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Polarography in hexamethylphosphoramide: Effect of supporting electrolyte cations
J. Electroanal. Chem., 90 (1977) 315--324
315
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
POLAROGRAPHY IN H E X A M E T H Y L P H O S P H O R A M I D E : SUPPORTING ELECTROLYTE CATIONS
EFFECT OF
SACHIKO S A K U R A *
Department of Chemistry, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606 (Japan) (Received 27th June 1975, in final form 12th August 1976)
ABSTRACT Polarographic reductions of various metal ions such as the silver, cupric, zinc, cobaltous, nickel, ferric, ferrous ions and hydrogen ion in hexamethylphosphoramide (HMPA), have been investigated in the supporting electrolytes with various perchlorates. The reduction of most of these ions is strongly influenced by the cation of the supporting electrolyte. In the presence of the tetraethylammonium ion, when the size of the cation of the supporting electrolyte is small and easily adsorbed on the negatively charged electrode surface, the reductions of metal ions are controlled by some preceding processes and are naturally irreversible. The rate of reduction becomes more rapid with the increase of the size of the cation. Thus, in Hex4NC104 or LiC10 solutions, the reduction of these various metal ions takes place almost totally under diffusion control, although the waves of most of metal ions show a maximum. These effects of the cation of the supporting electrolytes on reduction can be explained as a phenomenon occurring on the electrode surface. This phenomenon has been reported in previous papers [1 ] on the reductions of the alkali and alkaline earth metal ions. The difference in the electrocapillary curves in these solutions is rarely shown at the potential around the electrocapillary maximum, but it is very obviously shown at more negative potential. The difference in the effect of the size of the cation of the supporting electrolyte on reduction of metal ion coincides well with the difference in the electrocapillary curves in these solutions: the effect of the size of the supporting electrolyte cation on the polarographic reduction is rarely shown at the potential around the electrocapillary maximum, but it is very obviously shown at more negative potential; therefore this effect is due to the electrode double-layer difference.
INTRODUCTION In p r e v i o u s papers [1,2], we r e p o r t e d t h a t the p o l a r o g r a p h i c r e d u c t i o n s of the a l k a l i a n d a l k a l i n e e a r t h m e t a l i o n s in h e x a m e t h y l p h o s p h o r a m i d e ( H M P A ) ar e s t r o n g l y i n f l u e n c e d b y t h e c a t i o n o f t h e s u p p o r t i n g e l e c t r o l y t e . F o r e x a m p l e , in 0 . 0 5 M t e t r a e t h y l a m m o n i u m p e r c h l o r a t e s o l u t i o n , t h e s o d i u m i o n is n o t r e d u c e d b e f o r e t h e p o t e n t i a l o f t h e r e d u c t i o n o f t h e s u p p o r t i n g e l e c t r o l y t e is o b s e r v e d . I n l i t h i u m p e r c h l o r a t e s o l u t i o n , h o w e v e r , t h e r e d u c t i o n o f t h e s o d i u m i o n is reversible and d i f f u s i o n - c o n t r o l l e d . T h e a u t h o r s f o u n d t h a t these effects are mainly due to double-layer phenomena.
* Present address: Department of Chemistry, Faculty of Medicine, Hamamatsu University School of Medicine, Hamamatsu 431-31, Japan.
316
Polarographic reductions of various metal ions in HMPA have already been studied by Luehrs and Leddy [ 3], who used t e t r a e t h y l a m m o n i u m perchlorate as the supporting electrolyte. They reported that most metal ions and hydrogen ion are reduced irreversibly, but that the reduction of the cadmium ion is reversible. The present communication will show that, by using either various tetraalkyla m m o n i u m or lithium salts for the supporting electrolyte, many transition metal cations yield polarographic waves with diffusion-controlled limiting currents, especially at potentials more negative than the potential of zero charge. EXPERIMENTAL
Apparatus The dropping mercury electrode used had the following characteristics in 0.05 M Et4NC104--HMPA at h -- 62 cm: the A electrode, m = 1.760 mg s-1 with open circuit and t = 1.450 s at --2.6 V vs. Ag/0.1 M AgC104(HMPA) electrode, and the B electrode, m = 1.88 mg s-1 and t = 1.39 s. Except when otherwise described, the A electrode was used. Otherwise, the equipment used is the same as that used in a previous study [1]. All experiments were carried out at 25 -+ 0.1°C.
Reagents Cupric perchlorate, zinc perchlorate, cobaltous perchlorate, nickel perchlorate, and ferric perchlorate were carefully dehydrated by heating at 60°C under vacuum. Silver nitrate was carefully dehydrated without heating under vacuum, to avoid the reduction of the silver ion. Thallous perchlorate was prepared as described in ref.4, p-Toluenesulphonic acid was dried at 60°C under vacuum. Other salts used, and HMPA, were prepared as described previously [1]. RESULTS AND DISCUSSION The polarographic behavior of various metal ions and hydrogen ion in HMPA was examined by the use of Et4NC104, ButNC104, Hex4NC104 and LiC104 as the supporting electrolytes. The results are summarized in Figs. 1--9.
Ferric and ferrous ions In 0.05 M Et4NC104 solution, the ferric and ferrous ions are reduced apparently irreversibly, because of very drawn-out waves. Their half-wave potentials are --0.886 and --1.87o V, respectively. In 0.05 M LiC104, Bu4NCIO4, Hex4NC104 solutions, however, the half-wave potentials are shifted towards more positive values than the half-wave potential of the Et4NC104 solution, and the ferrous waves have small maxima (Fig. 1). In Fig. 2, the relationship between the limiting current and the square root of the mercury height is shown. The limiting currents were measured at --1.1 V for the first wave; for the second wave, the limiting currents were measured at --2.1 V in Et4NC104 solution, and at --2.2 V in Bu4NC104 solution.
