraday i n coulombs. = Diffusion coefficient cm.  of the i o n i n  sec. .  2  - 1  3 C  = Concentration  m = t n  i n moles per cm. .  gms. per s e c . of mercury  •= time i n seconds. = Number of e l e c t r o n s i n v o l v e d i n the reduction.  The  constant  constants.  0.732 i s a combination I t i s more convenient  of s e v e r a l numerical  t o express  the c u r r e n t i n  microamperes, the c o n c e n t r a t i o n i n terms of m i l l i m o l e s per l i t r e , and m asrag.sec."" .  The e x p r e s s i o n then becomes,  1  i.  =706 nD^ Cm / t 2  3  1  /  6  -  21 -  The maximum current i n the l i f e t i s maximum.  of a drop i s given by i ^ . when  The average current i n the l i f e  given by performing  of a drop i s  the integration over the drop time and i s  given by I = 607 nD* C m / 2  3  t  ^  This i s referred to as the 'Ilkovic equation'. ing  d i f f u s i o n current, therefore, i  Thus,  = 607 nD* C m  d  i /Cm / 2  d  cular ion. m  For the l i m i t -  2/3  mg / 2  .f.l/6 3  v  t^ 1  6  t  l  /  = 607 nD  ¥  6  = Constant f o r any p a r t i -  I t i s only approximately so i n cases where the i  a  sec / ; 1  3  2/3  6  The  u  e  s  f o r two c a p i l l a r i e s d i f f e r by more than 0.5  otherwise they agree to within 2%. o r i g i n a l equation  of I l k o v i c has been subject t o  c r i t i c i s m because of certain omissions and oversimplifications (27) and corrections have been introduced. equations  have not been found completely  Even the corrected  s a t i s f a c t o r y from the  theoretical standpoint, even though they hold to a f a i r degree of accuracy i n p r a c t i c e .  Factors that govern d i f f u s i o n current  From a study  of the Ilkovic equation, one immediately  recognises the factors that govern the d i f f u s i o n current: a) The d i f f u s i o n current i s d i r e c t l y proportional to the concentration of the reducible ion. of a l l a n a l y t i c a l work i n polarography.  This forms the basis  - 22 b) The d i f f u s i o n c u r r e n t i s p r o p o r t i o n a l t o the square r o o t of the d i f f u s i o n c o e f f i c i e n t of the i o n . c) The d i f f u s i o n c u r r e n t i s p r o p o r t i o n a l t o m  2/3  t  l/6^  T  h  i  s  f  a  c  t  o  r  m  lary characteristics.  2/3 l / 6 t  i  g  d  e  p  e  n  d  e  n  t  o  n  Hence i t i s necessary  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  t  h  e  c  a  p  i i _  t o determine  the p o l a r o g r a p h i c i n -  v e s t i g a t i o n s are made. Capillary  characteristics  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 t o be of uniform  c i r c u l a r bore, the r a d i u s and the l e n g t h of the  c a p i l l a r y c o n s t i t u t e 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 u n c e r t a i n t y as r e gards the u n i f o r m i t y of the bore. l a r y Constant'" designated  In such cases,  "Capil-  by 'K' i s employed t o c h a r a c t e r i s e  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, m = ft r  4  c  we have,  d P  8lT)  where m = m i l l i g r a m s r  and  c  of mercury f l o w i n g per second  = r a d i u s of c a p i l l a r y i n cm.  d  = d e n s i t y of mercury i n gms./cm. .  1  = l e n g t h of c a p i l l a r y i n cm.  T|  = Coefficient  P  = ' E f f e c t i v e * pressure -2 dynes, cm.  of v i s c o s i t y  of mercury  on the dropping  mercury i n  23 When the mercury i s dropping  slowly from the c a p i l l a r y , the  e f f e c t i v e pressure P i s s m a l l e r than the t o t a l h y d r o s t a t i c pressure  of the mercury column because the i n t e r f a c i a l  ten-  s i o n a t the s u r f a c e of the drop e x e r t s a back p r e s s u r e . back pressure P_,.  =  has been shown by Kucera (19) t o be g i v e n by 2<r/r  where  H  o~ = I n t e r f a c i a l t e n s i o n i n dynes cm  and When expressed P  back  i  s  S  The  i v e n  r  -1  = r a d i u s of the drop i n cm.  d  i n terms of m and t , the average value of b  v  <  Average P  1 7  » P S a  e  8 1  =  b a c k  >  3.1/m / 1  3  t / 1  3  P i s t h e r e f o r e given by: p — -  P  _  T3  * applied  d  q '  From the P o i s e u i l l e ' s formula  -1  / 1/3 l / 3  1  /  +  m  t  above, we have,  P/m = 8T|l/flr d = K c • K ' can be used t o c h a r a c t e r i s e a l l kinds 4  of c a p i l l a r i e s ,  whether or not they are uniform. d) The i n f l u e n c e of the p o t e n t i a l of the dropping electrode  on the d i f f u s i o n c u r r e n t i s another f a c t o r of im-  portance . I t has been shown (28) that the i n t e r f a c i a l at a mercury-electrolyte potential  s o l u t i o n i n t e r f a c e depends on the  of the mercury.  p o t e n t i a l s a t the dropping i s found a t f i r s t  tension  On a p p l y i n g i n c r e a s i n g l y negative e l e c t r o d e , the i n t e r f a c i a l  tension  t o i n c r e a s e , pass through a maximum and  - 24 then s t e a d i l y  decrease.  maximum i s c a l l e d the  The  potential  'Electrocapillary  b o l i c curve obtained by p l o t t i n g the a g a i n s t the  potential  corresponding to Zero* and  the  the  para-  i n t e r f a c i a l tension  i s c a l l e d the  'Electrocapillary  Curve*  of mercury. The to the  drop time i s found to be  i n t e r f a c i a l t e n s i o n as mt  =  c  d i r e c t l y proportional  g i v e n by  the  expression,  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 v a r i e s of the  dropping e l e c t r o d e .  diffusion i n the  same way,  the  values of m ^ 2  of the  t /  3  cathodically  1  to the  effect  have shown the  i o n that the potentials  polarised  The  same  If  1  against  mercury, a p a r a maxima i n  potential.  Orlemann  (18)  method of making c o r r e c t i o n s  c o r r e c t i o n s are  independent of the  c a p i l l a r y f o r any  t ^ .  plotted  K b l t h o f f and  r e l a t i v e values of the  are  are  the  c u r r e n t i s s i g n i f i c a n t l y changed  of p o t e n t i a l ,  importance and  for this effect.  6  3  to the above i s obtained, the  diffusion  potential  a l r e a d y been seen  cases corresponding t o the Thus the  due  has  2  b o l i c curve s i m i l a r the  As  with the  c u r r e n t i s d i r e c t l y p r o p o r t i o n a l to m /  the p o t e n t i a l  all  0"  i n a s i m i l a r manner to  given supporting  based on the  observat-  drop time at two  characteristics electrolyte.  of  different the  - 25 -  e) The  effect  of temperature on the d i f f u s i o n  r e n t i n v o l v e s the v a r i a t i o n temperature, the v a r i a t i o n  of the d i f f u s i o n  important  of m.  i s the temperature c o e f f i c i e n t  coefficient.  The  coefficient  of d e n s i t y with temperature  f i n a l l y the temperature c o e f f i c i e n t  with  and  By f a r the most of the  magnitude of t h i s v a r i a t i o n  about 2% per degree f o r most of the i o n s .  cur-  diffusion  i s found to he  The  other  two  v a r i a t i o n s are found t o be so s m a l l that they could be neglected  (17 p. 92 ).  Residual The  current measured d i f f u s i o n  current i s usually greater  than the a c t u a l c u r r e n t because i t i n c l u d e s the r e s i d u a l rent.  R e s i d u a l c u r r e n t i s the c u r r e n t that flows when no  e l e c t r o r e d u c i b l e substance i s present is actually  the sum  of:  i n the s o l u t i o n .  :  side of the e l e c t r o c a p i l l a r y  charged at p o t e n t i a l s l e s s negative z e r o and negative c u r r e n t due  at more negative  zero.  It is positively  than the  electrocapillary  potentials.  2) a F a r a d a y i c  like  oxygen, e t c . (17, p. 151).  measured d i f f u s i o n c u r r e n t s must be c o r r e c t e d f o r the current.  on  t o the r e d u c t i o n of t r a c e s of r e d u c i b l e impur-  i t i e s i n the s o l u t i o n ,  ual  It  1) a non-Faradayic condenser c u r -  r e n t , a r i s i n g as a r e s u l t of the mercury being charged either  cur-  The resid-  - 26 Potential The  of tbe dropping e l e c t r o d e : p o t e n t i a l of the dropping e l e c t r o d e i s  given by, p d.e  - p " *  E  0-0591 ~  B  i n %  i Tprj  p  l0  where i = c u r r e n t a t any i n s t a n t i n microamperes, i^  = l i m i t i n g c u r r e n t i n microamperes,  n = number of e l e c t r o n s i n v o l v e d i n the r e d u c t i o n , and  Ex = h a l f wave p o t e n t i a l or the p o t e n t i a l i n v o l t s 2  when i = d/2. x  T h i s fundamental equation  of the p o l a r o g r a p h i c wave  shows that when the l o g a r i t h m  of j - ^ — * i s p l o t t e d against d " the p o t e n t i a l of the dropping e l e c t r o d e , the slope of the l i n e x  1  0 0591 would give the value  of • * ^  .  Prom the slope  of the l o g -  a r i t h m i c p l o t , t h e r e f o r e , one can get the value  of n, the  number of e l e c t r o n s i n v o l v e d i n the r e d u c t i o n .  S i n c e , a t the  h a l f wave p o t e n t i a l , i = &/2,log x  -—  T  = 0 so t h a t the  p o t e n t i a l corresponding t o the p o i n t on the l i n e where l o g -.— —v = 0, g i v e s the h a l f wave p o t e n t i a l . The h a l f wave d " 1  1  p o t e n t i a l i s constant ent  f o r any p a r t i c u l a r i o n and independ-  of the c o n c e n t r a t i o n The  i n reversible reductions.  