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Medium effects on the electroreduction of In(III) at DME
J EIectroanal. Chem., 115 ( 1 9 8 0 ) 2 4 7 - - 2 5 1
247
© Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in T h e N e t h e r l a n d s
MEDIUM EFFECTS ON THE ELECTROREDUCTION OF In(III) AT DME FORMATION OF ELECTROCAPILLARY ACTIVE In(III) SPECIES IN AQUEOUS MEDIA OF D I F F E R E N T ANIONS
A.A. M O U S S A a n d F. E L - T A I B H E A K A L *
Chemistry Department, Faculty of Science, Cairo University, Giza (Egypt) ( R e c e i v e d 2 0 t h N o v e m b e r 1 9 7 9 ; in revised f o r m 2 8 t h J u l y 1 9 8 0 )
ABSTRACT T h e f o r m a t i o n o f e l e c t r o c a p i l l a r y active I n ( I I I ) species was t r a c e d in various a q u e o u s m e d i a using surface t e n s i o n m e a s u r e m e n t s . I n all m e d i a s t u d i e d , e x c e p t iodide, t h e p r e s e n c e o f I n ( I I I ) at a c o n c e n t r a t i o n o f 10 -2 M caused a s h i f t o f Ema x t o w a r d s m o r e negative p o t e n tials. In M-azide, t h i o c y a n a t e , c h l o r i d e a n d b r o m i d e s o l u t i o n s , t h e s h i f t a m o u n t e d t o 30, 25, 8 a n d 4 m V respectively, w h i c h parallels t h e e x t e n t o f l o w e r i n g in Oma x in t h e s e media. T h e results in general i n d i c a t e t h a t in all m e d i a t h e e l e c t r o c a p i l l a r y active i n d i u m species is a negatively c h a r g e d one, a n d t h a t t h e f o r m a t i o n o f s u c h species a t t h e e l e c t r o d e surface is f a v o u r e d in t h e o r d e r : N3 > S C N - > C I - > B r - > I-, w h i c h is t h e same o r d e r f o r t h e tend e n c y o f I n ( I I I ) to ligand w i t h t h e s e a n i o n s in t h e b u l k o f s o l u t i o n .
INTRODUCTION
The polarographic wave o f In(III) exihibits a peculiar current minimum in several aqueous electrolytes. This conspicuous feature was first reported by Lingane [1] and confirmed subsequently by m a n y investigators [2--6]. Several theoretical interpretations taking account of adsorption--desorption phenomena of electrocapillary active In(III) complexes at the mercury surface have been advanced to account for this current m i n i m u m [7--9]. Experimental evidence for the existence of such complexes does not seem to have been envisaged. In the present paper the formation of electrocapillary active In(III) species in molar aqueous solution of NAN3, NaSCN, NaC1, NaBr and NaI was investigated using surface tension measurements. EXPERIMENTAL
Electrocapillary measurements were carried out using a modified form o f Grahame's streaming capillary electrometer. A full description of the apparatus, accessories, calibration and working procedure is given elsewhere [10]. * To w h o m all c o r r e s p o n d e n c e s h o u l d be addressed.
248
Here, 1 M solution of NAN3, NaSCN, NaC1, NaBr and NaI as supporting electrolytes were prepared as described previously [ 11 ]. Solutions of indium were prepared by appropriate dilution of a stock solution prepared by dissolving the specpure metal in the least a m o u n t of nitric acid, then diluting with distilled water. Measurements were all made in an air bath adjusted at 25 ° + 0.5°C. Solutions were deaerated with purified hydrogen gas. RESULTS AND DISCUSSION
Figure 1 shows the electrocapillary curves of mercury as established, first in pure 1 M NaN3 and in the presence of In(III) at the four different concentrations used. The potential of the electrocapillary m a x i m u m Emax is indicated on the curves by short vertical strokes. The half-wave potential of In(III) reduction as substantiated from separate measurements in 1 M NaN3 + 5 × 10 -4 M In(III) is also indicated. The estimated Emax values to within +2 mV, and the maxim u m surface tension values a m ~ to within +0.2 mN m -1, in the various media examined are collected in Table 1. The same table contains previously reported values in the pure electrolytes together with the reference electrode used. The polarising currents were monitored in the potential region of measurements,
