- Email: [email protected]
Reexamination of the theory of exaltation of the migration current by preceding electrode reactions
Electroanalytical Chemistry and Interfacial Electrochemistry, 62 (1975) 357-362 QElsevier Sequoia S.A., Lausanne - Printed in The Netherlands
357
REEXAMINATION OF THE THEORY OF EXALTATION OF THE MIGRATION CURRENT BY PRECEDING ELECTRODE REACTIONS
E. B. WEROI%KI
and J. CZERNIK
Department of Physical Chemistry, Polytechnic of Warsaw, Warsaw (Poland) (Received
16th December
1974; in revised form 18th March
1975)
ABSTRACT
The effect of the preceding current of Hg(CN),, CdS04, ZnSO,, and HCl on the K+ wave, of Hg(CN)2 and CuS04 on the Cd’+ wave, of CdS04 on the Zn2+ wave, and of Hg(CN), and CdS04 on the IO; wave was investigated. No exaltation of the Cd2+ wave was observed and only small variations of the Zn2+ and IO; waves. The increase of the K+ waves was roughly in accord with the prediction of the theory of the so-called exaltation of migration currents by preceding electrochemical reactions. However, the exaltation can be explained by concomitant reactions of K-amalgam with water and electrochemical reduction of hydrogen, which are accelerated by other ions (and also by the ions involved in the preceding reactions) and which, consequently, invalidate the equation of the exaltation of migration currents. INTRODUCTION
In the case of polarographic reduction of K+ ions occurring in the absence of a supporting electrolyte Kemula and Michalski’ observed an increase of the limiting current of K+ in the presence of the preceding reduction of oxygen or H+ ions. They called this increase an “exaltation of limiting currents”. Heyrovsky and Bures2 extended the experiments and obtained the exaltation effect with K+, Na+, Ba2+ and Mn’+ ions in the presence of oxygen or quinone. They explained it on the basis of theoretically derived quantitative relations as an “exaltation of the migration currents of the ions”. The exaltation i,, was found to be independent of the concentration of the reduced ions and was expressed by the current of the preceding reaction, ipr,multiplied by the ratio of the equivalent conductances of the ions2v3, e.g.: i,, = i,J+/A_
= i,,A,lA_
(1)
where io, is the limiting current of oxygen, and A+ and i- are the equivalent conductances of the reducible cations and anions, respectively. Von Stackelberg4 derived the relations in another way and suggested, as did Kolthoff and Lingane5, a correction in the calculations for the produced OH- ions. Because of some discrepancies between the experimental and the calculated values he suggested further investigations of this phenomenon. On the contrary, Kolthoff and Lingane5 found in their verifying experiments a very good agreement between the experi-
E. B. WEROlbKI,
358
J. CZERNIK
mentally determined and the theoretically calculated (by means of the Heyrovsky’s and Bures’ relation?) exaltation currents of K’ ions in the presence of oxygen and Tlf ions. In the latter case the total exaltation was ascribed to the increase in the migration current both of Tl+ and K’ ions, i,,,, = i,,. -,.,I + iex,K +. Reeves et ~1.~~~ investigated the electrochemical reduction of alkali metal ions at the dropping mercury electrode. They found that the primary electrochemical reduction of cations, e.~.~.’ : K+ +Hg+e was accompanied K(Hg)+H,O
-+ K(Hg), by a secondary
(2) chemical
reaction
of dissolution
of the alkali metal
+ K++(1/2)H2+OH-,
(34
or K(Hg)+H+
+ K++(l/2)H,,
(3b)
or by an electrochemical reduction of water catalysed by the amalgam’. Consequently, the secondary reactions increase directly (by the electrochemical reduction of water) or indirectly (by additional reduction of those K+ ions which are produced by the dissolution of the amalgam, with hydrogen evolution) the observed total current i,. The total current was determined as the sum of a contribution ialk due to the primary reduction of the alkali metal ions and of a contribution in to the hydrogen evolution currenP.‘. i, = ialk + i,