half-wave p o t e n t i a l has a d e f i n i t e  dynamic s i g n i f i c a n c e (17, page 199, 2 6 , Ei where f  = E °  a  a f t e r reduction  4 6 ) . E i i s given by  + RT/nFy In a ^ - RT/nF  i s the a c t i v i t y c o e f f i c i e n t on the surface  i s the a c t i v i t y c o e f f i c i e n t  thermo-  y  In  ^  of the amalgam formed  of the mercury drops and f  of the r e d u c i b l e i o n a t the  - 27 surface  of the mercury drops, and  the a c t i v i t y  cury iinn the amalgam and E ° , the standard p o t e n t i a l a amalgam. Consider  Let  a  s a  ^^  amalgam.  of merof the  the standard p o t e n t i a l of the c e l l ,  K (s)  M (Hg) s a t u r a t e d .  be the a c t i v i t y  of the metal  i n the s a t u r a t e d  The e.m.f. of t h i s c e l l , E , i s independent of  the c o n c e n t r a t i o n of the metal E  s  ions i n s o l u t i o n  = ° a " ° M " RT/nFy In E  E  !§§*L Hg  a  where E ^ = standard p o t e n t i a l 0  a!!  = activity  of the pure metal and  of mercury i n the s a t u r a t e d amalgam  r e l a t i v e t o pure mercury E  °a  = s E  l n  +  E  °M -  R T  /  n F  y  l  n  C  satd. satd. f  R T  /  n F  y  *Hg  a  s u b s t i t u t i n g t h i s back a g a i n i n the e x p r e s s i o n f o r E ^ , we obtain  _  „o• = E M  1„p The  +  ,_ E s  , 0.0591 „ — log C n  +  s  a  f  f  t  d  <  s  M  ^  - —  0.0591  ****  l a s t f a c t o r i n most cases i s n e g l i g i b l e and so, E  r  E  s  +B° *2^sa K  i„g  csatd-fsatd-  Thus, the E i values may be c a l c u l a t e d from standard of  potential  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.  between observed  S a t i s f a c t o r y agreement i s obtained  and c a l c u l a t e d values  (17, p. 201).  - 28 -  In the case of i r r e v e r s i b l e reductions, E i may be 2  constant and independent of concentration  only i n a few cases  but generally not so. But the slope of the logarithmic plot w i l l d i f f e r from the r e v e r s i b l e value.  Since the rates of  oxidation and reduction are d i f f e r e n t , the two waves genera l l y do not coincide with each other and p o l a r i s a t i o n eff e c t s , i n addition to concentration p o l a r i s a t i o n come i n t o play.  The equation  of the wave then involves the rate constant  of the slow step i n the electrode reaction. The p o t e n t i a l of the dropping electrode i s always reported  negative. Corrections t o the half-wave p o t e n t i a l have to be  made i n the case of c e l l s with high i n t e r n a l resistance. The apparent half-wave p o t e n t i a l i s always larger than the true value as a r e s u l t 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 p o t e n t i a l (17, p.374). Maxima and suppression The appearance of the maxima i n 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 c a l l e d •Suppressors'.  They are usually c a p i l l a r y active substances,  charged c o l l o i d s or coloured dyes.  The maxima are observed  -  29  -  to be sometimes acute and sometimes rounded but always reproducible.  In the case of acute maxima, i t has been observed  that the potential often remains constant from the beginning of the discharge u n t i l the maximum i s reached.  The slope of  the l i n e , then i s found to be the reciprocal of the c e l l resistance.  The height i s 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 e l e c t r i c f i e l d around the drop (17, p. 168).  Two d i f f e r e n t theories were advanced—  one by Heyrovsky and the other by I l k o v i c to account f o r the inhomogeneous e l e c t r i c f i e l d .  But the recent work of Ant-  weiler has shown that there i s a streaming of the l i q u i d around the mercury drop on the part of the current-voltage curve showing the maximum.  At potentials, where the l i m i t -  ing current was seen, there was no streaming and a well defined d i f f u s i o n 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 i n case of 'positive' maxima and sideward streaming, i n case of 'negative' maxima—the positive and negative r e f e r r i n g to whether the maxima was at positive potentials of the E l e c t r o c a p i l l a r y Zero or on the side of negative potentials.  - 30 The  streaming,  according t o Antweiler,  e l e c t r o k i n e t i c phenomenon.  i s an  The e l e c t r i c a l double l a y e r s can  migrate under the i n f l u e n c e of p o t e n t i a l g r a d i e n t s .  Since i t  i s the l i q u i d - l i q u i d i n t e r f a c e t h a t vie are d e a l i n g with and s i n c e mercury i s a good conductor,  the double l a y e r can move  very e a s i l y even due t o a s l i g h t d i f f e r e n c e i n s u r f a c e  ten-  s i o n between the top and bottom of the drops. In the case of the p o s i t i v e maximum, the mercury p o o l 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 i o n , there i s a l a r g e c o n c e n t r a t i o n of the p o o l .  Since  of anions  attract-  on the s u r f a c e  there i s a downward streaming, the  p o t e n t i a l a t the bottom of the mercury drop must be p o s i t i v e and a t the top, n e g a t i v e .  T h i s i s because the c u r r e n t den-  s i t y a t the bottom of the mercury drop i s g r e a t e r than a t the top as a r e s u l t  of the bottom s u r f a c e b e i n g f r e e t o the  r e d u c i b l e i o n s , whereas the c a p i l l a r y t i p above e x e r c i s e s a s c r e e n i n g e f f e c t a t the top. In the case of the negative maximum, the mercury p o o l i s n e g a t i v e l y charged and hence there accumulates a greater concentration  of c a t i o n s a t i t s s u r f a c e .  Since the  bottom of the mercury drop i s p o s i t i v e with r e s p e c t t o the top, there i s a m i g r a t i o n tom  of the double l a y e r from the bot-  t o the top ( i . e . , from h i g h . c a t i o n c o n c e n t r a t i o n  cation concentration).  There i s thus a sideward  t o low  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 e f f e c t i v e l y by anions and the negative maxima, more e f f e c t i v e l y by cations. Again, the bivalent  ones are more effective than the univalent ones  and so on. The effect of c a p i l l a r y active ions l i k e acid dyes and negative c o l l o i d s i s to suppress the positive maxima while the basic dyes and positive negative maxima.  c o l l o i d s are e f f e c t i v e on  However, such i s not always the case, and  exceptions are known (17, p. 164). The p o l a r i z a b i l i t y of the ions, their s p e c i f i c a b i l i t y to be adsorbed at various potentials  on mercury  and their effect of s h i f t i n g the  e l e c t r o c a p i l l a r y zero are factors t o be reckoned.  - 32 Historical  The polarographic may  studies i n non-aqueous solvents  be said to have started with Shikata  investigated sodium  ethoxide:  (41) i n 1923,  i n alcohol.  The  when he  e a r l i e r studies  were made mostly with alcohols because they were s i m i l a r to water i n many respects i n addition to being good solvents f o r a number of organic and inorganic compounds.  Gosman and  Heyrovsky (10) employed a methyl alcohol medium f o r determining the c a p i l l a r y - a c t i v e substances i n petroleum. acid was  Acetic  f o r the f i r s t time t r i e d as a polarographic  MacGillavry  (29) with r e l a t i v e l y l i t t l e success.  medium by  However, more  successful results were obtained i n that solvent by Bachman and Astle (3).  Later, Bergman and James (4) studied a number  of organic compounds i n that solvent to investigate the e f f e c t of substituents  on the reduction potentials of substances.  The use of mixtures of solvents was  made by several d i f f e r e n t  workers using several d i f f e r e n t mixtures.  Thus, mixtures of  methanol and benzene, dioxane and glycerine, and g l y c o l and glycerine were made use of f o r studying n i t r o compounds by Radin and De Vries (36).  The n i t r o compounds were also  studied i n isobutyl alcohol and methanol by the same workers. Lewis, Quackenbush and De Vries (25) employed methanolbenzene mixtures f o r investigating oxygen-containing compounds l i k e ketones, aldehydes and peroxides. Wagner (37) investigated the polarographic  Runner and  reduction  of  - 33 -  ortho-, meta-, and i n absolute  para- a c e t a n i l i d e s  alcohol.  and  nitro anilines  To determine t e t r a e t h y l l e a d i n gaso-  l i n e , Parks and Hansen (35) employed ethylene g l y c o l ethers. ermination  H a l l (11)  of sulphur  amounts of d i s s o l v e d determination and  c a r r i e d out a p o l a r o g r a p h i c  i n petroleum.  He  i n s e v e r a l organic  formamide.  been used t i l l  vents s t u d i e d which do not first  Thus, most -  c o n t a i n any  0H~  Among the  groups are  employed by Sanko and  s e v e r a l i n o r g a n i c c a t i o n s and  hydes and ketones.  organic  then were a l l OH "-containing  I t has been more r e c e n t l y used by Letaw and studying  made by Sanko  s o l v e n t s l i k e meth-  s o l v e n t s , which set up a s e r i o u s l i m i t a t i o n .  amide, which was  the  oxygen i n the petroleum f r a c t i o n s . The  g l y c e r i n e and  s o l v e n t s that had  det-  a l s o determined  of s e v e r a l i n o r g a n i c c a t i o n s was  Manussova (38)  anol, ethanol,  g l y c o l and  form-  Manussova  (38).  