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and fortunately the polarographic maxima were absent with the technique employed. By comparing our results in the pure media with those previously reported, it is noticed that the agreement is generally satisfactory. By considering the potentials of the saturated-, normal- and decinormal-calomel electrodes as being equal to 2 4 5 . 8 , 2 8 1 . 9 and 336.9 mV (NHE), respectively, the difference in Emax is within the experimental error except in azide where the nitrate concentration was raised to 0.4 M. As to amax, our values in the different media agree to within 1.0 mN m -1 with those previously reported. At one and same concentration of In(III), viz. 0.01 M, the electrocapillary curve is affected to an extent depending on the nature of the medium, particularly on the positive side of the electrocapillary maximum potential. While the negative branch of the electrocapillary curve is n o t markedly affected, the positive branch is noticeably shifted in the negative direction in azide, thiocyanate and to some extent in chloride and bromide media. In the iodide medium, however, the positive branch is also almost unaffected. Our results in thiocyanate, particularly with respect to the positive branch of the electrocapillary curve, resemble to a great extent those previously published b y Takahashi and Shirai [ 5] who were interested only in the change of surface tension of mercury at the region of the polarographic current minimum. In bromide and iodide, our results agree with the previous observations of O ' D o m and Murray that In(III) in those media gives no detectable adsorption on the mercury surface [12]. With the same In(III) concentration, E m a x in all media, except iodide, is shifted towards more negative potentials. The amount of shift, however, varies according to the nature o f the medium. In azide, thiocyanate, chloride and bromide the shift amounts to 30, 25, 8 and 4 mV respectively. The amount of shift runs parallel with the extent of lowering in (/max and which amounts respectively to, 1.6, 1.1, 1.0 and 0.3 mN m -1. By increasing the In(III) concentration in azide medium from 5.0 X 10 -3 M to 1.0 X 10 -1 M (Fig. 1), the positive branch of the electrocapillary curve is progressively shifted towards more negative potentials, with a gradual lowering of amax and a shift in E m a x towards more negative potentials. The above results indicate in general that in all media the electrocapillary active indium species is a negatively charged one. In the light of these results, the formation of the electrocapillary active indium species at the electrode surface is favoured in the order N~ > SCN- > C1- > Br- > I-. A rough estimate of the excess of the electrocapillary active indium species F at E m a x may be obtained for any In(III) concentration from (~ O/~P)Emax = --F, where/1 is the chemical potential of the prevailing indium species. Here, F would be expected to follow the order N~ > SCN- > CV > Br-. This order parallels that for the ten dency of In(III) to complex with the different anions in the bulk of solution [111. From separate polarographic measurements, the half-wave potential of In(III) reduction referred to the same reference electrode (SCE) is --0.730, --0.635, --0.595, --0.580 and --0.555 V in azide, thiocyanate, chloride, bromide and iodide respectively. It is interesting to note that this order also parallels that for the tendency of complexation of In(III) with the different anions.
251
REFERENCES 1 J.J. L i n g a n e , thesis, Univ. o f M i n n e s o t a ( 1 9 3 8 ) , I.M. K o l t h o f f a n d J.J. L l n g a n e , P o l a x o g r a p h y , I n t e r science, N e w Y o r k , 1 9 4 1 , p. 2 7 5 . 2 D. Cozzl a n d S. V l v a r e l h , Z. E l e k t r o c h e m . , 57 ( 1 9 5 3 ) 4 0 8 ; 58 ( 1 9 5 4 ) 9 0 7 . 3 M. B u l o v o v a , Collect. C z e c h . C h e m . C o m m u n . , 19 ( 1 9 5 4 ) 1 1 2 3 . 4 H. Slurai, J. C h e m . Soc. J a p . , P u r e C h e m . Sect., 81 ( 1 9 6 0 ) 1 2 4 8 . 5 T. T a k a h a s h i a n d H. Shlrai, R e v . P o l a r o g r . K y o t o , 11 ( 1 9 6 3 ) 155. 6 A . G . S t r o m b e r g a n d K h . Z . Bralnina, Zh. Fiz. K h i m . , 35 ( 1 9 6 1 ) 2 0 1 6 . 7 R. de L e v l e a n d A . A . H u s o v s k y , J. E l e c t r o a n a l . C h e m . , 22 ( 1 9 6 9 ) 29. 8 R. de L e v l e a n d L. Posplsll, J. E l e c t r o a n a l . C h e m . , 22 ( 1 9 6 9 ) 2 7 7 . 9 L. Pospisll a n d R. de L e v l e , J. E l e c t r o a n a l . C h e m . , 25 ( 1 9 7 0 ) 2 4 5 . 10 A . A . M o u s s a a n d M.M. A b o u R o m l a , E l e c t r o c h l m . A c t a , 15 ( 1 9 7 0 ) 1 3 7 3 . 11 A . A . Moussa a n d F. E1 Taib t t e a k a l , T a l a n t a , s u b m i t t e d . 12 G.W. O ' D o m a n d R.W. M u r r a y , J. E l c c t r o a n a l . C h e m . , 16 ( 1 9 6 8 ) 3 2 7 .