317
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Fig. 1. P o l a r o g r a m s of 0 . 5 2 4 m M Fe 3+ ion a n d Fe 2+ ion in HMPA. S u p p o r t i n g e l e c t r o l y t e : (1) Et4NC104; (2) Bu4NC}O4; (3) LiC104; (4) Hex4NCIO 4. E a c h is 0.05 M. Fig. 2. C h a n g e o f t h e l i m i t i n g c u r r e n t s o f 0 . 5 2 4 m M Fe 3+ a n d Fe 2+ ions w i t h t h e h e i g h t o f t h e m e r c u r y c o l u m n . (1) T h e first wave in 0.D5 M Et4NC104; (2) t h e s e c o n d wave in 0.05 M Et4NC104; (3) t h e first wave in 0.05 M Bu4NC104; (4) the s e c o n d wave in 0.05 M Bu4NCIO 4.
Since in Et4NC104 solution the limiting currents for bot h waves are n o t proportional to the square r o o t of the mercury height, these reduction processes are kinetically controlled. However, in Bu4NC104 solution, the limiting currents for b o th waves are proportional to the square r o o t of the mercury height, indicating that the limiting currents are in this case diffusion-controlled. In Hex4NC104 and LiC104 solutions, the r e duc t ion processes of these ions are also diffusion-controlled, measured by the same test. Z i n c ion
The lack o f a proportional relationship between the limiting current and the square r o o t of the m e r c u r y height shows t hat in 0.05 M Et4NC104 solution the zinc wave has a small limiting current which is not diffusion-controlled. (The limiting current was measured at --2.43 V.) The wave height does not change linearly with the concentration. In Bu4NC104, LiC104 or Hex4NC104 solutions, see Fig. 3, the wave height is higher, showing a big m axi m um between --1.6 and --2.2 V. The lack of a proportional relationship between the square r o o t of the m e r cu r y height and the limiting current shows the process to be still kinetically controlled. However, it was f ound that the wave height is proportional to the concentration.
318
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Fig. 3. Polarograms of 0.981 mM Zn 2+ ion in HMPA. Supporting electrolyte: (1) Et4NClO4; (2) Bu4NC]O4; (3) LiClO4, (4) tiex4NCIO 4. Each in HMPA. Fig. 4. Polarograms of 0.895 mM Co 2+ ion in HMPA. Supporting electrolyte: (1) Et4NCJO4; (2) Bu4NCIO4; (3) Hex4NCIO4; (4) LiCIO 4. Each is 0.05 M.
Cobaltous ion
In 0.05 M Et4NC104 solution, the wave of the cobaltous ion is also fairly irreversible which is seen from its drawn-out shape. The limiting current is not proportional to the square r o o t of the m e r cur y height; t herefore the reduct i on is kinetically controlled. The limiting current was measured at --2.2 V. As is the case of the zinc ion with Et4NC104, the wave height is not proport i onal to its concentration. In Bu4NC104, Hex4NC104, or LiC104 solutions, however, the wave height is proportional to the concentration. The reduction is still kinetically controlled, based on the lack of linear relationship between the limiting current and the square r o o t of the mercury height. It exhibits a big m axi m um of the first kind between --1.3 and --2.0 V (Fig. 4). N i c k e l ion
In 0.05 M Et4NC104 solution, the nickel ion wave has a big m a x i m u m of the first kind between --1.0 and --1.2 V. It can be seen, from the relation of the square r o o t of the m e r cur y height to the limiting current, that the nickel ion is reduced under kinetic control. (The limiting current was measured at --1.5 V.) In Bu4NC104, Hex4NC104, and LiC1Ot solutions, the reduction wave changes to be diffusion-controlled, although it still has a big m a x i m u m at the same potential (Fig. 5).
319
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E~ v vs. ~ / A g •
Fig. 5. Polarograms of 0.888 mM Ni 2+ ion in HMPA. Supporting electrolyte: (1) Et4NClO4; (2) Bu4NCIO4; (3) Hex4NC104; (4) LiC104. Each is 0.05 M. Fig. 6. Polarograms of 0.955 mM Tl + ion in HMPA. Supporting electrolyte: (1) Et4NClO4; (2) Bu4NClO4; (3) LiCIO4; (4) Hex4NClO 4. Each is 0.05 M.
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Fig. 7. Polarograms of 0.935 mM C u 2+ ion in HMPA. Supporting electrolyte: (1) Et4NCIO4; (2) Bu4NC]O4; (3) LiCIO 4. Each is 0.05 M. Fig. 8. Polarograms of 1.281 mM Ag+ ion in HMPA. Supporting electrolyte: (1) Et4NC104; (2) Bu4NC104; (3) LiC104. Each is 0.05 M.