Gropp (24) f o r  some organic  alde-  L i q u i d ammonia i s another solvent  has been employed with success f o r the determination metals by L a i t i n e n and Nyman (20).  sol-  Laitenen  and  which of  alkali  Shoemaker  (22) used the same s o l v e n t i n the polarography of t h a l l i u m , copper, and ammonium ions and acetone has  molecular oxygen.  been s t u d i e d by A r t h u r  t r a t e d s u l p h u r i c a c i d by V l c e k inorganic cations.  Lyons (1) and  Sargent, C l i f f o r d and  electrode.  concen-  (45) f o r the polarography of some  hydrogen f l u o r i d e as p o l a r o g r a p h i c r o t a t i n g platinum  and  Anhydrous  solvent  Lemmon (39) have used making use  of a  34 In a c e t o n i t r i l e i t s e l f there has been very work done. solvent  little  The only work r e p o r t e d i n l i t e r a t u r e u s i n g  this  has been t h a t of Wawzonek and Runner (47) who i n -  v e s t i g a t e d the p o l a r o g r a p h i c r e d u c t i o n of some i n o r g a n i c cations.  They r e p o r t e d the h a l f wave p o t e n t i a l s of some  i n o r g a n i c c a t i o n s and the d i f f u s i o n c u r r e n t s f o r a m i l l i m o l a r s o l u t i o n of each. and  They employed t e t r a b u t y l ammonium i o d i d e  tetrabutylammonium p e r c h l o r a t e s e p a r a t e l y as s u p p o r t i n g  e l e c t r o l y t e s and found t h a t both of them were in acetonitrile.  satisfactory  They r e f e r r e d the p o t e n t i a l s measured t o  the p o t e n t i a l of the mercury pool and found t h a t the maxima observed i n some cases could not be suppressed by any of the commonly used suppressors.  No organic compounds have been  r e p o r t e d s t u d i e d i n the l i t e r a t u r e t i l l The scope of the present  now.  r e s e a r c h has been t o study  the p o l a r o g r a p h i c behaviour of s o l u t i o n s i n a c e t o n i t r i l e and t o i n v e s t i g a t e the c a t h o d i c r e d u c t i o n compounds i n t h a t s o l v e n t .  of some organic  nitro-  35  -  Experimental  Apparatus and m a t e r i a l s The p o l a r o g r a p h i c c e l l  consisted  of an H-shaped  v e s s e l , the two limbs of which were connected t o 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  The two were p r o v i d e d with ground g l a s s j o i n t s $ 50/50 and $ 19/38  respectively.  2  cms.  of s i z e  The s m a l l e r limb  was  designed t o accommodate the r e f e r e n c e calomel e l e c t r o d e desc r i b e d i n s e c t i o n 1, i f necessary.  The l a r g e r limb  was  f i l l e d with a ground g l a s s top i n the centre of which  was  an open g l a s s tube j u s t wide enough f o r the c a p i l l a r y  end  of the dropping mercury  e l e c t r o d e t o be i n s e r t e d .  a l s o c a r r i e d a s i d e tube with a ground $ 10/30  and s t o p p e r , through which mercury  i n t o the v e s s e l . was  glass joint  A small gas e x i t  The top of s i z e  could be poured  tube, with a narrow bore,  a l s o p r o v i d e d i n the t o p , f o r the n i t r o g e n t o escape.,;;.  J o i n e d t o the l a r g e r limb almost a t the base, and  on op-  p o s i t e s i d e s of i t , were two narrow tubes bent a t r i g h t angles t o the v e s s e l .  One  of them was  through and i t c a r r i e d a t h r e e way  t o pass n i t r o g e n  stop-cock by means of  which n i t r o g e n c o u l d e i t h e r be bubbled through the s o l u t i o n or streamed  i n t o the v e s s e l over the s o l u t i o n .  Through  other tube, a l o n g p i e c e of p l a t i n u m wire could be  the  inserted  - 36 so as t o make contact with the mercury pool a t the bottom of the v e s s e l . The  dropping mercury e l e c t r o d e c o n s i s t e d of a l o n g  tube, about 80 cms. i n l e n g t h , connected t o a s h o r t e r and narrower g l a s s t u b i n g at the end of which there was a c a p i l l a r y tube, 4.7 cms. i n l e n g t h , provided  of very narrow bore.  at the bottom of the g l a s s tube was connected t o a  r e s e r v o i r of mercury by means of a p l a s t i c The  A s i d e tube  r e s e r v o i r was a g l a s s bulb s t a n d i n g  whose height  could be a d j u s t e d .  'Tygon' t u b i n g .  on a r i n g  A platinum  stand,  wire, fused i n t o  a g l a s s tube dipped under the mercury s u r f a c e and made the electrical  contact. The  voltage which was a p p l i e d t o the p o l a r o g r a p h i c  c e l l c o u l d be r e g u l a t e d by means of a r h e o s t a t and was approximately i n d i c a t e d by a voltmeter Any  i n the c i r c u i t .  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 a p p l i e d by  a d j u s t i n g two r e s i s t a n c e c o i l s i n s e r i e s — o n e  f o r coarse  adjustments and the other f o r f i n e adjustments.  The ap-  p l i e d p o t e n t i a l , however, could be a c c u r a t e l y 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 t o 1.6 v o l t s . standard  By s t a n d a r d i s i n g a g a i n s t a Weston  c e l l , a f t e r s e t t i n g the potentiometer at h a l f the  p o t e n t i a l of the standard from 0 t o 3.2 v o l t s .  c e l l , the range could be extended  The n u l l p o i n t d e t e c t o r i n the p o t e n t i o -  m e t r i c setup was a galvanometer s u p p l i e d by the Rubicon Co.  - 37 (Catalogue number 3002-SYS) w i t h a s e n s i t i v i t y ampere per mm.  of 1 micro-  division.  The  current measuring device  galvanometer provided  was  a calibrated  with an Ayrton shunt made of two  pre-  c i s i o n decade r e s i s t a n c e boxes s u p p l i e d by the General Radio Company.  The  galvanometer was  pany, P h i l a d e l p h i a , and and  a p e r i o d of 4.9  had  s u p p l i e d by the Rubicon Com-  a sensitivity  seconds.  of 0.0014 mm/  (aa.  By a d j u s t i n g 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 a d j u s t the maximum s e n s i t i v i t y  an i n t e g r a l  value  a l s o t o provide  resistance.  The  K o l t h o f f and  Lingane (17, p.  and  method of c a l i b r a t i o n  to  the proper damping was  t h a t given  by  301).  There were p r o v i s i o n s made i n the apparatus t o reverse  the p o l a r i t y  verse the  of the p o t e n t i a l a p p l i e d , and  current flow i n the galvanometer.  n a t i v e method of measuring the ing  standard  resistances  current was  of 1000  i n t o the  a f f o r d e d by  of the  measuring the IR-drop by means of the  By a p p r o p r i a t e  alter-  ohms, 10,000 ohms or  d e s i r e d e x t e r n a l r e s i s t a n c e , i n place and  An  to re-  havany  galvanometer  potentiometer.  switches these r e s i s t a n c e s could be  brought  circuit.  Materials The purified  solvent  as d e s c r i b e d  employed was in section  acetonitrile 1.  which  was  38 -  The the work was  s u p p o r t i n g e l e c t r o l y t e employed  throughout  tetrabutylammonium i o d i d e which had been used  p r e v i o u s l y i n the same s o l v e n t by Wawzonek and Runner and r e p o r t e d s u i t a b l e .  I t was  prepared  (47)  by a method given  by L a i t i n e n and Wawzonek (23), s l i g h t l y m o d i f i e d .  A mix-  t u r e of equal volumes of n - b u t y l i o d i d e and t r i b u t y l a m i n e were taken i n a b o t t l e and the a i r i n s i d e d i s p l a c e d by dry nitrogen.  I t was  f o r about 3 days. a little  t i g h t l y stoppered The  and kept  s o l i d t h a t separated was  I t was  washed with  f i l t e r e d and the f i l t r a t e  t i a l l y d i s t i l l e d under reduced separated  on c o o l i n g was  The r e c r y s t a l l i s a t i o n was  pressure.  The  of the c r y s t a l s was  par-  s o l i d that  r e c r y s t a l l i s e d from e t h y l a c e t a t e . repeated three times.  t a l s were then d r i e d i n a vacuum d e s i c c a t o r .  was  oven  e t h y l a c e t a t e and d i s s o l v e d i n the l e a s t amount  of c o l d e t h a n o l .  point  i n a hot  The c r y s -  The  found t o be 144-45°C.  melting The  solid  preserved i n a w e l l - s t o p p e r e d brown b o t t l e . The  substances  t h a t were i n v e s t i g a t e d p o l a r o -  g r a p h i c a l l y were sodium i o d i d e , ortho-, meta-, and n i t r o p h e n o l s and  ortho-, meta-, and para-  Sodium i o d i d e used was grade.  I t was  used without  nitroanilines.  the a n a l y t i c a l  reagent  further purification.  O r t h o - n i t r o p h e n o l and meta-nitrophenol products  para-  were both  of the Eastman Kodak Company and both were used  without f u r t h e r p u r i f i c a t i o n .  39 -  p a r a - n i t r o p h e n o l used was product.  the Eastman Kodak Co.  Since the marketed product i s supposed  t a i n a mixture the substance.  t o con-  of two forms (12) i t was necessary t o p u r i f y I t was  r e c r y s t a l l i s e d from t o l u e n e below  63°C t o obtain the y e l l o w needles of p - n i t r o p h e n o l , s t a b l e at room temperature  and somewhat s e n s i t i v e t o l i g h t . They  were p r e s e r v e d i n a b o t t l e wrapped round with aluminium  foil  i n dark u n t i l they were used. ortho- and p a r a - n i t r o a n i l i n e s were both p r o d u c t s 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. m e t a - n i t r o a n i l i n e was I t was  an Eastman Kodak Co. p r o d u c t .  