247
© Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in T h e N e t h e r l a n d s
MEDIUM EFFECTS ON THE ELECTROREDUCTION OF In(III) AT DME FORMATION OF ELECTROCAPILLARY ACTIVE In(III) SPECIES IN AQUEOUS MEDIA OF D I F F E R E N T ANIONS
A.A. M O U S S A a n d F. E L - T A I B H E A K A L *
Chemistry Department, Faculty of Science, Cairo University, Giza (Egypt) ( R e c e i v e d 2 0 t h N o v e m b e r 1 9 7 9 ; in revised f o r m 2 8 t h J u l y 1 9 8 0 )
ABSTRACT T h e f o r m a t i o n o f e l e c t r o c a p i l l a r y active I n ( I I I ) species was t r a c e d in various a q u e o u s m e d i a using surface t e n s i o n m e a s u r e m e n t s . I n all m e d i a s t u d i e d , e x c e p t iodide, t h e p r e s e n c e o f I n ( I I I ) at a c o n c e n t r a t i o n o f 10 -2 M caused a s h i f t o f Ema x t o w a r d s m o r e negative p o t e n tials. In M-azide, t h i o c y a n a t e , c h l o r i d e a n d b r o m i d e s o l u t i o n s , t h e s h i f t a m o u n t e d t o 30, 25, 8 a n d 4 m V respectively, w h i c h parallels t h e e x t e n t o f l o w e r i n g in Oma x in t h e s e media. T h e results in general i n d i c a t e t h a t in all m e d i a t h e e l e c t r o c a p i l l a r y active i n d i u m species is a negatively c h a r g e d one, a n d t h a t t h e f o r m a t i o n o f s u c h species a t t h e e l e c t r o d e surface is f a v o u r e d in t h e o r d e r : N3 > S C N - > C I - > B r - > I-, w h i c h is t h e same o r d e r f o r t h e tend e n c y o f I n ( I I I ) to ligand w i t h t h e s e a n i o n s in t h e b u l k o f s o l u t i o n .
INTRODUCTION
The polarographic wave o f In(III) exihibits a peculiar current minimum in several aqueous electrolytes. This conspicuous feature was first reported by Lingane [1] and confirmed subsequently by m a n y investigators [2--6]. Several theoretical interpretations taking account of adsorption--desorption phenomena of electrocapillary active In(III) complexes at the mercury surface have been advanced to account for this current m i n i m u m [7--9]. Experimental evidence for the existence of such complexes does not seem to have been envisaged. In the present paper the formation of electrocapillary active In(III) species in molar aqueous solution of NAN3, NaSCN, NaC1, NaBr and NaI was investigated using surface tension measurements. EXPERIMENTAL
Electrocapillary measurements were carried out using a modified form o f Grahame's streaming capillary electrometer. A full description of the apparatus, accessories, calibration and working procedure is given elsewhere [10]. * To w h o m all c o r r e s p o n d e n c e s h o u l d be addressed.
248
Here, 1 M solution of NAN3, NaSCN, NaC1, NaBr and NaI as supporting electrolytes were prepared as described previously [ 11 ]. Solutions of indium were prepared by appropriate dilution of a stock solution prepared by dissolving the specpure metal in the least a m o u n t of nitric acid, then diluting with distilled water. Measurements were all made in an air bath adjusted at 25 ° + 0.5°C. Solutions were deaerated with purified hydrogen gas. RESULTS AND DISCUSSION
Figure 1 shows the electrocapillary curves of mercury as established, first in pure 1 M NaN3 and in the presence of In(III) at the four different concentrations used. The potential of the electrocapillary m a x i m u m Emax is indicated on the curves by short vertical strokes. The half-wave potential of In(III) reduction as substantiated from separate measurements in 1 M NaN3 + 5 × 10 -4 M In(III) is also indicated. The estimated Emax values to within +2 mV, and the maxim u m surface tension values a m ~ to within +0.2 mN m -1, in the various media examined are collected in Table 1. The same table contains previously reported values in the pure electrolytes together with the reference electrode used. The polarising currents were monitored in the potential region of measurements,
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0 800