320 Thallous ion
In 0.05 M Et4NC104 solution, the thallous ion is reduced at El~ 2 = --0.83 V. It is also semireversibly reduced, as E l l 4 -- E3/4 = 65 mV shows. No differences are observed with the other supporting electrolytes, see Fig. 6. Cupric ion
In 0.05 M Et4NC104 solution, the half-wave potential is --0.45 V. The halfwave potentials and the limiting currents are almost the same with the other supporting electrolytes. The waves sometimes show maxima depending on the electrolyte, as seen in Fig. 7. Silver ion
In 0.05 M Et4NC104 solution, the silver ion has a wave with a fairly big maximum of the first kind between --0.1 and --0.3 V. When the larger supporting electrolyte cations are used, the m a x i m u m becomes larger, although the limiting current does not change (Fig. 8). H y d r o g e n ion
p-Toluenesulphonic acid is reduced in two waves. This acid was reported [5] to be completely dissociated in HMPA. The author considers that the first wave may be the reduction of the hydrogen ion. In 0.05 M Et4NC104 solution the hydrogen ion is reduced at El/2 = --1.91 V. The first wave is drawn-out and the value of El/4 -- E3/4 is 190 mV, suggesting that the reduction process is irreversible. In 0.05 M Bu4NC104 solution, however, the half-wave potential shifted to a more positive value: --1.81 V. The value of El/4 -- E3/4 is 80 mV, indicating that the electron transfer process is promoted more in this solution than in Et4NC104 solution, although its process is still irreversible. In 0.05 M LiC104 solution the reduction wave has a big m a x i m u m and its height is greater than that obtained in Bu4NC104 solution (Fig. 9). The electrocapillary curves obtained by measuring the drop time are shown in Fig. 10. The drop time at extremely negative potentials changes considerably with the supporting electrolyte used; it is largest in 0.05 M LiC104 solution, smallest in 0.05 M Et4NC104 solution, and between these two extremes in 0.05 M Bu4NC104 solution and in 0.05 M Hex4NC104 solution. The charge densities on the electrode, calculated from the slopes of these electrocapillary curves, are plotted in Fig. 11. The negative charge on the electrode is much larger in 0.05 M Et4NC104 solution than in 0.05 M LiC104 solution, showing that the t e t r a e t h y l a m m o n i u m ion is more easily attracted to the electrode surface. This difference is probably due to the difference in the sizes of the solvated ions. Figure 12 shows the effective radii of mono-valent cations in HMPA, calculated from the results of conductivity measurements [ 2]. Though it is questionable whether or not the effective radii calculated from the Stokes' law radii are direct measures of the sizes of solvated ions, it is reasonable to assume that,
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Fig: 9. Polarograms of 1.161 mM p-toluenesulph0nic acid in HMPA. Supporting electrolyte: (1) Et4NCIO4; (2) Bu4NC104; (3) LiCIO 4. Each is 0.05 M. Fig. 10. Drop time-potential curves. (1) In 0.05 M LiC104; (2) in 0.05 M Hex4NC104; (3) in 0.05 M Bu4NClO4; (4) in 0.05 M Et4NC10 4. For conversion of drop time t to surface tension ?, the values of t and ? at the ecm in aqueous 0.1 M KC1 were used as reference. With the DME B.
among the ions in Fig. 12, the tetraethylammonium ion is the smallest, the tetrahexylammonium ion is the largest and the lithium ion is also very large, but slightly smaller than the tetrahexylammonium ion. It is natural that the tetraethylammonium ion, which is the smallest cation, is the most easily adsorbed to the negatively charged electrode surface. 6
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Fig. 11. Charges on the electrode surface obtained from the electrocapillary curves in Fig. 10. (1) In 0.05 M LiCIO4; (2) in 0.05 M Hex4NClO4; (3) in 0.05 M Bu4NC104; (4) in 0.05 M Et4NCIO 4. Fig. 12. Effective radii of various mono-valent cations in HMPA [2]. reff the effective radius; r c = the crystal radius. (1) Li+; (2) Na+; (3) K+; (4) Rb+; (5) Cs+; (6) Me4N+; (7) Et4N+; (8) Pr4N+; (9) Bu4N+; (10) Hex4 N+. =
322 The cation effect on the electrode reactions can tentatively be explained by means of the double-layer effect which takes into account the differences in the distance of the closest approach to the electrode for various cations. The distance (r2) of the closest approach for the cations of the supporting electrolytes, i.e., the distance to the outer Helmholtz plane, is expected to increase with the increase of the size of the solvated cations. For a given depolarizing cation with a distance of the closest approach equal to r~, the potential Cr at the distance rr will become more negative with the increase of the distance r 2 or the size of the cation of the supporting electrolyte. (The ¢2-potential calculated from the electrocapillary data in HMPA by applying the simple Gouy-Chapman theory, assuming no specific adsorption, does not change appreciably with the species of the cation of the supporting electrolytes.) Thus the increase of the size of the cation of the supporting electrolytes accelerates the electrode reactions in two ways: (1) the acceleration of a preceding reaction (desolvation [6]) and (2) the