r e c r y s t a l l i s e d from hot water t o o b t a i n orange-red  f l a k e s of m - n i t r o a n i l i n e . The n i t r o g e n used f o r degassing the e l e c t r o l y t e s o l u t i o n was  of premium grade.  of Meites and Meites (31). sulphate was  I t was  An 0.1  p u r i f i e d by the method  molar s o l u t i o n  prepared i n d i l u t e s u l p h u r i c a c i d and  50 m i l l i l i t r e s  of the s o l u t i o n was  first  about  p l a c e d i n each of two  gas washing b o t t l e s , each c o n t a i n i n g 100 gms. amalgamated z i n c .  of vanadyl  of l i g h t l y  The n i t r o g e n gas from the c y l i n d e r  was  bubbled through these and then through a wash b o t t l e  c o n t a i n i n g d i s t i l l e d water. through a d r y i n g tower packed  I t was with  then d r i e d by p a s s i n g 'Drierite . 1  I t was  passed through an empty b o t t l e t o d e p o s i t any dust  and  next  - 40 finally  i n t o another wash b o t t l e through a s i n t e r e d  tube d i p p i n g i n t o pure a c e t o n i t r i l e .  glass  T h i s l a s t wash b o t t l e  was designed t o e l i m i n a t e c o n c e n t r a t i o n changes i n t h e p o l a r o g r a p h i c v e s s e l due t o the vapours being c a r r i e d away by n i t r o g e n gas.  of the s o l v e n t  To prevent the uneven  v a p o u r i s a t i o n of the s o l v e n t due t o changes i n temperature,,, t h i s wash b o t t l e was maintained at the same temperature as the  polarographic c e l l . The s o l u t i o n  of the s u p p o r t i n g e l e c t r o l y t e  throughout the work was an 0.1 molar s o l u t i o n butylammonium i o d i d e i n a c e t o n i t r i l e .  employed  of t e t r a -  Only 250 ml. of the  s o l u t i o n was prepared each time and the s o l u t i o n was kept i n a w e l l stoppered b o t t l e . The mercury  used f o r both the dropping  e l e c t r o d e and the mercury i a l l y p u r i f i e d as f o l l o w s .  mercury  p o o l i n each experiment The commercial  o x i d i s e d u s i n g 4N n i t r i c a c i d .  mercury  was specwas  first  I t was thoroughly a g i t a t e d  i n c o n t a c t with n i t r i c a c i d by b u b b l i n g a i r through i t . I t was f i l t e r e d through a porous wooden f u n n e l under p r e s sure and washed thoroughly i n r u n n i n g water. d i s t i l l e d under reduced p r e s s u r e .  The d i s t i l l a t i o n was r e -  peated t h r i c e and the t r i p l e - d i s t i l l e d mercury a l l the experiments.  I t was then  was used i n  - 41 -  Procedure i ) C h a r a c t e r i s a t i o n of the  capillary:  As a l r e a d y d e s c r i b e d i n the i n t r o d u c t i o n , the 'Capillary Constant capillaries.  1  serves t o c h a r a c t e r i s e a l l kinds  I t r e p r e s e n t s the pressure r e q u i r e d t o f o r c e  1 m i l l i g r a m of mercury through the c a p i l l a r y per The  determination  made by the method of O.H. The graphic  joint and  of the c a p i l l a r y constant Muller  apparatus used was  T h i s s i d e tube was of s i z e  ip 24/40.  very s i m i l a r t o the  polaro-  through the centre  45°  p r o v i d e d with a ground g l a s s  of the cap, a g l a s s rod having a could be moved up and  e x a c t l y below the dropping  removed and  of about  A ground g l a s s cap f i t t e d i n t o i t  F i r s t l y , the top  mercury or outside i t .  the mercury allowed  vessel  was  t o drop f r e e l y i n a i r .  c o l l e c t e d i n the spoon the weight.  time r e q u i r e d f o r the drops t o form was  determined by  p r e c i s i o n stop watch r e a d i n g t o 0.1  second.  A  and  t r a n s f e r r e d t o a beaker f o r determining  peated with d i f f e r e n t heights  tiny  down, so t h a t i t  of the p o l a r o g r a p h i c  d e f i n i t e number of drops was  The  was  (32,33).  t o the l a r g e r limb, at an angle  g l a s s spoon at i t s end was  second.  c e l l a l r e a d y d e s c r i b e d , except that i t had a s i d e  tube attached to i t .  of  T h i s was  The a re-  of mercury i n the r e s e r v o i r .  r e s u l t s are t a b u l a t e d i n Table  I.  - 42  Next an 0.1 molar s o l u t i o n of potassium c h l o r i d e i n water was p l a c e d i n the p o l a r o g r a p h i c v e s s e l and the c a p i l l a r y dipped i n t o i t . the  With the c a p i l l a r y dropping i n  s o l u t i o n , the same procedure was  repeated again a t d i f -  f e r e n t h e i g h t s of the mercury r e s e r v o i r .  The r e s u l t s are  shown i n Table I. [ i t was n o t i c e d d u r i n g t h i s experiment the  that i f  c a p i l l a r y i s allowed t o stand above a c e t o n i t r i l e , the  drop time observed w i l l not correspond t o that i n a i r any more.  The a c e t o n i t r i l e vapour seems t o a f f e c t the s u r f a c e  t e n s i o n of mercury  so t h a t the drop times are c o n s i d e r a b l y  smaller. ] ii) For  Electrocapillary  Curve:  d e t e r m i n i n g the v a r i a t i o n  of drop time with  a p p l i e d p o t e n t i a l , the apparatus used was  the s p e c i a l  p o l a r o g r a p h i c c e l l d e s c r i b e d i n s e c t i o n i ) on the c h a r a c t erisation  of the c a p i l l a r y .  A 0.1 molar s o l u t i o n  of the  s u p p o r t i n g e l e c t r o l y t e was p l a c e d i n the p o l a r o g r a p h i c v e s s e l over some mercury s e r v i n g as the p o o l anode. dropping mercury e l e c t r o d e was  The  lowered i n t o i t and the  height of the mercury r e s e r v o i r a d j u s t e d t o a convenient height.  The mercury p o o l anode and the dropping mercury  cathode were then s h o r t e d .  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  c o l l e c t e d i n the spoon  t r a n s f e r r e d t o beaker, washed, d r i e d and weighed.  The  and time  - 43 -  r e q u i r e d f o r the formation determined  of the drops was accurately-  by means of a p r e c i s i o n stop watch.  The  drop  time and the number of m i l l i g r a m s of mercury p a s s i n g the c a p i l l a r y per second (m) were c a l c u l a t e d .  through  Increasing  negative p o t e n t i a l s were a p p l i e d t o the dropping mercury e l e c t r o d e and the corresponding drop times and the m v a l v e s were determined. at  The  values of m ^ 2  each a p p l i e d p o t e n t i a l .  The  ions are shown i n Table I I . and m / 2  The  t /  3  1  6  The  3  t / 1  6  were  r e s u l t s of the values  determined determinat-  of the drop  times  have been p l o t t e d a g a i n s t p o t e n t i a l i n F i g . 1.  curve r e p r e s e n t s 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 s o l u t i o n used and the curve corresponds  t o the  the maximum i n  'Electrocapillary  Zero'.  i i i ) Measurement of the p o l a r o g r a p h i c wave: A known volume of the s u p p o r t i n g e l e c t r o l y t e s o l u t i o n , about 50  or 60 ml. was  a c c u r a t e l y measured out  i n t o the p o l a r o g r a p h i c c e l l and the v e s s e l i n t r o d u c e d i n t o the constant temperature bath maintained 25 - 0.2°C. bubbled  The n i t r o g e n gas, p u r i f i e d as d e s c r i b e d ,  was  i n t o the e l e c t r o l y t e at a steady r a t e f o r about  h a l f an hour b e f o r e every d e t e r m i n a t i o n . addition for  at  of the t r i p l y  the pool anode was  distilled  As a r u l e ,  mercury i n t o the  the  cell  made a f t e r degassing f o r at l e a s t  h a l f an hour t o guard a g a i n s t the p o s s i b i l i t y  of complex  formation  suggested  of mercury with the i o n s and  by Arthur and Lyons ( 1 ) .  N i t r o g e n was  oxygen  again bubbled f o r  - 44 -  -  45  -  another h a l f an hour t o secure v i r t u a l removal of a l l d i s solved  oxygen from the  of n i t r o g e n  was  electrolyte solution.  then d i r e c t e d  The  stream  over s o l u t i o n by means of  the  two-way stopcock, before the measurements were begun.  The  dropping mercury e l e c t r o d e  p o l a r o g r a p h i c v e s s e l so t h a t  was  then lowered i n t o  the t i p of the  as near the p o o l of mercury as p o s s i b l e . of mercury was  then a d j u s t e d so t h a t the  was  oms.  exactly  30  The cury p o o l . the  r e s i d u a l current  galvanometer at g r a d u a l l y  sensitivity  The  reservoir  height  of the  was  of mercury  measured by means of  increasing potentials.  galvanometer was  applied  the  currents  p o t e n t i o m e t r i c setup d e s c r i b e d  was  then p l o t t e d as a f u n c t i o n  the r e s i d u a l current  already.  measured  The  of p o t e n t i a l .  curve of the  at  galvanometer  p o t e n t i a l s were a c c u r a t e l y  the  The  a d j u s t e d by means of  each a p p l i e d p o t e n t i a l were read off on the The  was  were r e f e r r e d t o the mer-  the Ayrton shunt to a d e f i n i t e value and  scale.  capillary  from the t i p of the c a p i l l a r y .  potentials applied  The  the  by  current  F i g . 2 shows  electrolyte solution  used. In the d e t e r m i n a t i o n of the p o l a r o g r a p h i c r e d u c t i o n waves of any v e s t i g a t i o n was correct  t o 0.1  substance, the  substance under i n -  taken i n a s m a l l weighing piggy and mg.  