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and fortunately the polarographic maxima were absent with the technique employed. By comparing our results in the pure media with those previously reported, it is noticed that the agreement is generally satisfactory. By considering the potentials of the saturated-, normal- and decinormal-calomel electrodes as being equal to 2 4 5 . 8 , 2 8 1 . 9 and 336.9 mV (NHE), respectively, the difference in Emax is within the experimental error except in azide where the nitrate concentration was raised to 0.4 M. As to amax, our values in the different media agree to within 1.0 mN m -1 with those previously reported. At one and same concentration of In(III), viz. 0.01 M, the electrocapillary curve is affected to an extent depending on the nature of the medium, particularly on the positive side of the electrocapillary maximum potential. While the negative branch of the electrocapillary curve is n o t markedly affected, the positive branch is noticeably shifted in the negative direction in azide, thiocyanate and to some extent in chloride and bromide media. In the iodide medium, however, the positive branch is also almost unaffected. Our results in thiocyanate, particularly with respect to the positive branch of the electrocapillary curve, resemble to a great extent those previously published b y Takahashi and Shirai [ 5] who were interested only in the change of surface tension of mercury at the region of the polarographic current minimum. In bromide and iodide, our results agree with the previous observations of O ' D o m and Murray that In(III) in those media gives no detectable adsorption on the mercury surface [12]. With the same In(III) concentration, E m a x in all media, except iodide, is shifted towards more negative potentials. The amount of shift, however, varies according to the nature o f the medium. In azide, thiocyanate, chloride and bromide the shift amounts to 30, 25, 8 and 4 mV respectively. The amount of shift runs parallel with the extent of lowering in (/max and which amounts respectively to, 1.6, 1.1, 1.0 and 0.3 mN m -1. By increasing the In(III) concentration in azide medium from 5.0 X 10 -3 M to 1.0 X 10 -1 M (Fig. 1), the positive branch of the electrocapillary curve is progressively shifted towards more negative potentials, with a gradual lowering of amax and a shift in E m a x towards more negative potentials. The above results indicate in general that in all media the electrocapillary active indium species is a negatively charged one. In the light of these results, the formation of the electrocapillary active indium species at the electrode surface is favoured in the order N~ > SCN- > C1- > Br- > I-. A rough estimate of the excess of the electrocapillary active indium species F at E m a x may be obtained for any In(III) concentration from (~ O/~P)Emax = --F, where/1 is the chemical potential of the prevailing indium species. Here, F would be expected to follow the order N~ > SCN- > CV > Br-. This order parallels that for the ten dency of In(III) to complex with the different anions in the bulk of solution [111. From separate polarographic measurements, the half-wave potential of In(III) reduction referred to the same reference electrode (SCE) is --0.730, --0.635, --0.595, --0.580 and --0.555 V in azide, thiocyanate, chloride, bromide and iodide respectively. It is interesting to note that this order also parallels that for the tendency of complexation of In(III) with the different anions.
251
REFERENCES 1 J.J. L i n g a n e , thesis, Univ. o f M i n n e s o t a ( 1 9 3 8 ) , I.M. K o l t h o f f a n d J.J. L l n g a n e , P o l a x o g r a p h y , I n t e r science, N e w Y o r k , 1 9 4 1 , p. 2 7 5 . 2 D. Cozzl a n d S. V l v a r e l h , Z. E l e k t r o c h e m . , 57 ( 1 9 5 3 ) 4 0 8 ; 58 ( 1 9 5 4 ) 9 0 7 . 3 M. B u l o v o v a , Collect. C z e c h . C h e m . C o m m u n . , 19 ( 1 9 5 4 ) 1 1 2 3 . 4 H. Slurai, J. C h e m . Soc. J a p . , P u r e C h e m . Sect., 81 ( 1 9 6 0 ) 1 2 4 8 . 5 T. T a k a h a s h i a n d H. Shlrai, R e v . P o l a r o g r . K y o t o , 11 ( 1 9 6 3 ) 155. 6 A . G . S t r o m b e r g a n d K h . Z . Bralnina, Zh. Fiz. K h i m . , 35 ( 1 9 6 1 ) 2 0 1 6 . 7 R. de L e v l e a n d A . A . H u s o v s k y , J. E l e c t r o a n a l . C h e m . , 22 ( 1 9 6 9 ) 29. 8 R. de L e v l e a n d L. Posplsll, J. E l e c t r o a n a l . C h e m . , 22 ( 1 9 6 9 ) 2 7 7 . 9 L. Pospisll a n d R. de L e v l e , J. E l e c t r o a n a l . C h e m . , 25 ( 1 9 7 0 ) 2 4 5 . 10 A . A . M o u s s a a n d M.M. A b o u R o m l a , E l e c t r o c h l m . A c t a , 15 ( 1 9 7 0 ) 1 3 7 3 . 11 A . A . Moussa a n d F. E1 Taib t t e a k a l , T a l a n t a , s u b m i t t e d . 12 G.W. O ' D o m a n d R.W. M u r r a y , J. E l c c t r o a n a l . C h e m . , 16 ( 1 9 6 8 ) 3 2 7 .