acceleration of the charge transfer proper [7]. If the metal ions are lightly solvated and are moderate in size, both of these two processes can occur rapidly even in EtaNC104 solution. For the heavily solvated cation, such as zinc ion, the negative value of ~r in Et4NC104 solution is too small for the reactions to proceed rapidly enough to give the reduction waves. In this case, the use of the supporting electrolytes with larger cations, such as LiC104 or Hex4NC104, makes the reactions easier. In the case of the silver, cupric and thallous ion, the size of the cation of the supporting electrolyte has little or no effect on the limiting current. The potentials of the reduction of these ions are between --0.1 and --0.83 V. These values are around the electrocapillary m a x i m u m potential. (The electrocapillary maxim u m potential is --0.40 V, as seen in Fig. 10.) The effect of the size of the cation, however, can be observed clearly at the negative potential. At more negative potentials than --1.5 V, most metal ions are not reduced completely with the Et4N + ion as the supporting electrolyte cation, as in the case of the ferrous or zinc ions. Table 1 shows half-wave potentials of alkali and alkaline earth metal ions in HMPA solutions [ 1]. These metal ions are reduced at more negative potential than --2.0 V. The sodium, lithium, barium, strontium and calcium ions are not reduced before the potential of the reduction of the supporting electrolyte is observed in 0.05 M Et4NC104 solution. In Pr4NC104 solution, the reduction waves of the sodium and barium ions appear at --2.42 V, although the limiting currents are very small. On the other hand, the reductions of the strontium and calcium ions are neither observed in Et4NC104 solution nor in Pr4NC104 solution. Even in Bu4NC104 solution,£he wave heights are quite small. The half-wave potentials of the strontium and calcium ions are --2.70, and --2.82 V, respectively. By the relationship between the square root of the mercury height and the limiting current, the reduction process appears to be still kinetically controlled. Only in Hex4NC104 solution are these metal ions reduced under diffusion control, as is seen in ref. 1. This polarographic p h e n o m e n o n coincides well with the fact (Fig. 10) that electrocapillary curves of the various supporting electrolytes are almost the same at the potential of zero charge, and become different at more negative potentials. The adsorption of the Et4 N÷ ion to the negative electrode surface becomes stronger with the shift of negative potential from the electrocapillary
1
--2.32 a
E t 4 N+ Pr 4 N+ B u 4 N+ H e x 4 N+ Li +
--2.35 a
--2.35 a
Rb +
--2.37 --2.39 --2.38 ,2.39 --2.35
K+
b b. a a, a*
a Diffusion-controlled wave. b Kinetically ~ontrolled wave. c Not-reduced wave. a, D i f f u s i o n - c o n t r o l l e d w a v e , b u t w i t h a p o l a r o g r a p h i c m a x i m u m . * R e f e r e n c e 8, t h i s i s s u e p p . 3 2 5 - - 3 3 5 .
Cs +
Supporting electrolyte cation N.R. c --2.42 --2.42 --2.49 --2.46
Na +
T h e c o n c e n t r a t i o n o f t h e s u p p o r t i n g e l e c t r o l y t e is 0 . 0 5 M i n e a c h c a s e .
H a l f - w a v e p o t e n t i a l s o f a l k a l i a n d a l k a l i n e e a r t h m e t a l i o n s i n H M P A [1 ]
TABLE
b. b b. a*
E1/2/V vs.
N.R. N.R. N.R. N.R. __
Li +
c c c c
c
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c
N.R. c --2.82 b - - 2 . 7 6 a,
N.R.
b b a, a'
N.R. c
N.R,
--2.42 --2.45 --2.41 --2.40
C a 2+
S r 2+
B a 2+
Ag/0.1 M AgC104
¢o
b~
324 maximum potential, as shown by the electrocapillary curve. Therefore, its adsorption is inhibiting the reduction of bulky ions (due to strong solvation) more strongly at the negative potentials. The reduction of the calcium ion, whose half-wave potential is the most negative, is inhibited also by Pr4N÷ and Et4N÷, although Pr4N+ is larger than Et4N+. ACKNOWLEDGEMENTS The author should like to express her sincere thanks to Professor Drs. Taitiro Fujinaga at Kyoto University and Kosuke Izutsu at Shinshu University for their helpful suggestions and discussions. This work is supported in part by the Grantin-Aid for Scientific Research from the Ministry of Education. REFERENCES 1 2 3 4 5 6 7 8
K. I z u t s u , S. S a k u r a a n d T. F u j i n a g a , Bull. C h e m . Soc. J a p . , 4 5 ( 1 9 7 2 ) 4 4 5 ; 4 6 ( 1 9 7 3 ) 4 9 3 , 2 1 4 8 . T. F u j i n a g a , K. I z u t s u a n d S. S a k u r a , N i p p o n K a g a k u K a i s h i , ( 1 9 7 3 ) 1 9 1 . D.C. L u e h r s a n d D . G . L e d d y , J. E l e c t r o a n a l . C h e m . , 41 ( 1 9 7 3 ) 1 1 3 . D.C. L u e h r s , J. I n o r g . N u c l . C h e m , , 31 ( 1 9 6 9 ) 3 5 1 7 . C. M a d i c a n d B. T r ~ m i U o n , Bull. Soc. C h i m . F t . , ( 1 9 6 8 ) 1 6 3 4 . J. H e y r o v s k y a n d J. K ~ t a in P r i n c i p l e s o f P o l a r o g r a p h y , A c a d e m i c Press, N e w Y o r k , 1 9 6 6 , p. 3 5 1 . J. H e y r o v s k ~ a n d J. K ~ t a in P r i n c i p l e s of P o l a r o g r a p h y , A c a d e m i c Press, N e w Y o r k , 1 9 6 6 , p. 2 2 9 . S. S a k u r a , J. E l e c t r o a n a l . C h e m , 8 0 ( 1 9 7 7 ) 3 2 5 ( t h i s issue).