(4)
The values of ialk and i, were of the same order of magnitude (e.g., the ratio i,/i, was found 0.48-0.75)6.9. In industrial electrochemistry the current efficiency of the electrolytic process represented by eqn. (2) is usually estimated as 50 to 60Y0’ O. In this respect also the results of our similar investigations” are in good accord with the latter conclusionsbP1 O,Independently of whether the rate of hydrogen evolution is only equivalent to the secondary electrochemical reaction connected with the chemical amalgam dissolution according to eqns. (3a) and (3b) or is equal to the direct electrochemical water reduction. Taking into account the complexity of the electrode reactions accompanying the reduction of alkali metal ions it appears that the emphasized good accordance3.5 of experiments and the theory of the “exaltation of the migration currents” in such cases can be accidental only and due just to the secondary reactions. The more so, amongst others, because the rate of hydrogen evolution with decomposition of water can be strongly affected by the preceding reactions’ ‘, the paradoxical independence of the exaltation current of the ionic depolarizer concentration (emphasized by Heyrovsky et ~1.~) seems to be little probable, and the interpretation of the exaltation current observed in the presence of Tl+ and K + ions suggested by Kolthoff and Lingane’ seems to be, in fact, in direct contradiction with that suggested for this phenomenon by Heyrovsky and Bures2.3. Therefore, a verification of this theory in the case of other metals is the purpose of the following investigations. EXPERIMENTAL
The measurements
have been
made
applying
the standard
polarographic
THEORY
OF “EXALTATION”
359
CURRENTS
method with a dropping mercury electrode (DME) used as the cathode, and a bottom pool mercury electrode as the reference electrode. Other conditions were as described in the preceding paper’ ‘. Since for the final conclusions the heights of the compared waves of the succeeding reactions are more essential, the concentrations of the corresponding compounds were kept constant, whereas the concentrations of those involved in the preceding reactions are different and are given as approximate only. For instance, curve 2 (solution 2) in Fig. 2 was obtained after an addition by means of a micropipette of 0.10 ml of the solution of 0.0001 M KIO,+0.02 M Hg(CN), to 10.0 ml of 0.0001 M KIO, (solution 1). RESULTS
AND DISCUSSION
From Fig. 1 it is evident that the preceding reactions cause a considerable increase of the height of K + wave ( CJ the heights of the corresponding waves on curves 2, 3 and 4 with l), but it is negligible in the case of Cd*+ (cJ curve 5 with 4) or Zn*+ (cf: curve 4 with 3) waves. At first sight the observed exaltation current appears to be in accord with that predicted by eqn. (1) although only in the case of K+ waves. Thus, the calculated exaltation of the current is iex.ca,c= i,& i /iso: = 13 x 64.5/68.3 = 12.4 and the observed value is iex,obs= i,.k2 - i,.K1= 42 - 32 = 10 for curves 1 and 2. We shall perform the calculations of the exaltation of the currents in the presence of some other ions in the same way as Kolthoff and Lingane5 who found a very good agreement of the theory* with experiment. In the case of cation reduction the limiting current i, is equal to the sum of the diffusion current i, and the migration current i,, in the absence of foreign electrolytes
(5)
i, = i, + i, I
,
0
-0.0
-1.6
Fig. 1. Comparison of the effect waves. f 1) Solution of 0.0001 M ZnSO,: (4) solution 3+0.0002 arrows denote the heights of the
E/V (BME)
-2.4
-3.2
of preceding reactions on potassium, zinc and cadmium polarographic K,SO,; (2) solution 1+0.0002 M Hg(CN),; (3) solution 1 +O.OOOl M M CdSO,; (5) solution 4+0.0002 M Hg(CN),. The numbers at the waves in comparative units.