I t was  c a r e f u l l y tipped  volume of e l e c t r o l y t e s o l u t i o n p r e v i o u s l y described  above.  The  weighed  i n t o the known  degassed  amount of substance thus  as  transferred  Figure 2.  Residual current curve  - 47 was  such that an approximately millimolar solution was  obtained.  thus  Nitrogen gas was again bubbled f o r about 10  minutes to ensure thorough mixing. Measurements were again made of the current with gradually increasing potentials.  The potential increase  was  done i n steps of 0.1 volt except i n the region of sudden increase i n the current.  In t h i s region of r i s i n g current,  the potentials were increased 0.02 step and the corresponding  v or 0.05  volt at each  increase in the current measured.  Experiments were repeated with at least three d i f ferent concentrations i n each case.  Typical measurements  have been shown f o r every compound studied. wave potentials of the solutions at different were within - 0.01  volt of each other.  The h a l f concentration  The values of the  slopes of the logarithmic plot remained constant - 0.005 v.  within  48  Results  The  p o l a r o g r a p h i c c a p i l l a r y made use of through-  out was 4.7 cms. l o n g and had a drop time of 4.24 seconds with the mercury pool and the dropping mercury e l e c t r o d e shorted.  The height  of the mercury was always  maintained  at 30 cms. throughout the measurements. i ) The r e s u l t s of the c h a r a c t e r i s a t i o n 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  mean of these two average v a l u e s . According  to Kuller,  as the  K =  22.79  < =  2.1567 x 1 0 ~ *  l/r  4 0  1 0  .  Hence, the r a d i u s of the c a p i l l a r y could be given by r  Whence  4 _ c ~  r  c  =  2.1567 X 1 0 " 22.79 0.002583  of m ^ 2  3  t ^ l y  x 4.7  cm.  i i ) The experimental the v a r i a t i o n  1 0  6  results  obtained  i n studying  with p o t e n t i a l were as shown  i n Table I I . F i g u r e 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 i o d i d e s o l u t i o n in  acetonitrile. The  maximum value  of m ^ 2  3  t ^ 1  6  occurs a t a  p o t e n t i a l of -0.18 v when the drop time i s l o n g e s t .  The  -  Table I .  49 -  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 a i r : p  app (cm)  40.0 50.0 60.0 70.0  t (sec) 5.15 41.6 34.8 30.0  W (mg) 90.26 90.98 91.04 91.00  m (mg/ sec) 1. 753 2. 187 2. 616 3. 033  Average  app  w  22.82 22.87 22.94 23.08 =  l/3  P  4.485 4.498 4.499 4.498  baclc 0.5 0.5 0.5 0.5  P  39.5 49.5 59.5 69.5  22. 54 22. 63 22. 75 22. 92  22.71  b) C a p i l l a r y dropping i n 0.1 molar potassium chloride P app (cm) 30.0 40.0 50.0 60.0 70.0  solution:  t (sec)  W (mg)  m (mg/ sec)  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  app  w  23.40 23.43 23.54 23.47 23.31 23.31 23.37 23.36 23.52 23.45  l/3  1.793 1.719 1.813 1.769 1.839 1.802 1.846 1.833 1.848 1.829  P  back 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3  P  28.7 38.7 48.7 58.7 68.7  Observations made d u r i n g the l o w e r i n g of mercury Average  K =22.87  22.39 22.37 23.17 22.72 22.70 22.64 22.81 22.88 23.08 23.02  —  Table E (volts)  0.000 0.100 0.150 0.180 0.200 0.400 0.500 0.700 0.900 1.100 1.300 1.500  II.  Effect  t (seconds) 5 drops  1 drop  21.2 21.7 21.8 21.9 21.7 21.4 20.8 19.6 18.0 16.4 14.8 13.0  4.24 4.34 4.36 4.38 4.34 4.28 4.16 3.92 3.60 3.28 2. 96 2. 60  50  of p o t e n t i a l on m / 2  t / 1  m  0.1465 0.1449 0.1458 0.1467 0.1449 0.1409 0.1413 0.1391 0.1409 0.1422 0.1436 0.1474  0.0976 0.0966 0.0972 0.0978 0.0968 0.0939 0.0942 0.0927 0.0939 0.0948 0.0957 0.0983  0.1046 0.1062 0.1066 0.1069 0.1062 0.1052 0.1021 0.0989 0.0927 0.0860 0.0785 0.0692  e l e c t r o c a p i l l a r y z e r o f o r the system t h e r e f o r e t o -0.18 v o l t . value  6  2/3 l o g 1/6 l o g 2/3 ?l/6 m t  W log m (rag)  29.7 30.3 30.5 30.7 30.3 29.6 28.8 27.0 24.9 22.75 20.60 18.25  3  1. 593 1. 595 1. 599 1. 602 1. 595 1. 581 1. 571 1. 555 1. 537 1. 516 1.493 1. 471  corresponds  T h i s i s i n agreement with the r e p o r t e d  of Wawzonek and Runner ( 4 7 ) . i i i ) Residual The  current:  r e s i d u a l current at gradually i n c r e a s i n g  p o t e n t i a l s measured i n a i r - f r e e s o l u t i o n of the s u p p o r t i n g e l e c t r o l y t e s o l u t i o n were as shown i n Table I I I . Figure  2 shows t h e r e s i d u a l current curve.  The  s m a l l wave o c c u r r i n g i n the r e s i d u a l current curve could not  be e l i m i n a t e d i n s p i t e of repeated  of the s u p p o r t i n g  electrolyte.  recrystallisations  Hence i n the f o l l o w i n g  measurements, c o r r e c t i o n s have been made i n the d i f f u s i o n currents  accordingly.  - 51 -  Table I I I .  Residual  - Potential (volts)  current  Current  0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 1.100 1.200 1.300 1.400 1.500 1.600 1.700 1.800 1.900 2.000  (microamperes)  -0.09 0.32 0.78 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.90 1.05 1.15 1.30 1.40 1.50 1.60 1.70 1.90 2.60 3.40  i v ) Sodium i o d i d e : P r e l i m i n a r y t o the study of the nitrocompounds, a study of the e l e c t r o r e d u c t i o n a c e t o n i t r i l e was made. been given below.  of the sodium i o n i n  A t y p i c a l s e t of measurements has  The c o n c e n t r a t i o n  of the s o l u t i o n s t u d i e d  was 2.974 m i l l i m o l a r . The  c o r r e c t e d values  the p o t e n t i a l i n F i g u r e  3.  have been p l o t t e d a g a i n s t  We f i n d there  i s a w e l l de-  f i n e d wave whose half-wave p o t e n t i a l corresponds t o -1.28  - 0.01 v o l t s . The  r e s i s t a n c e of the c e l l was measured by means  of an audio-conductance bridge  and was found t o be of the  -128  F i g u r e 3.  P o l a r o g r a p h i c r e d u c t i o n wave of sodium i o n (2.974 m i l l i m o l a r ) i n a c e t o n i t r i l e  - 53 -  Table  IV.  Measurements with sodium i o d i d e s o l u t i o n  (2.974 millimolar) -E  (volts)  i (observed) (microamperes) -0.09 0.32 0.78 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.90 1.05 1.58 3.30 5.50 7.0 8.3 9.3 10.0 10.1 10.2 10.2  0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 1.100 1.200 1.250 1.280 1.300 1.320 1.350 1.400 1.500 1.600 1.700  order  i ( c o r r e c t e d ) microamperes  of 500 ohms.  0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.43 2.10 4.30 5.7 7.0 8.0 8.6 8.6 8.6 8.5  The IR-drop at the h a l f wave, c a l c u l a t e d  on t h a t b a s i s comes out t o be 0.002 v o l t .  The magnitude of  t h i s c o r r e c t i o n i s very s m a l l and hence i t may be n e g l e c t e d . The  constant  1  d/ 2/3 l/6 e m  t  was found t o be  1.96 - 0.0% F i g u r e 4 shows the l o g a r i t h m i c p l o t against  the p o t e n t i a l a p p l i e d .  having a slope  1  I t gives a s t r a i g h t  of 0.062 v. constant  T h i s i n d i c a t e s a one-electron  of / . line  t o w i t h i n - 0.002 v.  r e d u c t i o n , as expected.  •0-80  .0-401-  0*0  -0  80  -100  Figure 4.  Logarithmic plot of ~ T o r sodium ion  vs. potential —SAIIII  II  - 55 -  Nitrophenols The  n i t r o p h e n o l s show some abnormal behaviour  e l e c t r o l y t i c r e d u c t i o n ; a l l of them show d e f i n i t e r e d u c t i o n - - i n some the waves are w e l l separated o t h e r s , r a t h e r c l o s e l y spaced. there i s formation  stepwise  and  In a l l cases, we  on  in  find  that  of maxima, on a l l o w i n g the s o l u t i o n s t o  stand i n contact with mercury f o r some hours. are a l s o found t o develop deep c o l o u r s .  The  solutions  The maxima are  formed on the p o s i t i v e s i d e of the e l e c t r o c a p i l l a r y z e r o i n all  cases and  suppressors,  cannot be suppressed by the commonly used such as  a- Naphthol,  | 3 - Naphthol, f u c h s i n e ,  and methyl r e d . v)  o-nitrophenol:  Table V shows the r e s u l t s of measurements made with a 1.018 represents  m i l l i m o l a r s o l u t i o n of o-nitrophenol  and  the t y p i c a l behaviour of t h a t Substance  electrolytic  during  reduction.  F i g u r e 5 shows the p o l a r o g r a p h i c waves of o-nitrophenol. t o the two  There are two  stages  d i s t i n c t waves  of r e d u c t i o n .  corresponding  There i s a l s o observed a  s m a l l wave about 1 microampere i n height preceding two waves. are -0.38 The  The - 0.01  the  h a l f wave p o t e n t i a l s of the two waves v o l t and  -1.38  - 0.01  volt respectively.  c o r r e c t i o n f o r the IR-drop i n t h i s and  the f o l l o w i n g  cases have been n e g l e c t e d because of t h e i r s m a l l magnitude.  Figure 5.  o-nitrophenol (1.018 millimolar) in aceJfcVonitrile, immediately after solution  Polarographic wave of  -  Table V.  57 -  Reduction of o-nitrophenol (1.018 millimolar)  -E ( v o l t s )  Current  (Observed)  Current ( c o r r e c t e d )  (microamperes) 0.000 0.100 0.200 0.300 0.340 0.380 0.400 0.440 0.480 0.500 0.540 0.580 0.600 0.700 0.800 0.900 1.000 1.100 1.200 1.240 1.280 1.300 1.340 1.380 1.400 1.440 1.480 1.500 1.540 1.600 1.640 1.700 1.800  -0.09 1.22 1.78 2.10 2.70 3.90 4.60 5.20 5.40 5.60 5.70 5.70 5.70 5.70 5.70 5.70 5.70 5.85 6.95 8.55 10.5 11.5 14.2 16.7 17.7 20.8 22.8 23.6 24.9 25.8 26.6 26.6 26.6  F i g u r e 6 shows the l o g a r i t h m i c p l o t a g a i n s t the p o t e n t i a l .  