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On the nature of a pre-step observed in the electroreduction of cobalt(II)-thiocyanate complexes
J. ElectroanaL Chem., 106 (1980) 11--22 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
11
ON THE NATURE OF A PRE-STEP OBSERVED IN THE ELECTROREDUCTION OF COBALT(II)-THIOCYANATE COMPLEXES
L A U R A JANISZEWSKA and ZBIGNIEW GALUS
Laboratory of Electroanalytical Chemistry, Institute of Fundamental Problems of Chemistry, University, 02093 Warsaw (Poland) (Received 6th April 1979; in revised form 9th July 1979)
ABSTRACT Using the method of stationary electrode voltammetry with hanging mercury drop and glassy carbon electrodes, the electroreduction of Co(II)-thiocyanate complexes occurring at smaller cathodic potentials than the formal potential of the Co(II)/Co(Hg) couple was studied. This process results in the formation of CoS and CN- ions, and occurs only on electrode surfaces modified by the presence of CoS with an admixture of metallic cobalt. The maximum efficiency of this reaction, calculated with respect to the charge passed through the cell, was found to be 75%, on the basis of macroelectrolysis carried out at a mercury pool cathode. The influence of various factors on the electroreduction of SCN- was also studied.
INTRODUCTION
The mechanism of the electroreduction of Co(II), and also of the subsequent oxidation of the cobalt deposits produced in thiocyanate solutions is complicated by the formation of CoS, which adsorbs on the electrode surfaces. The papers published up to now have not paid much attention to the role of CoS, which may be produced as a result of the electroreduction of SCN- ions in the course of the both cathodic and anodic processes [1--5]. Our earlier studies [6--8] have stressed the influence of CoS on amalgam formation, on crystal formation of cobalt at the electrode surfaces and on the rate of Co(II) electroreduction. Similarly, the electroreduction of Ni(II) in the presence of thiocyanates also proceeds with the formation of NiS and CN- ions [9]. At higher temperatures, direct electroreduction of SCN- ions was observed, at potentials less cathodic than the potential of electroreduction of the Ni(II)-thiocyanate complexes [10]. Barafski and Galus [11] have proposed two possible routes for the electroreduction of sulphur-containing ligands formed during the electroreduction of cations of transition metals. Recently Itabashi and coworkers [12,13] have suggested a mechanism of reduction of Co(II) in the presence of thiocyanates which involves the participation of cobalt sulphide. They explained the current observed at --0.6 V on cyclic voltammetric curves as due to electroreduction of HgS, which results
12 from transformation of CoS during the anodic polarisation of the electrode, which was initially kept at negative potentials. Our own explanation, described in the present paper, is different: the appearance of this current is thought to be related to the modification of the electrode caused b y the presence of metallic cobalt and cobalt sulphide on its surface. EXPERIMENTAL
Reagents N a S C N (produced by POCh) and NaCIO4 (produced by Merck) used as background electrolytes were p.a. reagents. Co(CIO4)2 (produced by Ventrom) was used for the Co(II) stock solution. All solutions were prepared with triply distilledwater. The second distillation of water was carried out from an alkaline solution of permanganate and the third from an all-quartz still. Mercury was twice distilledunder reduced pressure and chemically purified by prolonged shaking with a solution of Hg2(NO3)2 acidified with HNO3. All the solutions were carefully deoxygenated by bubbling electrolytically generated hydrogen, which was freed from traces of oxygen by passage through a palladium catalyst bed.
Apparatus Voltammetric, chronoamperometric and various microelectrolyses were carried o u t with a hanging mercury drop electrode (HMDE; surface area 1.5 × 10 -2 cm 2) or with glassy carbon electrodes (GCE; 7 × 10 -2 cm2). Before each experiment the GCE surface was chemically cleaned with a mixture of HC1 and HNO3, followed b y rubbing with c o t t o n wool. Macroelectrolysis were carried o u t on a large mercury pool cathode. In all experiments, a NaCl-saturated calomel electrode was used as reference electrode. A platinum wire, separated from the solution b y a sintered glass was used as an auxiliary electrode. The electrolysis cell equipped with a water jacket was kept at a constant temperature (if n o t stated otherwise at 25 + 0.2 ° C). Macroelectrolyses were performed at r o o m temperatures of the order of 20 ° C. Voltammetric curves were recorded on a Radelkis polarograph type OH-102. For macroelectrolysis, TE-1C Or Radelkis type OH-404/A potentiostats fed to an O H 404/C integrator were used. Ion-selective electrodes were used for the analytical determination of sulphide and cyanide ions. Spectra in the visible region were recorded with a Carl Zeiss U V Vis spectrophotometer. RESULTS The cyclic polarisation of the electrode in solutions of Co(II) in thiocyanate medium at 25°C results in the appearence of a prestep in the potential region --0.60 V to - 0 . 6 5 V, during the second and following cathodic runs. This additional current (ipd) is observed with b o t h HMDE and GCE, and does n o t appear
13
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Fig. 1. M u l t i c y c l i c v o l t a m m e t r i c c u r v e s o f 2 X 10 -3 M C o ( I I ) in 1.0 M N a S C N : (a) G C E , a n d ( b ) H M D E . T h e s e q u e n c e o f c u r v e s is i n d i c a t e d b y n u m b e r s : B e f o r e r e c o r d i n g c u r v e 20 t h e e l e c t r o d e was k e p t a t - - 0 . 1 0 V f o r 5 m i n .