360
E. B. WERO$SKI,
J. CZERNIK
The migration current can then be determined by the relation i, = i, T+
(6)
where T+ = l+/(A+ + A_) is the transference number of the reducible cations. In the solution of K,S04 T+ is equal to 0.486x0.49, hence from eqn. (6) and the curve 1 we obtain &= 32 x 0.49 m 16 and further from (5) i,, x 16 for all the curves in Fig. 1. In solution 3 (curve 3) containing equal concentrations of K,SO, and ZnS04 the transference numbers of K+, Zn2+ and SOi- are 64.5/(45.0+ 64.5 + 2 x 68.3) = 64.5/246.1= 0.262, (45/246.1) = 0.183 and ( 136.6/246.1) = 0.555, respectively. Since the total limiting current is 42+ lO= 52, and according to Kolthoff and Lingane’ the apparent increase in the i,,, current should be due both to the exaltation of the K+ ion migration current iex.k+ and to the exaltation of the migration current i,,. Zn~ of Zn2+ ions when they are discharging simultaneously with K+ ions, their individual contributions are: iex.K+=0.262x 52= 13.5 and iex,Znl+ =0.183 x 52=9.5, giving for the total migration current itm= 23 instead of 52- (16+ lo)= 26. The agreement is still reasonable. Analogous calculations performed for 0.001 M KC1 without (curve 4) and with 0.001 M HCl (curve 5 in Fig. 2) give &+ =309, ‘ex+K+ = 63 and it,,,= 372 instead of the experimental value of it,,,= 294, and for b.0095 M KC1 and 0.0048 M HCI (evaluated from Fig. 2 in ref. 1) i,,=48 instead of 42. Thus, these results obtained under so different conditions, independently of the discrepancies, roughly seem to confirm the conclusions of the authors’. However, according to the interpretation of this phenomenon suggested by Heyrovsky et ~1.~ the increase in i, should be proportional to the increase in the ohmic potential drop i,R in the electrolyte. With the increase of electrolyte concentration the current i, increases but the resistance R of the electrolyte decreases. Hence, the effect of a preceding reaction of a non-electrolyte should be large compared with that of an electrolyte, both giving the same current value for the preceding wave. In fact rather a reverse phenomenon is observed (cJ also curves 2 and 3 with 1 in Fig. 1). The reduction of Cd2+ ions was repeated in the absence of any succeeding
I
cl
I
I
I
-0.8
/,,,/,,/,,
-1.6
E/V(BME)
-2.4
-3
Fig. 2. Comparison of the effect of preceding reactions on cadmium and potassium polarographic waves. (1) Solution of 0.0001 A4 CdS04; (2) solution 1 f0.0002 M Hg(CN),; (3) solution 1 +O.OOOl M CUSO,; (4) solution of 0.001 A4 KCl; (5) solution 4+0.001 A4 HCl. The scale for curves 4 and 5 should be about l4-fold of that shown in the Figure.
THEORY
OF “EXALTATION”
361
CURRENTS
reactions (Fig. 2). No exaltation of the Cd2+ current is observed either in the case of the preceding reduction of Hg(CN), or of Cu 2f ions (cf. curves 2 and 3 with 1). In the former case it should be according to the theory2-’ (eqn. 1): i,, ca,c= 13 x 45.1168.3 = 8.6 and is ,iex.obs --0. The difference is too large to be in the limits of a usual experimental error in the applied method. On the other hand, the wave appearing at more negative potentials (at - 2.1 V, curve 1) and reminiscent of a K+ wave, although no other reducible ions except of Cd2+ ions should be present, seems to confirm the “paradoxical independence of the exaltation current of the ionic depolarizer concentration” suggested by Heyrovsky et ~1.~. Some traces of alkali metal ions could be extracted from the glass vessel but this effect is probably negligible. But it can be due also to an accelerating effect of the amalgam and of the electrolyte on the electrochemical decomposition of water6.8’“. In Fig. 3 the two reduction waves of KI03 are represented. In the reaction of IO; 6 electrons are involved, in the following reaction only 1 per KIO, molecule, but both waves are almost equal (curve 1). Taking into account that A,+ =64.5 is about twice that of &, = 33.9, the K+ wave is still about 3 times too high. The more so, as in the first reaction IO; +6e+3H20
--f II+60H-
(7)
reduction of each IO; anion produces 7 other anions, and in accord with the theory3-’ the anions produced should strongly diminish the exaltation current of K+, similarly as was explained in the case of reduction of K+ ions in the presence of a preceding wave of oxygei?. In the presence of the preceding reactions of Cd2+ ions(curve 2)or Hg(CN),( curve 3) the increase of K+ wave is about 9 (c$ curve 2 with 1) or 4.5 times (cj. curve 3 with l), respectively, greater than the corresponding decrease of the IO; waves. In addition, the exaltation current of K+ ions, despite the counteraction of the simultaneously produced anions I- and OH-, is practically the same as in the case of their absence (cf. iex.K+ on curves 2 and 3 in Fig. 3 with + on curves 2 and 3 in Fig. 1). Thus, the small decrease of IO; waves in the i;e:ence of Cd’+ or Hg(CN), can be explained rather by electrostatic interaction
)
-0.6
-1.6
EI"mlE~-2A
-3.2
Fig. 3. Comparison of the effect of preceding reactions on potassium and iodate polarographic waves. (1) Solution of 0.0001 M KIO,; (2) solution 1 +O.OOOl M CdSO,; (3) solution 1+0.0002 M Hg(CN),.