0.00 0.90 1.00 1.30 1.90 3.10 3.80 4.40 4.60 4.80 4.90 4.90 4.90 4.90 4.90 4.90 4.80 4.80 5.80 7.30 9.2 10.2 12.8 15.3 16.3 19.3 21.3 22.1 23.3 24.2 24.9 24.9 24.7  of -.  The slope of the s t r a i g h t  corresponding t o the f i r s t  line  wave i s found t o be 0.086  ± 0.005 v and t h a t corresponding t o the second wave 0.154 * 0.005 v.  The values of n c a l c u l a t e d a c c o r d i n g  to these s l o p e s a r e 0.73 and 0.38 r e s p e c t i v e l y .  yjffure 6.  L o g a r i t h m i c p l o t corresponding t o f i g u r e  5  - 59  In f i n d i n g the d i f f u s i o n current f o r the second wave, corrections were made f o r the change of drop time with potential by taking the. r a t i o of t i ^ A j ^ s  b  y  t  h  e  m e t n o d  of Kolthoff and Orlemann (18). On allowing the solution to stand i n contact with mercury f o r 36 hours, the solution changed colour from yellow to orange.  Measurements were again made a f t e r 36 hours,  a f t e r further degassing and the results are shown i n Table VI. Table VI. -E volts  o-nitrophenol (after 36 hours' standing) (1.018 millimolar) i (observed) i (corrected) Microamperes  0.020 0.040 0.060 0.080 0.100 0.140 0.180 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 1.100 1.200 1.300 1.400 1.500 1.600 1.700 1.800  14.5 30.8 40.4 47.6 51.9 44.5 5.35 5.40 5.50 5.60 5.60 5.60 5.60 5.60 5.60 5.60 6.05 8.45 12.7 18.6 24.9 28.7 29.5 29.7  14.4 30.6 40.2 47.3 51.6 44.0 4.70 4.60 4.70 4.80 4.80 4.80 4.80 4.80 4.80 4.70 5.00 7.30 11.4 17.2 23.4 27.1 27.8 27.8  cn  o  -20  Figure 7.  Polarographic wave of o-nitrophenol (1.018 millimolar) i n a c e t ^ o n i t r i l e , a f t e r standing f o r 36 hours  volts  Figure 8.  Logarithmic plot corresponding to figure 7  - 62 -  Figure 7 shows these values plotted.  I t was found  that there was a d i s t i n c t maximum which could not be suppressed by the commonly used suppressors fuchsine, and methyl red.  like  oc-Naphthol,  Most of the inorganic  |3-Naphthol,  suppressors  and gelatin were unsuitable because of i n s o l u b i l i t y i n the e l e c t r o l y t e solution.  I t was noticed that the half wave poten-  t i a l of the second wave was unaffected i n spite of the disappearance of the f i r s t wave and the formation  of the maximum.  The logarithmic plot i s shown i n Figure 8. The slope of this l i n e was found to be 0.204 - 0.005 and the corresponding  value of n was 0.29.  This was d i s t i n c t l y d i f f e r e n t  from the o r i g i n a l slope and the corresponding n.  The results  indicate an i r r e v e r s i b l e reduction (17, p. 266). vi)  m-nitrophenol:  Table VII contains a t y p i c a l set of r e s u l t s  obtained  with a 1.503 millimolar solution of m-nitrophenol i n the supporting e l e c t r o l y t e solution. Figure 9 shows the polarographic waves of m-nitrophenol. It i s found that there are two closely-spaced waves preceded by a tiny wave about 0.4 |*a. high.  The half wave p o t e n t i a l s , 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 d / 2 / 3 l / 6 f o r the f i r s t wave was 1  C m  t  2.82 - 0.05 and that for the second, a f t e r correcting f o r the  o  F i g u r e 9.  P o l a r o g r a p h i c wave of m-nitrophenol i n a c e t f o n i t r i l e , immediately a f t e r  (1.503 m i l l i m o l a r ) solution""  Figure 10.  Logarithmic plot corresponding to figure 9  - 65 -  Table VII. -E (volts)  Reduction of m-nitrophenol (1.503 milllmolarl i (observed)  solution i (corrected)  microamperes 0.000 0.100 0.200 0.300 0.400 0.500 0.550 0.600 0.650 0.700 0.750 0.800 0.850 0.900 1.000 1.100 1.200 1.300 1.400  -0.09 0.42 1.18 1.20 1.30 2.60 3.90 5.50 6.60 7.10 7.40 8.50 9.40 11.2 11.3 11.5 11.6 11.7 11.7  0.00 0.10 0.40 0.40 0.50 1.80 3.10 4.70 5.80 6.30 6.60 7.70 8.60 10.4 10.4 10.45 10.45 10.4 10.3  change i n drop time with potential was 1.76 - 0.05. Figure 10 shows the logarithmic plot of against p o t e n t i a l .  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 i r r e v e r s i b l e reductions. On allowing the solution to stand overnight i n contact with the mercury pool, the solution turned orange in colour. 24 hours.  Measurements were again made a f t e r a period of 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 0.050 0.075 0.100 0.150 0.200 0.300 0.400 0.500 0.600 0.620 0.650 0.680 0.700 0.800 0.850 0.900 0.950 1.000 1.100 1.200 1.300  9.3 18.7 25.1 22.1 5.10 4.50 4.60 4.60 4.60 4.90 5.20 5.60 5.70 5.80 5.90 6.70 8.70 10.4 10.4 10.5 10.6 10.5  9.2 18.5 24.8 21.8 4.60 3.70 3.80 3.80 3.80 4.10 4.40 4.80 4.90 5.00 5.10 5.90 7.90 9.5 9.5 9.45 9.45 9.2  Figure 11 shows that there i s the formation of a maximum on the positive side of the e l e c t r o c a p i l l a r y zero. The waves are now observed to be well separated and d i s t i n c t with their half wave potentials showing a s h i f t towards more negative values. respectively.  They were found to be -0.63 v and -0.89 v  The maximum was again found  non-suppressible.  Figure 12 shows the logarithmic plot of against potential.  "Vi^-i  The slope of the l i n e corresponding to  F i g u r e 11.  P o l a r o g r a p h i c wave of m-nitrophenol (1.503 m i l l i m o l a r ) i n a c e t o n i t r i l e , a f t e r standing f o r 24 hours  Figure  12.  Logarithmic plot corresponding to figure 11  - 69 -  the f i r s t wave was found to be 0.057 and that to the second was 0.069.  corresponding  Both the slopes indicate one  electron reductions. vii)  p-nitrophenol  The results of the polarographic study of pnitrophenol are given i n Table IX. Measurements were made with a 3.197 millimolar s o l u t i o n . Table IX. Reduction of p-nitrophenol (3.197 millimolar) •E (volts)  i (observed)  i (corrected)  (Microamperes) 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 1.100 1.200 1.300 1.400 1.500 1.600  -0.09 0.84 2.28 2.40 2.70 5.20 8.40 11.5 13.3 15.3 17.4 21.2 28.3 29.4 29.4 29.6 29.9  0.00 0.52 1.50 1.60 1.90 4.40 7.60 10.7 12.5 14.5 16.5 20.1 27.1 28.1 28.0 28.1 28.3  The results have been plotted i n Figure 13. The waves were found to be rather closely spaced and the l i m i t i n g current hardly distinguishable. There was again v i s i b l e a small wave preceding the two reduction waves.  The waves were  rather i l l defined f o r the half-wave potentials to be  00  -0-2  -0-4  -0-6  -0-8  -10  H-2  -1-4  -1-6  volts  F i g u r e 13.  P o l a r o g r a p h i c wave of p - n i t r o p h e n o l (3.3.97 m i l l i m o l a r ) i n a c e t o n i t r i l e . immediately a f t e r s o l u t i o n  - 71 -  determined with any accuracy.  Even the graphical method  of Zimmermann and Gropp (50) was not quite s a t i s f a c t o r y . They were however found to be i n the region of -0.62 volt and -1.12 volt respectively. The f i n a l l i m i t i n g current was observed at a potential of -1.30 v o l t . The constant  2/3 l/6 '  t a k : J L n K  as equal to this  f i n a l d i f f u s i o n current was found to be £.98 - 0.0£. On allowing the solution to stand i n contact with the mercury pool f o r several hours, i t was found that the colour of the solution deepened.  Measurements of the current  were again made a f t e r 24 hours at gradually increasing negative potentials. The results are shown i n Table X and plotted i n Figure 14. Table X.  p-nitrophenol (after 24 hours' standing) 7*3.197 m i l l i m o l a r )  -B ( v o l t s )  i (observed)  i (corrected) (microamperes)  0.050 0.100 0.150 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900 1.000 1.100 1.200 1.300 1.400 1.500  34.0 43.3 11.0 6.8 6.8 6.8 7.3 10.2 13.3 15.3 16.1 16.4 19.8 28.2 30.4 30.5 30.6  33.8 43.0 10.5 6.0 6.0 6.0 6.5 9.4 12.5 14.5 15.3 15.5 18.7 27.0 29.1 29.1 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 d i s t i n c t waves.  I t was noticed  that the maximum obtained i n 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 f o r the IR-drop were again  neglected. Figure 15 shows the logarithmic plot of  *x  against the p o t e n t i a l .  d "  x  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 respectively.  These values indicate that the reductions tak-  ing place are i r r e v e r s i b l e processes.  F i g u r e 15.  L o g a r i t h m i c p l o t corresponding t o f i g u r e  14  - 75 -  Nitroanilines  U n l i k e the n i t r o p h e n o l s , the n i t r o a n i l i n e s were p o l a r o g r a p h i c a l l y w e l l behaved. behaviour.  They showed no abnormal  They a l l produced w e l l d e f i n e d waves and there  was no i n d i c a t i o n  of any maxima.  No a d d i t i o n  mum suppressor was t h e r e f o r e necessary.  of any maxi-  No prewaves of any  k i n d were observed but only two d i s t i n c t waves corresponding to the stepwise r e d u c t i o n of the nitrocompound. were i n 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  Their slopes  of the e l e c t r o d e  processes. viii)  ortho-nitroaniline  The r e s u l t s of measurements made with a 1.084 millimolar solution  of o - n i t r o a n i l i n e i n the s u p p o r t i n g  e l e c t r o l y t e have been g i v e n i n Table "EL and they have been p l o t t e d i n F i g u r e 16. F i g u r e 16 shows that there are two d i s t i n c t waves corresponding t o half-wave p o t e n t i a l s of -0.67 - 0.01 v and -1.59  - 0.01 v r e s p e c t i v e l y . The  found  constants  / Q ^ / ^ I  1  /  6  f  or  t  h  e  t  w  o  waves were  t o be 4.87- 0.05 and 1 1 . 