315
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
POLAROGRAPHY IN H E X A M E T H Y L P H O S P H O R A M I D E : SUPPORTING ELECTROLYTE CATIONS
EFFECT OF
SACHIKO S A K U R A *
Department of Chemistry, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606 (Japan) (Received 27th June 1975, in final form 12th August 1976)
ABSTRACT Polarographic reductions of various metal ions such as the silver, cupric, zinc, cobaltous, nickel, ferric, ferrous ions and hydrogen ion in hexamethylphosphoramide (HMPA), have been investigated in the supporting electrolytes with various perchlorates. The reduction of most of these ions is strongly influenced by the cation of the supporting electrolyte. In the presence of the tetraethylammonium ion, when the size of the cation of the supporting electrolyte is small and easily adsorbed on the negatively charged electrode surface, the reductions of metal ions are controlled by some preceding processes and are naturally irreversible. The rate of reduction becomes more rapid with the increase of the size of the cation. Thus, in Hex4NC104 or LiC10 solutions, the reduction of these various metal ions takes place almost totally under diffusion control, although the waves of most of metal ions show a maximum. These effects of the cation of the supporting electrolytes on reduction can be explained as a phenomenon occurring on the electrode surface. This phenomenon has been reported in previous papers [1 ] on the reductions of the alkali and alkaline earth metal ions. The difference in the electrocapillary curves in these solutions is rarely shown at the potential around the electrocapillary maximum, but it is very obviously shown at more negative potential. The difference in the effect of the size of the cation of the supporting electrolyte on reduction of metal ion coincides well with the difference in the electrocapillary curves in these solutions: the effect of the size of the supporting electrolyte cation on the polarographic reduction is rarely shown at the potential around the electrocapillary maximum, but it is very obviously shown at more negative potential; therefore this effect is due to the electrode double-layer difference.
INTRODUCTION In p r e v i o u s papers [1,2], we r e p o r t e d t h a t the p o l a r o g r a p h i c r e d u c t i o n s of the a l k a l i a n d a l k a l i n e e a r t h m e t a l i o n s in h e x a m e t h y l p h o s p h o r a m i d e ( H M P A ) ar e s t r o n g l y i n f l u e n c e d b y t h e c a t i o n o f t h e s u p p o r t i n g e l e c t r o l y t e . F o r e x a m p l e , in 0 . 0 5 M t e t r a e t h y l a m m o n i u m p e r c h l o r a t e s o l u t i o n , t h e s o d i u m i o n is n o t r e d u c e d b e f o r e t h e p o t e n t i a l o f t h e r e d u c t i o n o f t h e s u p p o r t i n g e l e c t r o l y t e is o b s e r v e d . I n l i t h i u m p e r c h l o r a t e s o l u t i o n , h o w e v e r , t h e r e d u c t i o n o f t h e s o d i u m i o n is reversible and d i f f u s i o n - c o n t r o l l e d . T h e a u t h o r s f o u n d t h a t these effects are mainly due to double-layer phenomena.
* Present address: Department of Chemistry, Faculty of Medicine, Hamamatsu University School of Medicine, Hamamatsu 431-31, Japan.
316
Polarographic reductions of various metal ions in HMPA have already been studied by Luehrs and Leddy [ 3], who used t e t r a e t h y l a m m o n i u m perchlorate as the supporting electrolyte. They reported that most metal ions and hydrogen ion are reduced irreversibly, but that the reduction of the cadmium ion is reversible. The present communication will show that, by using either various tetraalkyla m m o n i u m or lithium salts for the supporting electrolyte, many transition metal cations yield polarographic waves with diffusion-controlled limiting currents, especially at potentials more negative than the potential of zero charge. EXPERIMENTAL
Apparatus The dropping mercury electrode used had the following characteristics in 0.05 M Et4NC104--HMPA at h -- 62 cm: the A electrode, m = 1.760 mg s-1 with open circuit and t = 1.450 s at --2.6 V vs. Ag/0.1 M AgC104(HMPA) electrode, and the B electrode, m = 1.88 mg s-1 and t = 1.39 s. Except when otherwise described, the A electrode was used. Otherwise, the equipment used is the same as that used in a previous study [1]. All experiments were carried out at 25 -+ 0.1°C.
Reagents Cupric perchlorate, zinc perchlorate, cobaltous perchlorate, nickel perchlorate, and ferric perchlorate were carefully dehydrated by heating at 60°C under vacuum. Silver nitrate was carefully dehydrated without heating under vacuum, to avoid the reduction of the silver ion. Thallous perchlorate was prepared as described in ref.4, p-Toluenesulphonic acid was dried at 60°C under vacuum. Other salts used, and HMPA, were prepared as described previously [1]. RESULTS AND DISCUSSION The polarographic behavior of various metal ions and hydrogen ion in HMPA was examined by the use of Et4NC104, ButNC104, Hex4NC104 and LiC104 as the supporting electrolytes. The results are summarized in Figs. 1--9.
Ferric and ferrous ions In 0.05 M Et4NC104 solution, the ferric and ferrous ions are reduced apparently irreversibly, because of very drawn-out waves. Their half-wave potentials are --0.886 and --1.87o V, respectively. In 0.05 M LiC104, Bu4NCIO4, Hex4NC104 solutions, however, the half-wave potentials are shifted towards more positive values than the half-wave potential of the Et4NC104 solution, and the ferrous waves have small maxima (Fig. 1). In Fig. 2, the relationship between the limiting current and the square root of the mercury height is shown. The limiting currents were measured at --1.1 V for the first wave; for the second wave, the limiting currents were measured at --2.1 V in Et4NC104 solution, and at --2.2 V in Bu4NC104 solution.
317
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-10
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2 ...0.o.o--
0.2
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- 2.