362
E. B.
WERONSKI.
J. CZERNIK
between IO; and the anionic products being in 7 times larger concentration in the reaction layer. Although the theoretical verification of eqn. (1) derived by Heyrovsky et a1.2.3 and the explanation of the mechanism of formation of the exaltation of migration currents given by Von Stackelberg4 seems to be convincing if the attention is limited to the aspects there considered, similarly as the results of the verifying experiments and comments of Kolthoff and Lingane5 and some results of our experiments, in fact the theory is incorrect. Roughly it seems to be valid only in the case of reactions accompanied by some secondary processes, such as those in the case of electroreduction of the most chemically active metals reacting with water, but which are accelerated by the ions present in the solution and also by the ions involved in the preceding electrochemical reactions”. Thus the increase of the current can be explained by decomposition of the amalgam by water and the concomitant secondary reactions6p9,1’, still rather unknown by the authors2-5. Probably therefore nobody could observe the exaltation currents of trace contaminations of all the metals less chemically active than alkali metals and which should be, according the theory3.4, independent of their concentrations and therefore the exaltation currents should mask completely any electrochemical process in the absence of a supporting electrolyte. Similarly, as no exaltation was observed in the presence of Hg(CN), or CuSO, and of CdSO, (cf: curves 4 and 5 in Fig. 1 and 2 and 3 with 1 in Fig. 2). Before our investigations for a long time no other investigations (to our knowledge) of the exaltation phenomena have been undertaken, despite of the suggestions of Von Stackelberg4. Probably they were assumed as explained already and estimated as more of theoretical than practical value5. Here and in the previous investigations’ i, inspired by the results of the investigations of other authors’~“.“, it is proved that a correct interpretation of the phenomena can be of considerable practical importance for industrial electrochemistry of alkali metals. REFERENCES 1 2 3 4 5 6 7 8 9 IO 1I 12
W. Kemula and M. Michalski, Roczn. Chem., 16 (1936) 535. J. Heyrovsky and M. Bures, Collect. Czech. Chem Commun., 8 (1936) 446. J. Heyrovsky and J. Kdta, Grundlugen der Polaroyraphie, Akademie-Verlag, Berlin, 1965, pp. 53 55. M. von Stackelberg, Z. Elektrochern. Anger. Phys. Chem., 45 (1939) 466. I. M. Kolthoff and J. J. Linganq, J. Amer. Chem. Sot., 61 (1939) 1045. R. M. Reeves, M. Sluyters-Rehbach and J. H. Sluyters, J. ElectroanaL Chew., 34 (1972) 55. R. M. Reeves, M. Sluyters-Rehbach and J. H. Sluyters, J. Elecrroanal. Chem., 36 (1972) 287. R. M Reeves, M. Sluyters-Rehbach and J. H. Sluyters, J. Electrout&. Chem., 34 (1972) 69. R. M. Reeves, M. Sluyters-Rehbach and J. H. Sluyters, J. Electroanul. Chew, 36 (1972) 101. C. L. Mantell, Electrochemicul Engineering, McGraw-Hill, New York, 4th edn.. 1960. chapt. XI. E. B. Weronski and J. Szewczyk, J. Elecrroanul. Chem., 60 (1975) 197. A. Frumkin, V. Korshunov and I. Bagotzkaya, Elecrrochim. Actu, 15 (1970) 289.