8 0 - 0.15 r e s p e c t i v e l y . The  l o g a r i t h m i c p l o t i s shown i n F i g u r e 17.  The  slopes of the l i n e s corresponding t o the two waves were 0.078 - 0.005 and 0.125 - 0.005. were 0.756 and 0.472 r e s p e c t i v e l y .  The corresponding n values The e l e c t r o d e processes  Figure 16.  P o l a r o g r a p h i c wave of o - n i t r o a n i l i n e milliinolar) in acetronitrile  (1.084  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 X I .  Reduction of o - n i t r o a n i l i n e (1.081j. m i l l i m o l a r )  -B ( v o l t s )  i (observed)  i (corrected)  (microamperes) 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.650 0.700 0.750 0.800 0.850 0.900 0.950 1.000 1.100 1.200 1.300 1.400 1.450 1.500 1.550 1.600 1.650 1.700 1.750 1.800 1.900 2.000  -0.09 0.32 0.78 0.80 0.80 0.80 1.70 3.80 6.40 8.30 9.00 9.00 9.00 9.00 9.00 9.10 9.10 9.20 9.60 10.4 12.3 15.5 19.2 22.3 25.2 26.2 27.5 28.3 28.8  ix)  0.00 0.00 0.00 0.00 0.00 0.00 0.90 3.00 5.60 7.50 8.20 8.20 8.20 8.20 8.10 8.05 7.95 7.90 8.20 9.00 10.8 14.0 17.6 20.7 23.5 24.4 25.6 25.7 25.4  para-nitroaniline  The r e s u l t s  obtained with a 2.1 m i l l i m o l a r  of p - n i t r o a n i l i n e have been g i v e n i n Table X I I .  soluti  - 79 -  Table X I I .  -E ( v o l t s )  Reduction of p - n i t r o a n i l i n e (2.1 m i l l i m o l a r ) i (observed)  i (corrected)  (microamperes) 0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.650 0.700 0.750 0.800 0.850 0.900 1.000 1.100 1.200 1.300 1.400 1.450 1.500 1.550 1.600 1.650 1.700 1.750 1.800 1.850 1.900 2.000  -0.09 0.32 0.78 0.80 0.80 0.80 1.00 2.10 4.45 7.00 10.8 12.3 12.8 12.8 12.8 12.9 13.5 14.1 15.9 19.4 23.5 29.5 33.5 37.0 41.0 41.6 42.0 42^3 42.8  0.00 0.00 0.00 0.00 0.00 0.00 0.20 1.30 3.65 6.20 10.0 11.5 12.0 11.9 11.8 11.7 12.2 12.7 14.5 17.9 22.0 27.9 31.9 35.3 39.2 39.7 39.7 39.7 39.4  F i g u r e 18 shows t h a t there a r e two d i s t i n c t l y  sep-  a r a t e d r e d u c t i o n waves corresponding t o h a l f wave p o t e n t i a l s of -0.74 - 0.01 v and -1.58 ± 0.01 v r e s p e c t i v e l y . The constants f o r the two waves were found  , i : after  c o r r e c t i n g the d i f f u s i o n c u r r e n t s f o r the change i n drop with p o t e n t i a l .  The constants thus  and 9.64 - 0.05 r e s p e c t i v e l y .  time  obtained were 3.71 - 0.05  F i g u r e 18,  P o l a r o g r a p h i c wave of p - n i t r o a n i l i n e in acetonitrile  (2.1  millimolar)  - 81 -  The  logarithmic plot  has been given i n F i g u r e 19.  of i / i ^ - i  against p o t e n t i a l  The slopes corresponding  to  the two waves were found t o be 0.092 - 0.005 v and 0.151 - 0.005 v r e s p e c t i v e l y .  I r r e v e r s i b l e r e d u c t i o n s were thus  indicated. x)  meta-nitroaniline  Table X I I I g i v e s the r e s u l t s  of a t y p i c a l s e t of  measurements i n the p o l a r o g r a p h i c r e d u c t i o n of m - n i t r o a n i l i n e . The  c o n c e n t r a t i o n of the s o l u t i o n employed was 1.062  milli-  molar,.. F i g u r e 20 shows the p o l a r o g r a p h i c waves of mnitroaniline.  Two w e l l d e f i n e d waves corresponding  to half  wave p o t e n t i a l s of -0.61 - 0.01 v and -1.37 - 0.01 v were found.  There was found no t r a c e of any maximum. The  constants  1  d/  e n i  2 / 3 l / 6 were determined as before t  f o r both waves, c o r r e c t i o n being made i n the d i f f u s i o n f o r the change i n drop time with p o t e n t i a l .  current  They were 3.28  - 0.05 and 7.69 - 0.05 r e s p e c t i v e l y . The  l o g a r i t h m i c p l o t , shown i n F i g u r e 21, gave the  slopes of the l i n e s corresponding  t o the two waves as  0.071 - 0.005 v and 0.155 - 0.005 v r e s p e c t i v e l y .  Figure 19.  Logarithmic plot corresponding to figure 18  18  0  I-  Figure  20.  P o l a r o g r a p h i c wave of (1.062 m i l l i m o l a r ) i n  m-nitroaniline acetif'onitrile  - 84 -  Table XIII. S (volts) 0.000 0.100 0.200 0.300 0.400 0.500 0.550 0.600 0.620 0.640 0.660 0.680 0.700 0.720 0.740 0.780 0.800 0.900 1.000 1.100 1.200 1.225 1.250 1.275 1.300 1.325 1.350 1.375 1.400 1.425 1.450 1.475 1.500 1.550 1.600 1.700 1.800 1.900 2.000  Reduction of m-nitroaniline (1.062 millimolar) i  (observed) -0.09 0.32 0.78 0.80 0.80 0.80 1.40 2.90 3.60 4.50 5.20 5.80 5.90 6.20 6.20 6.20 6.15 6.10 6.20 6.30 7.00 7.50 7.90 8.60 9.20 10.1 11.0 11.8 13.1 14.2 15.1 16.0 16.4 17.4 18.3 18.4 18.5 19.2 20.0  i  (correc 0.00 0.00 0.00 0.00 0.00 0.00 0.60 2.10 2.80 3.70 4.40 5.00 5.10 5.40 5.40 5.40 5.35 5.30 5.30 5.25 5.85 6.30 6.70 7.30 7.90 8.8 9.7 10.4 11.7 12.8 13.7 14.5 14.9 15.8 16.7 16.7 16.6 16.6 16.6  - 86 -  Discussion  The  e f f e c t of the s u b s t i t u e n t  p o t e n t i a l s has been given by S h i k a t a n e g a t i v i t y Rule'.  According  on the r e d u c t i o n  (42) i n the ' E l e c t r o -  t o t h i s r u l e , organic  are more e a s i l y reduced as more e l e c t r o n e g a t i v e  compounds  groups a r e  s u b s t i t u t e d i n the same molecule.  The OH group i s known t o  be more e l e c t r o n e g a t i v e  2  That r e q u i r e s  than the NH  that the n i t r o p h e n o l s  group (15, p. 10). must be more e a s i l y  r e d u c i b l e than the n i t r o a n i l i n e s or that the h a l f wave p o t entials  of the n i t r o a n i l i n e s must be more negative  those of the corresponding n i t r o p h e n o l s . d i c a t e that S h i k a t a ' s Astle  than  The r e s u l t s i n -  e l e c t r o n e g a t i v i t y r u l e i s obeyed.  (8) has extended t h i s theory  that where s u b s t i t u t i o n occurs causing  by p o s t u l a t i n g  the N of the n i t r o -  group t o be l e f t more p o s i t i v e or a t a lower e l e c t r o n dens i t y than the normal n i t r o g r o u p , within  the group i s decreased and hence the compound i s more  e a s i l y reduced. fects  then, as a r e s u l t , resonance  In a d d i t i o n t o the negative  of the OH and NH  effect.  2  inductive ef-  groups, they a l s o e x e r c i s e a + T  I t i s a l s o known that the NH  2  has a g r e a t e r  + T  e f f e c t than the OH group (15, p. 7 7 ) . Hence, the normal nitrogroups  e x h i b i t resonance as shown below:  - 87 -  As a r e s u l t  of the e l e c t r o m e r i c s h i f t s , we f i n d  that the N of the n i t r o g r o u p becomes more p o s i t i v e  than  i n the normal n i t r o g r o u p ; t h i s e f f e c t i s more  the case  in  of the NHg group which has a s t r o n g e r + T e f f e c t . result  As a  of t h i s , the N of the n i t r o g r o u p becomes more  t i v e with r e s p e c t t o the normal n i t r o g r o u p but l e s s t i v e than the N i n the n i t r o p h e n o l s .  posiposi-  Hence we f i n d that  the n i t r o p h e n o l s are more e a s i l y reduced  than the n i t r o -  anilines. An examination case  of the r e s u l t s  shows that i n the  of both the n i t r o p h e n o l s and the n i t r o a n i l i n e s , the  ease of r e d u c t i o n i s i n the order meta In other words, the half-wave p o t e n t i a l s negative i n the order meta  )> ortho  >  ortho are  ) para.  )  para.  increasingly An e x p l a n a t i o n  of t h i s could be given on the b a s i s of the + T e f f e c t s of NH  2  and OH groups. As a l r e a d y shown, i n the case  and  of both  the ortho-  the para- compounds, resonance l e a v e s the N more  positive.  88 -  Hence, both compounds should be expected t o be more e a s i l y r e d u c i b l e than n i t r o b e n z e n e .  But i n the case of the ortho-  compounds, there i s another a d d i t i o n a l f e a t u r e — t h e f o r m a t i o n 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 These H-bonds h i n d e r the resonance and hence we f i n d  factors.  of the normal n i t r o g r o u p  tha^t the ortho-compounds are more e a s i l y  reduced than the c o r r e s p o n d i n g para-compounds.  In the case  of the meta-compounds, however, there i s no resonance s i b l e and hence they are most e a s i l y reduced.  pos-  Thus, the ease  of r e d u c t i o n i n the f i r s t stage of r e d u c t i o n should be meta } ortho  )  para, and the r e s u l t s show that t h i s i s t r u e .  This  i s t o be expected i n a l l media, - a c i d i c , n e u t r a l or a l k a l i n e . Experiments  of Bergman and James (4) i n a c e t i c a c i d and  and co-workers  (8) i n aqueous b u f f e r s bear out the t r u t h  of A s t l e of t h i s  assertion. The r e s u l t s  of the experiments show that i n the case  of a l l three nitrocompounds  s t u d i e d , there are two r e d u c t i o n  waves seen, i n d i c a t i n g that the nitrocompounds  undergo a  two-step r e d u c t i o n .  made by Bergman "  A s i m i l a r o b s e r v a t i o n was  - 89 -  and James (4) d u r i n g the p o l a r o g r a p h i c nitrocompounds i n a c e t i c a c i d . wave without  any e x p l a n a t i o n  w e l l formed i n most cases. present study,  reduction  of some  But they d i s m i s s e d  the second  on the ground that they were not The second waves, obtained  however, are w e l l formed and d i s t i n c t  i n the  and  merit f u r t h e r c o n s i d e r a t i o n . Haber, i n 1900, has shown , that the mechanism of 1  e l e c t r o - r e d u c t i o n of the nitrocompounds i n a c i d medium was as f o l l o w s :  0 * 0 0 -ff 0 N0  N=0  2  NHOH  NH  8  TT  According  t o Haber, the r e d u c t i o n i n n e u t r a l or  a l k a l i n e medium i s given as f o l l o w s :  Presumably, the f i r s t  stage  step g i v i n g the intermediate  of r e d u c t i o n i s a f o u r - e l e c t r o n product,, azoxybenzene.  