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0"0"--
:'=---i--
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Fig. 1. P o l a r o g r a m s of 0 . 5 2 4 m M Fe 3+ ion a n d Fe 2+ ion in HMPA. S u p p o r t i n g e l e c t r o l y t e : (1) Et4NC104; (2) Bu4NC}O4; (3) LiC104; (4) Hex4NCIO 4. E a c h is 0.05 M. Fig. 2. C h a n g e o f t h e l i m i t i n g c u r r e n t s o f 0 . 5 2 4 m M Fe 3+ a n d Fe 2+ ions w i t h t h e h e i g h t o f t h e m e r c u r y c o l u m n . (1) T h e first wave in 0.D5 M Et4NC104; (2) t h e s e c o n d wave in 0.05 M Et4NC104; (3) t h e first wave in 0.05 M Bu4NC104; (4) the s e c o n d wave in 0.05 M Bu4NCIO 4.
Since in Et4NC104 solution the limiting currents for bot h waves are n o t proportional to the square r o o t of the mercury height, these reduction processes are kinetically controlled. However, in Bu4NC104 solution, the limiting currents for b o th waves are proportional to the square r o o t of the mercury height, indicating that the limiting currents are in this case diffusion-controlled. In Hex4NC104 and LiC104 solutions, the r e duc t ion processes of these ions are also diffusion-controlled, measured by the same test. Z i n c ion
The lack o f a proportional relationship between the limiting current and the square r o o t of the m e r c u r y height shows t hat in 0.05 M Et4NC104 solution the zinc wave has a small limiting current which is not diffusion-controlled. (The limiting current was measured at --2.43 V.) The wave height does not change linearly with the concentration. In Bu4NC104, LiC104 or Hex4NC104 solutions, see Fig. 3, the wave height is higher, showing a big m axi m um between --1.6 and --2.2 V. The lack of a proportional relationship between the square r o o t of the m e r cu r y height and the limiting current shows the process to be still kinetically controlled. However, it was f ound that the wave height is proportional to the concentration.
318
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I
-20
-lO
E/v s. O/Ag"
- 1.0
I
I
-20
-30
ElVv. %/%"
Fig. 3. Polarograms of 0.981 mM Zn 2+ ion in HMPA. Supporting electrolyte: (1) Et4NClO4; (2) Bu4NC]O4; (3) LiClO4, (4) tiex4NCIO 4. Each in HMPA. Fig. 4. Polarograms of 0.895 mM Co 2+ ion in HMPA. Supporting electrolyte: (1) Et4NCJO4; (2) Bu4NCIO4; (3) Hex4NCIO4; (4) LiCIO 4. Each is 0.05 M.
Cobaltous ion
In 0.05 M Et4NC104 solution, the wave of the cobaltous ion is also fairly irreversible which is seen from its drawn-out shape. The limiting current is not proportional to the square r o o t of the m e r cur y height; t herefore the reduct i on is kinetically controlled. The limiting current was measured at --2.2 V. As is the case of the zinc ion with Et4NC104, the wave height is not proport i onal to its concentration. In Bu4NC104, Hex4NC104, or LiC104 solutions, however, the wave height is proportional to the concentration. The reduction is still kinetically controlled, based on the lack of linear relationship between the limiting current and the square r o o t of the mercury height. It exhibits a big m axi m um of the first kind between --1.3 and --2.0 V (Fig. 4). N i c k e l ion
In 0.05 M Et4NC104 solution, the nickel ion wave has a big m a x i m u m of the first kind between --1.0 and --1.2 V. It can be seen, from the relation of the square r o o t of the m e r cur y height to the limiting current, that the nickel ion is reduced under kinetic control. (The limiting current was measured at --1.5 V.) In Bu4NC104, Hex4NC104, and LiC1Ot solutions, the reduction wave changes to be diffusion-controlled, although it still has a big m a x i m u m at the same potential (Fig. 5).
319
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T 0.75~A
f _/
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l
- 1.0
,,i
-I.5
-
0.5
I
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-I.0
E/V vs. Ag lag +
E~ v vs. ~ / A g •
Fig. 5. Polarograms of 0.888 mM Ni 2+ ion in HMPA. Supporting electrolyte: (1) Et4NClO4; (2) Bu4NCIO4; (3) Hex4NC104; (4) LiC104. Each is 0.05 M. Fig. 6. Polarograms of 0.955 mM Tl + ion in HMPA. Supporting electrolyte: (1) Et4NClO4; (2) Bu4NClO4; (3) LiCIO4; (4) Hex4NClO 4. Each is 0.05 M.
__ ...... . 4
2
/ I
0
-0.2
-
i
-/.0 E~ v vs. Ag/ Ag ~ -Q6
-Q8
-/.2
0
I
-0.5
_/I
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W v ~. Ag l,~"
Fig. 7. Polarograms of 0.935 mM C u 2+ ion in HMPA. Supporting electrolyte: (1) Et4NCIO4; (2) Bu4NC]O4; (3) LiCIO 4. Each is 0.05 M. Fig. 8. Polarograms of 1.281 mM Ag+ ion in HMPA. Supporting electrolyte: (1) Et4NC104; (2) Bu4NC104; (3) LiC104. Each is 0.05 M.