357
REEXAMINATION OF THE THEORY OF EXALTATION OF THE MIGRATION CURRENT BY PRECEDING ELECTRODE REACTIONS
E. B. WEROI%KI
and J. CZERNIK
Department of Physical Chemistry, Polytechnic of Warsaw, Warsaw (Poland) (Received
16th December
1974; in revised form 18th March
1975)
ABSTRACT
The effect of the preceding current of Hg(CN),, CdS04, ZnSO,, and HCl on the K+ wave, of Hg(CN)2 and CuS04 on the Cd’+ wave, of CdS04 on the Zn2+ wave, and of Hg(CN), and CdS04 on the IO; wave was investigated. No exaltation of the Cd2+ wave was observed and only small variations of the Zn2+ and IO; waves. The increase of the K+ waves was roughly in accord with the prediction of the theory of the so-called exaltation of migration currents by preceding electrochemical reactions. However, the exaltation can be explained by concomitant reactions of K-amalgam with water and electrochemical reduction of hydrogen, which are accelerated by other ions (and also by the ions involved in the preceding reactions) and which, consequently, invalidate the equation of the exaltation of migration currents. INTRODUCTION
In the case of polarographic reduction of K+ ions occurring in the absence of a supporting electrolyte Kemula and Michalski’ observed an increase of the limiting current of K+ in the presence of the preceding reduction of oxygen or H+ ions. They called this increase an “exaltation of limiting currents”. Heyrovsky and Bures2 extended the experiments and obtained the exaltation effect with K+, Na+, Ba2+ and Mn’+ ions in the presence of oxygen or quinone. They explained it on the basis of theoretically derived quantitative relations as an “exaltation of the migration currents of the ions”. The exaltation i,, was found to be independent of the concentration of the reduced ions and was expressed by the current of the preceding reaction, ipr,multiplied by the ratio of the equivalent conductances of the ions2v3, e.g.: i,, = i,J+/A_
= i,,A,lA_
(1)
where io, is the limiting current of oxygen, and A+ and i- are the equivalent conductances of the reducible cations and anions, respectively. Von Stackelberg4 derived the relations in another way and suggested, as did Kolthoff and Lingane5, a correction in the calculations for the produced OH- ions. Because of some discrepancies between the experimental and the calculated values he suggested further investigations of this phenomenon. On the contrary, Kolthoff and Lingane5 found in their verifying experiments a very good agreement between the experi-
E. B. WEROlbKI,
358
J. CZERNIK
mentally determined and the theoretically calculated (by means of the Heyrovsky’s and Bures’ relation?) exaltation currents of K’ ions in the presence of oxygen and Tlf ions. In the latter case the total exaltation was ascribed to the increase in the migration current both of Tl+ and K’ ions, i,,,, = i,,. -,.,I + iex,K +. Reeves et ~1.~~~ investigated the electrochemical reduction of alkali metal ions at the dropping mercury electrode. They found that the primary electrochemical reduction of cations, e.~.~.’ : K+ +Hg+e was accompanied K(Hg)+H,O
-+ K(Hg), by a secondary
(2) chemical
reaction
of dissolution
of the alkali metal
+ K++(1/2)H2+OH-,
(34
or K(Hg)+H+
+ K++(l/2)H,,
(3b)
or by an electrochemical reduction of water catalysed by the amalgam’. Consequently, the secondary reactions increase directly (by the electrochemical reduction of water) or indirectly (by additional reduction of those K+ ions which are produced by the dissolution of the amalgam, with hydrogen evolution) the observed total current i,. The total current was determined as the sum of a contribution ialk due to the primary reduction of the alkali metal ions and of a contribution in to the hydrogen evolution currenP.‘. i, = ialk + i,
(4)