during the first cathodic run. The potential of this pre-step is approximately 250 mV more positive than the potential of electroreduction of the Co(II)thiocyanate complexes at the HMDE. Deposition of CoS + Co is observed during the first cycle. There is no distinct anodic current which could correspond to these processes except for a small anodic step at potentials less negative than --0.6 V. Typical cyclic curves obtained with the use of the HMDE are shown in Fig. 1. In order to observe the pre-step, the cathodically deposited cobalt should be polarised anodically, at least for a short time. Moreover, the magnitude of this pre-step depends on several parameters, such as the potential and the duration of cobalt deposition (the charge passed during the cathodic process), the concentrations of Co(II) and thiocyanate, the potential and duration of the anodic oxidation o f deposited cobalt. These factors will be examined shortly.
Influence of the electroreduction potential For HMDE as well as GCE ipd decreases if the initial potential of electroIysis of Co(II)-thiocyanate complexes is more negative than that of electroreduction of CoS. This effect, which is more pronounced for HMDE than for GCE, is quantitatively illustrated b y the values given in Table 1. HMDE with 2 X 10 -3 M Co(II) in 1 M NaSCN and GCE with 10 -3 M Co(II) in 1 M NaSCN were used in
14 TABLE 1 D e p e n d e n c e o f ipd a n d ipd/ipc for Co(II) in t h i o c y a n a t e s o n e l e c t r o r e d u c t i o n p o t e n t i a l , charges passed in e l e c t r o r e d u c t i o n a n d t y p e o f e l e c t r o d e used Ered]V
HMDE Qred/PC
GCE ipd]PA
(ipd]ipc)]%
--1.00
--
--
--
--1.10
200 700 200 700 --
1.0 3.0 1.1 3.1 --
17 50 18 51 --
200 700
0.2 0.25
2.5 3.0
--1.20 --1.30 --1.40
Qred]PC
ipd]PA
(ipd]ipc)]%
350 1150 700 2100 850 2500 1000 3000 1500 3800
2.1 4.6 2.4 4.5 2.6 4.8 2.4 4.2 1.5 2.8
15 34 18 33 19 35 15 30 11 21
these experiments. Table 1 also shows the dependence of ipd on the charge passed during the electroreduction (Qr~d), as well as the ratio of ip~ to the main peak current of Co(II)-thiocyanate, ip¢. The decrease of ipd observed when the initial electroreduction is carried o u t at negative potentials is probably due to the electroreduction of CoS at such potentials. The deposited cobalt is then wetted b y mercury (in case of the HMDE) and the electrode surface is n o t modified. With GCE, CoS is also unstable on the electrode surface b e y o n d --1.40 V. However, deposition of metallic cobalt modifies the electrode properties to some extent. Larger Qred at reduction potentials more negative than --1.3 V results from the participation of hydrogen ions in the cathodic process. This side reaction m a y also change the properties of the electrode surface to some extent. On the basis of these results, one may conclude that a pre-peak is observed on the voltammetric curves only when CoS is cathodically deposited on the electrode surfaces.
Influence o f the Co(II) concentration In the cyclic voltammetric experiments presented the HMDE was polarized at a scan rate of 1.5 V/min from --0.5 V to --1.1 V, kept at this potential for some time and then scanned back to --0.10 V; ipd was recorded on the next cathodic scan. The concentration of thiocyanate ion was 5 X 10 -2 M. The influence of the time of cathodic electrolysis at --1.10 V and that of the Co(II) concentration on the ratio ipd/ipc are given in Table 2. The increase o f Co(II) concentration results in some decrease of ipd/ipc. Such behaviour was also found in studies carried o u t at 95°C.