320 Thallous ion
In 0.05 M Et4NC104 solution, the thallous ion is reduced at El~ 2 = --0.83 V. It is also semireversibly reduced, as E l l 4 -- E3/4 = 65 mV shows. No differences are observed with the other supporting electrolytes, see Fig. 6. Cupric ion
In 0.05 M Et4NC104 solution, the half-wave potential is --0.45 V. The halfwave potentials and the limiting currents are almost the same with the other supporting electrolytes. The waves sometimes show maxima depending on the electrolyte, as seen in Fig. 7. Silver ion
In 0.05 M Et4NC104 solution, the silver ion has a wave with a fairly big maximum of the first kind between --0.1 and --0.3 V. When the larger supporting electrolyte cations are used, the m a x i m u m becomes larger, although the limiting current does not change (Fig. 8). H y d r o g e n ion
p-Toluenesulphonic acid is reduced in two waves. This acid was reported [5] to be completely dissociated in HMPA. The author considers that the first wave may be the reduction of the hydrogen ion. In 0.05 M Et4NC104 solution the hydrogen ion is reduced at El/2 = --1.91 V. The first wave is drawn-out and the value of El/4 -- E3/4 is 190 mV, suggesting that the reduction process is irreversible. In 0.05 M Bu4NC104 solution, however, the half-wave potential shifted to a more positive value: --1.81 V. The value of El/4 -- E3/4 is 80 mV, indicating that the electron transfer process is promoted more in this solution than in Et4NC104 solution, although its process is still irreversible. In 0.05 M LiC104 solution the reduction wave has a big m a x i m u m and its height is greater than that obtained in Bu4NC104 solution (Fig. 9). The electrocapillary curves obtained by measuring the drop time are shown in Fig. 10. The drop time at extremely negative potentials changes considerably with the supporting electrolyte used; it is largest in 0.05 M LiC104 solution, smallest in 0.05 M Et4NC104 solution, and between these two extremes in 0.05 M Bu4NC104 solution and in 0.05 M Hex4NC104 solution. The charge densities on the electrode, calculated from the slopes of these electrocapillary curves, are plotted in Fig. 11. The negative charge on the electrode is much larger in 0.05 M Et4NC104 solution than in 0.05 M LiC104 solution, showing that the t e t r a e t h y l a m m o n i u m ion is more easily attracted to the electrode surface. This difference is probably due to the difference in the sizes of the solvated ions. Figure 12 shows the effective radii of mono-valent cations in HMPA, calculated from the results of conductivity measurements [ 2]. Though it is questionable whether or not the effective radii calculated from the Stokes' law radii are direct measures of the sizes of solvated ions, it is reasonable to assume that,
321
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i
o
-2.5 E /V ¢.s. Ag /A9 ÷ -20
-I.0
-2.'0
-3.1
E/V vs.Ag/Ag÷(HMPA)
Fig: 9. Polarograms of 1.161 mM p-toluenesulph0nic acid in HMPA. Supporting electrolyte: (1) Et4NCIO4; (2) Bu4NC104; (3) LiCIO 4. Each is 0.05 M. Fig. 10. Drop time-potential curves. (1) In 0.05 M LiC104; (2) in 0.05 M Hex4NC104; (3) in 0.05 M Bu4NClO4; (4) in 0.05 M Et4NC10 4. For conversion of drop time t to surface tension ?, the values of t and ? at the ecm in aqueous 0.1 M KC1 were used as reference. With the DME B.
among the ions in Fig. 12, the tetraethylammonium ion is the smallest, the tetrahexylammonium ion is the largest and the lithium ion is also very large, but slightly smaller than the tetrahexylammonium ion. It is natural that the tetraethylammonium ion, which is the smallest cation, is the most easily adsorbed to the negatively charged electrode surface. 6
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Fig. 11. Charges on the electrode surface obtained from the electrocapillary curves in Fig. 10. (1) In 0.05 M LiCIO4; (2) in 0.05 M Hex4NClO4; (3) in 0.05 M Bu4NC104; (4) in 0.05 M Et4NCIO 4. Fig. 12. Effective radii of various mono-valent cations in HMPA [2]. reff the effective radius; r c = the crystal radius. (1) Li+; (2) Na+; (3) K+; (4) Rb+; (5) Cs+; (6) Me4N+; (7) Et4N+; (8) Pr4N+; (9) Bu4N+; (10) Hex4 N+. =
322 The cation effect on the electrode reactions can tentatively be explained by means of the double-layer effect which takes into account the differences in the distance of the closest approach to the electrode for various cations. The distance (r2) of the closest approach for the cations of the supporting electrolytes, i.e., the distance to the outer Helmholtz plane, is expected to increase with the increase of the size of the solvated cations. For a given depolarizing cation with a distance of the closest approach equal to r~, the potential Cr at the distance rr will become more negative with the increase of the distance r 2 or the size of the cation of the supporting electrolyte. (The ¢2-potential calculated from the electrocapillary data in HMPA by applying the simple Gouy-Chapman theory, assuming no specific adsorption, does not change appreciably with the species of the cation of the supporting electrolytes.) Thus the increase of the size of the cation of the supporting electrolytes accelerates the electrode reactions in two ways: (1) the acceleration of