The values of ialk and i, were of the same order of magnitude (e.g., the ratio i,/i, was found 0.48-0.75)6.9. In industrial electrochemistry the current efficiency of the electrolytic process represented by eqn. (2) is usually estimated as 50 to 60Y0’ O. In this respect also the results of our similar investigations” are in good accord with the latter conclusionsbP1 O,Independently of whether the rate of hydrogen evolution is only equivalent to the secondary electrochemical reaction connected with the chemical amalgam dissolution according to eqns. (3a) and (3b) or is equal to the direct electrochemical water reduction. Taking into account the complexity of the electrode reactions accompanying the reduction of alkali metal ions it appears that the emphasized good accordance3.5 of experiments and the theory of the “exaltation of the migration currents” in such cases can be accidental only and due just to the secondary reactions. The more so, amongst others, because the rate of hydrogen evolution with decomposition of water can be strongly affected by the preceding reactions’ ‘, the paradoxical independence of the exaltation current of the ionic depolarizer concentration (emphasized by Heyrovsky et ~1.~) seems to be little probable, and the interpretation of the exaltation current observed in the presence of Tl+ and K + ions suggested by Kolthoff and Lingane’ seems to be, in fact, in direct contradiction with that suggested for this phenomenon by Heyrovsky and Bures2.3. Therefore, a verification of this theory in the case of other metals is the purpose of the following investigations. EXPERIMENTAL
The measurements
have been
made
applying
the standard
polarographic
THEORY
OF “EXALTATION”
359
CURRENTS
method with a dropping mercury electrode (DME) used as the cathode, and a bottom pool mercury electrode as the reference electrode. Other conditions were as described in the preceding paper’ ‘. Since for the final conclusions the heights of the compared waves of the succeeding reactions are more essential, the concentrations of the corresponding compounds were kept constant, whereas the concentrations of those involved in the preceding reactions are different and are given as approximate only. For instance, curve 2 (solution 2) in Fig. 2 was obtained after an addition by means of a micropipette of 0.10 ml of the solution of 0.0001 M KIO,+0.02 M Hg(CN), to 10.0 ml of 0.0001 M KIO, (solution 1). RESULTS
AND DISCUSSION
From Fig. 1 it is evident that the preceding reactions cause a considerable increase of the height of K + wave ( CJ the heights of the corresponding waves on curves 2, 3 and 4 with l), but it is negligible in the case of Cd*+ (cJ curve 5 with 4) or Zn*+ (cf: curve 4 with 3) waves. At first sight the observed exaltation current appears to be in accord with that predicted by eqn. (1) although only in the case of K+ waves. Thus, the calculated exaltation of the current is iex.ca,c= i,& i /iso: = 13 x 64.5/68.3 = 12.4 and the observed value is iex,obs= i,.k2 - i,.K1= 42 - 32 = 10 for curves 1 and 2. We shall perform the calculations of the exaltation of the currents in the presence of some other ions in the same way as Kolthoff and Lingane5 who found a very good agreement of the theory* with experiment. In the case of cation reduction the limiting current i, is equal to the sum of the diffusion current i, and the migration current i,, in the absence of foreign electrolytes
(5)
i, = i, + i, I
,
0
-0.0
-1.6
Fig. 1. Comparison of the effect waves. f 1) Solution of 0.0001 M ZnSO,: (4) solution 3+0.0002 arrows denote the heights of the
E/V (BME)
-2.4
-3.2
of preceding reactions on potassium, zinc and cadmium polarographic K,SO,; (2) solution 1+0.0002 M Hg(CN),; (3) solution 1 +O.OOOl M M CdSO,; (5) solution 4+0.0002 M Hg(CN),. The numbers at the waves in comparative units.