p o s s i b l e that the f i n a l product electron reduction.  It i s  i s aniline after a six-  As the r e d u c t i o n mechanism i n d i c a t e s ,  - 90 -  some of the i n t e r m e d i a t e steps might be r a t e c o n t r o l l e d , making the ional  o v e r a l l process i r r e v e r s i b l e .  order, which i s p e r s i s t e n t l y  The  'n' of f r a c t -  seen i n a l l r e d u c t i o n s  can thus be e x p l a i n e d . The  i n t e r m e d i a t e compound i n the r e d u c t i o n of the  n i t r o p h e n o l s might presumably  be,  o  In the case  (iii)  of ( i ) , the i n t e r m e d i a t e corresponding t o the  ortho-compound there i s the p o s s i b i l i t y of H-bond f o r m a t i o n tending to hinder the removal of the oxygen, whereas, i n (iii),  the para-compound the resonance makes the N  almost  n e u t r a l or even s l i g h t l y negative so that the oxygen c o u l d be very r e a d i l y removed. more e a s i l y reduced  very p o s i t i v e .  f i n d that the p a r a - i s  than the ortho-compound.  mediate, corresponding e a s i l y reduced  Thus we  The  inter-  t o meta, ( i i ) i s of course  because i t has l i t t l e  the most  resonance so t h a t N i s  Hence, the ease of r e d u c t i o n of the  nitro-  compounds on the b a s i s of the' above mechanism must be meta  )  para  this fact.  ) The  ortho.  We  f i n d that the r e s u l t s  bear  half-wave p o t e n t i a l s corresponding  out  t o the  - 91  second stage the order  of r e d u c t i o n are i n c r e a s i n g l y more negative i n  meta  )> para  ^> ortho.  T h i s order, we f i n d i s d i f f e r e n t from the order observed i n the case  of the aqueous b u f f e r s .  Prom the r e s u l t s  of A s t l e and h i s co-workers (8) we f i n d t h a t the order i s e x a c t l y opposite t o that observed r e d u c t i o n , i . e . , meta ion.  y  ortho  i n the f i r s t  stage of  y para i n the case  of r e d u c t -  T h i s could be e x p l a i n e d on the b a s i s of the f o r m a t i o n  of phenylhydroxylamine  a t the i n t e r m e d i a t e stage, as Haber's  mechanism i n a c i d medium shows. Another i n t e r e s t i n g f a c t t h a t emerges from a study of the r e d u c t i o n of n i t r o p h e n o l s i s the g r a d u a l l y d e v e l o p i n g maxima that are observed  i n a l l cases.  The height of the  maximum i n c r e a s e s with time and i t i s a l s o found  t h a t when  the maxima have been formed, the half-wave p o t e n t i a l s have a l l s h i f t e d t o more n e g a t i v e v a l u e s .  Only i n the case of  o r t h o - n i t r o p h e n o l do we f i n d an e x c e p t i o n — t h e potential  of the second wave remains the same whereas the  f i r s t wave disappears almost maximum.  h a l f wave  completely g i v i n g p l a c e t o a  I t i s w e l l known that i n the case  of complex form-  a t i o n , the half-wave p o t e n t i a l s g e n e r a l l y s h i f t towards more n e g a t i v e values  (17, p. 214 ).  In a l l p r o b a b i l i t y , there i s  complex f o r m a t i o n between mercury and the r e d u c t i o n products i n the case  of para- and meta- n i t r o p h e n o l s .  I t i s a l s o well  known that the h e i g h t of a maximum depends on the c o n c e n t r a t i o n of the s o l u t i o n and i n c r e a s e s with i n c r e a s i n g c o n c e n t r a t i o n  - 92 -  (17, p.159).  The  f a c t t h a t the height  of the maxima i n  the case of para- and meta- n i t r o p h e n o l s i n c r e a s e s  with  time i n d i c a t e s that the complex formation must be a slow process.  The  growing height  of the maxima, most  probably  r e s u l t s from the gradual i n c r e a s e of c o n c e n t r a t i o n . most l i k e l y that the s m a l l waves preceding i n F i g u r e s 5, 9 and j u s t beginning  It is  the r e d u c t i o n waves  13, are p a r t s of the maxima, which are  to form.  In the case of o r t h o - n i t r o p h e n o l , however, the wave p o t e n t i a l  of the second wave i s not a l t e r e d  indicating  that i n a l l p r o b a b i l i t y , there i s no complex formation the intermediate  r e d u c t i o n product  i s p o s s i b l e that complex formation stage;  and mercury.  wave i s r a t h e r obscured by the presence In c o n c l u s i o n , the r e s u l t s  of the  regarding  compatible  of or-  no unusual  observations made with n i t r o p h e n o l s  with complex formation  with mercury.  the  n i t r o a n i l i n e s show w e l l d e f i n e d double  waves i n d i c a t i n g a two-step r e d u c t i o n and present f e a t u r e s , whereas, the  first  of t h i s work are i n har-  e f f e c t of the s u b s t i t u e n t s on the ease of r e d u c t i o n The  first  of the maximum.  mony with p r e v i o u s l y e s t a b l i s h e d observations  ganic compounds.  between  However, i t  does occur i n the  the change i n the h a l f wave p o t e n t i a l  half-  of the r e d u c t i o n  products  are  - 93 -  BIBLIOGRAPHY  1.  A r t h u r , P and Lyons, H.  2.  A s t l e , M.J. and McConnell, W.V. 35 (1943).  3.  Bachman, G.B. and A s t l e , M.J. 1303 (1942).  4.  Bergman, I . and James, J.C. Trans. Paraday Soc. 48, 956 (1952). ~~  5.  Conant, J.B., Small, L.F., and T a y l o r , B.S. Chem. Soc. 47,1959 (1945).  6.  Cruse, K.  7.  Cruse, K., G o e r t z , E.P., and P e t e r m u l l e r , H. Z. Electrochem. 55, 5 (1951).  8.  Dennis, S.F., P o w e l l , A.S., and A s t l e , M.J. Chem. Soc. 71, 1484, (1949).  9.  E l l i o t t , N., and Y o s t , D.M. 1057, (1934).  10.  A n a l . Chem. 24, 1422 (1952).  Z. Electrochem.  J . Am. Chem. Soc. 64, ~"  J . Am.  46, 571, (1940.  Gosman, B. and Heyrovsky, J . Soc. 59, 249 (1931). 1  J . Am. Chem. Soc. 65,  J . Am.  J . Am. Chem. Soc. 56, Trans. Electrochem. $  1  11.  H a l l , M.E.  A n a l . Chem. 22, 1137 (1950).  12.  H e i l b r o n , 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. C o l l e c t i o n Czechoslov. Chem. Communs. 6, 498, (1934) J . Chim. phys. 35, 129, (1938).  15.  I n g o l d , C.K. 'Structure and Mechanism i n Organic Chemi s t r y ' C o r n e l l U n i v e r s i t y P r e s s , I t h a c a , N.Y. (1953).  16.  Janz, G.J., and T a n i g u c h i , H. 397 (1953).  17.  K o l t h o f f , I.M., and Lingane, J . J . 'Polarography' I n t e r s c i e n c e P u b l i s h e r s , New York, London (1952).  Chem. Revs. 53,  94 -  18.  K M t h o f f , I.M., and Orlemann, E.F. 63, 2085 (1941).  J . Am. Chem. Soc.  19.  Kucera, G.  20.  L a i t i n e n , H.A. and Nyman, C.J. J . Am. Chem. Soc. 70, 2241, (1948).  21.  L a i t i n e n , H.A. and Shoemaker, C.E. 72, 663, (1950).  J . Am. Chem. Soc.  22.  L a i t i n e n , H.A. and Shoemaker, C.E. 72, 4975 (1950).  J . Am. Chem. Soc.  23.  L a i t i n e n , H.A. and Wawzonek, S. 1765 (1942).  24.  Letaw, H. and Gropp, A.H. (1953).  25.  Lewis, W.R., Quackenbush, F.M. and De V r i e s , T. A n a l . Chem. 21, 762 (1949).  26.  Lingane, J . J . J . Am. Chem. Soc.  27.  Lingane, J . J . and L o v e r i d g e , B.A.  Ann. Bhysik, 11, 529 (1903).  J . Am. Chem. Soc. 64,  J . Phys. Chem. 57, 964  61, 2099  (1939).  J . Am. Chem. Soc.  72, 438 (1950). 28.  Lippmann, G.  29.  M a c G i l l a v r y , D.  30.  M a c G i l l a v r y , D. and R i d e a l , E.K. 1013  Pogg. Ann. 149, 547 (1873). Trans. Faraday Soc. 32, 1447 (1936). Rec. t r a v . chim. 56,  (1937).  31.  M e i t e s , L . and M e i t e s , T.  32.  M f l l l e r , O.H.  J . Chem. Eds. 18, 172 (1941).  33.  M f l l l e r , O.H.  J . Am. Chem. Soc. 66, 1019 (1944).  34.  Nonhebel, G. and H a r t l e y , G.S. P h i l . Mag (6) 50, 729 (1925) Parks, T.D. and Hansen, K.A. A n a l . Chem. 22, 1268 (1950). Radin, N. and De V r i e s , T. A n a l . Chem. 24, 971 (1952).  35. 36. 37.  A n a l . Chem. 20, 984 (1948).  Runner, M.E. and Wagner, E.G. 2529 (1952).  J . Am. Chem. Soc. 74,  - 95 38.  Sanko, A.M. and Manussova, F.A. (U.S.S.R.) 10, 1171 (1946).  J . Gen. Chem.  39.  Sargent, J.W,, C l i f f o r d , A.F. and Lemmon, W.R. , A n a l . Chem. 25, 1727 (1953).  40.  S c h e r e r , G.A. and Newton, R.F. 18 (1934).  41.  S h i k a t a , M.  42.  S h i k a t a , M. and T a c h i , 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. Soc. 58, 179 (1930).  Trans. Am. E l e c t r o c h e m .  44.  U h l i c h , H. and S p i e g e l , G. 103 (1936).  Z. P h y s i k . Chem. 177,  45.  V l c e k , A.A. C o l l e c t i o n Czechoslov. Chem. Communs. V o l . 16, 2 3 0 - 3 8 , ( 1 9 5 1 ) .  46.  Von Stackellsrerg, M.  47.  Wawzonek, S. and Runner, M.E. No. 11, •l±57-.9, ( 1 9 5 2 ) .  48.  Woolcock, J.W. and H a r t l e y , H.  J . Am. Chem. Soc. 56,  T r a n s . Faraday Soc. 19, 721 (1924).  Z. E l e c t r ochem. 45, 466 (1939). J . Blectrochem. Soc. 99, P h i l . Mag. (7) 5, 1133  (1928). 49.  Y o s h i d a , T.  J . Chem. Soc. (Japan) 48, 435-41 (1927).  50.  Zimmermann, H.K. J r . and Gropp, A.H. 764 (1950).  J . Phy. Chem. 54,  

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