a preceding reaction (desolvation [6]) and (2) the acceleration of the charge transfer proper [7]. If the metal ions are lightly solvated and are moderate in size, both of these two processes can occur rapidly even in EtaNC104 solution. For the heavily solvated cation, such as zinc ion, the negative value of ~r in Et4NC104 solution is too small for the reactions to proceed rapidly enough to give the reduction waves. In this case, the use of the supporting electrolytes with larger cations, such as LiC104 or Hex4NC104, makes the reactions easier. In the case of the silver, cupric and thallous ion, the size of the cation of the supporting electrolyte has little or no effect on the limiting current. The potentials of the reduction of these ions are between --0.1 and --0.83 V. These values are around the electrocapillary m a x i m u m potential. (The electrocapillary maxim u m potential is --0.40 V, as seen in Fig. 10.) The effect of the size of the cation, however, can be observed clearly at the negative potential. At more negative potentials than --1.5 V, most metal ions are not reduced completely with the Et4N + ion as the supporting electrolyte cation, as in the case of the ferrous or zinc ions. Table 1 shows half-wave potentials of alkali and alkaline earth metal ions in HMPA solutions [ 1]. These metal ions are reduced at more negative potential than --2.0 V. The sodium, lithium, barium, strontium and calcium ions are not reduced before the potential of the reduction of the supporting electrolyte is observed in 0.05 M Et4NC104 solution. In Pr4NC104 solution, the reduction waves of the sodium and barium ions appear at --2.42 V, although the limiting currents are very small. On the other hand, the reductions of the strontium and calcium ions are neither observed in Et4NC104 solution nor in Pr4NC104 solution. Even in Bu4NC104 solution,£he wave heights are quite small. The half-wave potentials of the strontium and calcium ions are --2.70, and --2.82 V, respectively. By the relationship between the square root of the mercury height and the limiting current, the reduction process appears to be still kinetically controlled. Only in Hex4NC104 solution are these metal ions reduced under diffusion control, as is seen in ref. 1. This polarographic p h e n o m e n o n coincides well with the fact (Fig. 10) that electrocapillary curves of the various supporting electrolytes are almost the same at the potential of zero charge, and become different at more negative potentials. The adsorption of the Et4 N÷ ion to the negative electrode surface becomes stronger with the shift of negative potential from the electrocapillary
1
--2.32 a
E t 4 N+ Pr 4 N+ B u 4 N+ H e x 4 N+ Li +
--2.35 a
--2.35 a
Rb +
--2.37 --2.39 --2.38 ,2.39 --2.35
K+
b b. a a, a*
a Diffusion-controlled wave. b Kinetically ~ontrolled wave. c Not-reduced wave. a, D i f f u s i o n - c o n t r o l l e d w a v e , b u t w i t h a p o l a r o g r a p h i c m a x i m u m . * R e f e r e n c e 8, t h i s i s s u e p p . 3 2 5 - - 3 3 5 .
Cs +
Supporting electrolyte cation N.R. c --2.42 --2.42 --2.49 --2.46
Na +
T h e c o n c e n t r a t i o n o f t h e s u p p o r t i n g e l e c t r o l y t e is 0 . 0 5 M i n e a c h c a s e .
H a l f - w a v e p o t e n t i a l s o f a l k a l i a n d a l k a l i n e e a r t h m e t a l i o n s i n H M P A [1 ]
TABLE
b. b b. a*
E1/2/V vs.
N.R. N.R. N.R. N.R. __
Li +
c c c c
c
N.R. c --2.70 b - - 2 . 6 1 a'
c
N.R. c --2.82 b - - 2 . 7 6 a,
N.R.
b b a, a'
N.R. c
N.R,
--2.42 --2.45 --2.41 --2.40
C a 2+
S r 2+
B a 2+
Ag/0.1 M AgC104
¢o
b~
324 maximum potential, as shown by the electrocapillary curve. Therefore, its adsorption is inhibiting the reduction of bulky ions (due to strong solvation) more strongly at the negative potentials. The reduction of the calcium ion, whose half-wave potential is the most negative, is inhibited also by Pr4N÷ and Et4N÷, although Pr4N+ is larger than Et4N+. ACKNOWLEDGEMENTS The author should like to express her sincere thanks to Professor Drs. Taitiro Fujinaga at Kyoto University and Kosuke Izutsu at Shinshu University for their helpful suggestions and discussions. This work is supported in part by the Grantin-Aid for Scientific Research from the Ministry of Education. REFERENCES 1 2 3 4 5 6 7 8
K. I z u t s u , S. S a k u r a a n d T. F u j i n a g a , Bull. C h e m . Soc. J a p . , 4 5 ( 1 9 7 2 ) 4 4 5 ; 4 6 ( 1 9 7 3 ) 4 9 3 , 2 1 4 8 . T. F u j i n a g a , K. I z u t s u a n d S. S a k u r a , N i p p o n K a g a k u K a i s h i , ( 1 9 7 3 ) 1 9 1 . D.C. L u e h r s a n d D . G . L e d d y , J. E l e c t r o a n a l . C h e m . , 41 ( 1 9 7 3 ) 1 1 3 . D.C. L u e h r s , J. I n o r g . N u c l . C h e m , , 31 ( 1 9 6 9 ) 3 5 1 7 . C. M a d i c a n d B. T r ~ m i U o n , Bull. Soc. C h i m . F t . , ( 1 9 6 8 ) 1 6 3 4 . J. H e y r o v s k y a n d J. K ~ t a in P r i n c i p l e s o f P o l a r o g r a p h y , A c a d e m i c Press, N e w Y o r k , 1 9 6 6 , p. 3 5 1 . J. H e y r o v s k ~ a n d J. K ~ t a in P r i n c i p l e s of P o l a r o g r a p h y , A c a d e m i c Press, N e w Y o r k , 1 9 6 6 , p. 2 2 9 . S. S a k u r a , J. E l e c t r o a n a l . C h e m , 8 0 ( 1 9 7 7 ) 3 2 5 ( t h i s issue).