360
E. B. WERO$SKI,
J. CZERNIK
The migration current can then be determined by the relation i, = i, T+
(6)
where T+ = l+/(A+ + A_) is the transference number of the reducible cations. In the solution of K,S04 T+ is equal to 0.486x0.49, hence from eqn. (6) and the curve 1 we obtain &= 32 x 0.49 m 16 and further from (5) i,, x 16 for all the curves in Fig. 1. In solution 3 (curve 3) containing equal concentrations of K,SO, and ZnS04 the transference numbers of K+, Zn2+ and SOi- are 64.5/(45.0+ 64.5 + 2 x 68.3) = 64.5/246.1= 0.262, (45/246.1) = 0.183 and ( 136.6/246.1) = 0.555, respectively. Since the total limiting current is 42+ lO= 52, and according to Kolthoff and Lingane’ the apparent increase in the i,,, current should be due both to the exaltation of the K+ ion migration current iex.k+ and to the exaltation of the migration current i,,. Zn~ of Zn2+ ions when they are discharging simultaneously with K+ ions, their individual contributions are: iex.K+=0.262x 52= 13.5 and iex,Znl+ =0.183 x 52=9.5, giving for the total migration current itm= 23 instead of 52- (16+ lo)= 26. The agreement is still reasonable. Analogous calculations performed for 0.001 M KC1 without (curve 4) and with 0.001 M HCl (curve 5 in Fig. 2) give &+ =309, ‘ex+K+ = 63 and it,,,= 372 instead of the experimental value of it,,,= 294, and for b.0095 M KC1 and 0.0048 M HCI (evaluated from Fig. 2 in ref. 1) i,,=48 instead of 42. Thus, these results obtained under so different conditions, independently of the discrepancies, roughly seem to confirm the conclusions of the authors’. However, according to the interpretation of this phenomenon suggested by Heyrovsky et ~1.~ the increase in i, should be proportional to the increase in the ohmic potential drop i,R in the electrolyte. With the increase of electrolyte concentration the current i, increases but the resistance R of the electrolyte decreases. Hence, the effect of a preceding reaction of a non-electrolyte should be large compared with that of an electrolyte, both giving the same current value for the preceding wave. In fact rather a reverse phenomenon is observed (cJ also curves 2 and 3 with 1 in Fig. 1). The reduction of Cd2+ ions was repeated in the absence of any succeeding
I
cl
I
I
I
-0.8
/,,,/,,/,,
-1.6
E/V(BME)
-2.4
-3
Fig. 2. Comparison of the effect of preceding reactions on cadmium and potassium polarographic waves. (1) Solution of 0.0001 A4 CdS04; (2) solution 1 f0.0002 M Hg(CN),; (3) solution 1 +O.OOOl M CUSO,; (4) solution of 0.001 A4 KCl; (5) solution 4+0.001 A4 HCl. The scale for curves 4 and 5 should be about l4-fold of that shown in the Figure.
THEORY
OF “EXALTATION”
361
CURRENTS
reactions (Fig. 2). No exaltation of the Cd2+ current is observed either in the case of the preceding reduction of Hg(CN), or of Cu 2f ions (cf. curves 2 and 3 with 1). In the former case it should be according to the theory2-’ (eqn. 1): i,, ca,c= 13 x 45.1168.3 = 8.6 and is ,iex.obs --0. The difference is too large to be in the limits of a usual experimental error in the applied method. On the other hand, the wave appearing at more negative potentials (at - 2.1 V, curve 1) and reminiscent of a K+ wave, although no other reducible ions except of Cd2+ ions should be present, seems to confirm the “paradoxical independence of the exaltation current of the ionic depolarizer concentration” suggested by Heyrovsky et ~1.~. Some traces of alkali metal ions could be extracted from the glass vessel but this effect is probably negligible. But it can be due also to an accelerating effect of the amalgam and of the electrolyte on the electrochemical decomposition of water6.8’“. In Fig. 3 the two reduction waves of KI03 are represented. In the reaction of IO; 6 electrons are involved, in the following reaction only 1 per KIO, molecule, but both waves are almost equal (curve 1). Taking into account that A,+ =64.5 is about twice that of &, = 33.9, the K+ wave is still about 3 times too high. The more so, as in the first reaction IO; +6e+3H20
--f II+60H-
(7)
reduction of each IO; anion produces 7 other anions, and in accord with the theory3-’ the anions produced should strongly diminish the exaltation current of K+, similarly as was explained in the case of reduction of K+ ions in the presence of a preceding wave of oxygei?. In the presence of the preceding reactions of Cd2+ ions(curve 2)or Hg(CN),( curve 3) the increase of K+ wave is about 9 (c$ curve 2 with 1) or 4.5 times (cj. curve 3 with l), respectively, greater than the corresponding decrease of the IO; waves. In addition, the exaltation current of K+ ions, despite the counteraction of the simultaneously produced anions I- and OH-, is practically the same as in the case of their absence (cf. iex.K+ on curves 2 and 3 in Fig. 3 with + on curves 2 and 3 in Fig. 1). Thus, the small decrease of IO; waves in the i;e:ence of Cd’+ or Hg(CN), can be explained rather by electrostatic interaction
)
-0.6
-1.6
EI"mlE~-2A
-3.2
Fig. 3. Comparison of the effect of preceding reactions on potassium and iodate polarographic waves. (1) Solution of 0.0001 M KIO,; (2) solution 1 +O.OOOl M CdSO,; (3) solution 1+0.0002 M Hg(CN),.
362
E. B.
WERONSKI.
J. CZERNIK
between IO; and the anionic products being in 7 times larger concentration in the reaction layer. Although the theoretical verification of eqn. (1) derived by Heyrovsky et a1.2.3 and the explanation of the mechanism of formation of the exaltation of migration currents given by Von Stackelberg4 seems to be convincing if the attention is limited to the aspects there considered, similarly as the results of the verifying experiments and comments of Kolthoff and Lingane5 and some results of our experiments, in fact the theory is incorrect. Roughly it seems to be valid only in the case of reactions accompanied by some secondary processes, such as those in the case of electroreduction of the most chemically active metals reacting with water, but which are accelerated by the ions present in the solution and also by the ions involved in the preceding electrochemical reactions”. Thus the increase of the current can be explained by decomposition of the amalgam by water and the concomitant secondary reactions6p9,1’, still rather unknown by the authors2-5. Probably therefore nobody could observe the exaltation currents of trace contaminations of all the metals less chemically active than alkali metals and which should be, according the theory3.4, independent of their concentrations and therefore the exaltation currents should mask completely any electrochemical process in the absence of a supporting electrolyte. Similarly, as no exaltation was observed in the presence of Hg(CN), or CuSO, and of CdSO, (cf: curves 4 and 5 in Fig. 1 and 2 and 3 with 1 in Fig. 2). Before our investigations for a long time no other investigations (to our knowledge) of the exaltation phenomena have been undertaken, despite of the suggestions of Von Stackelberg4. Probably they were assumed as explained already and estimated as more of theoretical than practical value5. Here and in the previous investigations’ i, inspired by the results of the investigations of other authors’~“.“, it is proved that a correct interpretation of the phenomena can be of considerable practical importance for industrial electrochemistry of alkali metals. REFERENCES 1 2 3 4 5 6 7 8 9 IO 1I 12
W. Kemula and M. Michalski, Roczn. Chem., 16 (1936) 535. J. Heyrovsky and M. Bures, Collect. Czech. Chem Commun., 8 (1936) 446. J. Heyrovsky and J. Kdta, Grundlugen der Polaroyraphie, Akademie-Verlag, Berlin, 1965, pp. 53 55. M. von Stackelberg, Z. Elektrochern. Anger. Phys. Chem., 45 (1939) 466. I. M. Kolthoff and J. J. Linganq, J. Amer. Chem. Sot., 61 (1939) 1045. R. M. Reeves, M. Sluyters-Rehbach and J. H. Sluyters, J. ElectroanaL Chew., 34 (1972) 55. R. M. Reeves, M. Sluyters-Rehbach and J. H. Sluyters, J. Elecrroanal. Chem., 36 (1972) 287. R. M Reeves, M. Sluyters-Rehbach and J. H. Sluyters, J. Electrout&. Chem., 34 (1972) 69. R. M. Reeves, M. Sluyters-Rehbach and J. H. Sluyters, J. Electroanul. Chew, 36 (1972) 101. C. L. Mantell, Electrochemicul Engineering, McGraw-Hill, New York, 4th edn.. 1960. chapt. XI. E. B. Weronski and J. Szewczyk, J. Elecrroanul. Chem., 60 (1975) 197. A. Frumkin, V. Korshunov and I. Bagotzkaya, Elecrrochim. Actu, 15 (1970) 289.