Influence of the thiocyanate concentration The experiments were carried o u t in a manner analogous to that described above. The concentration of Co(II) was 2 × 10 -3 M and the concentration of
15 TABLE 2 D e p e n d e n c e o f ipd/ipc o n t h e Co(II) c o n c e n t r a t i o n a n d d u r a t i o n o f c a t h o d i c electrolysis ( t e l ( m i n ) ) at --1.1 V 103 X c o n c e n t r a tion of Co(II)/M
1 2 5 50
(ipd/ipc)/% tel = 0
tel = 0.5
tel = 1
tel = 2
tel = 4
6.5 5.0 4.0 3.0
6.5 6.0 5.0 .
10 10 8
15 14 11
27 23 20
.
.
.
thiocyanate was changed from 10 -3 M to 6.0 M. As in the previous section, the electrode was maintained at --1.10 V during various times (see Table 3), before reversing the polarization. During the anodic scan, the electrode was again polarised to - 0 . 1 0 V (--0.20 V in the case of 6 M NaSCN). The results obtained are given in Table 3. The increase of the pre-peak with concentration of thiocyanate is evident. This is probably due to larger amounts of CoS on the electrode surface. At very large thiocyanate concentrations (6.0 M) the current of the pre-peak decreases. For this solution, an additional very narrow peak appears on cyclic voltammetric curves at potentials less cathodic than the potential relative to the normal pre-peak. Its width at half-height was approximately equal to only 10 mV. One may assume that this new peak is due to the electroreduction of HgS, since it was never observed when using GCE.
Influence of the charge flowing during the electroreduction step (Q~) For 10 -3 M Co(II) solution in 1 M NaSCN, metallic cobalt and CoS were deposited at various potentials. The influence of the charge flowing at the electroreduction and d~position potential on the additional current (at a b o u t --0.6 V) is shown in Fig. 2. Voltammetric curves were recorded on GCE. The dependence o f ipd on the electroreduction charge is linear as long as the potential is n o t sufficient to reduce CoS. Since at --1.4 V CoS is n o t stable on
TABLE 3 D e p e n d e n c e o f ipd/ipc o n t h e c o n c e n t r a t i o n o f t h i o c y a n a t e and d u r a t i o n o f c a t h o d i c electrolysis, tel (rain) at - - 1 . 1 0 V Concentration of thiocyanate [SCN-]/M
(ipd/ipc)/% tel = 0
tel = 0.5
tel = 1.0
tel = 2.0
tel = 4.0
1 × 10 -3 5 × 10 -2 1 6
2.5 5.0 8.5 3.0
2.5 6.0 14 .
3.5 10 17
7 14 24
10 23 50
.
.
.
16 ipd/pA 2O16-
84-
260 4~50 660 ~o 1obo 12ooOr/pC Fig. 2. The dependence of ipd on the charge passed during electroreduction at different potentials: ~ --1.20 V, o --1.30 V and X--1.40 V. 10 -3 M Co(II) solution in 0.20 M NaSCN and 0.80 M NaCIO4; GCE. the electrode, after preliminary electroreduction at this potential ipd w a s relatively small and practically independent on time of electrolysis. Such behaviour is typical both for GCE and HMDE. The dependence of ipd on Qr (--1.1 V) for HMDE is given in Fig. 3. Similar measurements were also carried out at 60 and 95 ° C. Under these conditions, the ipd--Qr relationship was, in general, also linear. However, the deviations were larger. Inspection of all these results leads to the conclusion that the larger Qr the higher the additional current at --0.6 V. To test this conclusion for extended electroreduction times, measurements were carried out with 10 -3 M Co(II) in 1 M NaSCN at GCE with initial electrolysis times lasting up to 2 h. The oxidation of deposited cobalt was carried out by scanning the potential up - 0 . 1 0 V, where the electrode was kept for 5 min. The results obtained are shown in Table 4. Maximum values of/pd w e r e equal to the original cathodic current at - 0 . 9 0 V. When ipd was at its maximum, no current was observed at - 0 . 9 0 V nor any cobalt oxidation.
10-
a
86o
42-
46o s60 ~2bo ~6bo 2doo 2~0 28b0 3~00 3600 ~-l~,c Fig. 3. The d e p e n d e n c e o f ipd on the charge passed during e l e c t r o r e d u c t i o n at different potentials: L~ L~--1.0 V, O O --1.20 V, [] [] --1.40 V and × × results obtained f r o m multicyclic experiments. 2 × 10 -3 M Co(II) solution in 0.2 M NaSCN and 0.8 M NaC104; HMDE.
17 TABLE 4 T h e d e p e n d e n c e o f ipd, ip, tion
ipdlipc a n d ip/ipc o n
p o t e n t i a l , t i m e a n d charge o f e l e c t r o r e d u c -
Er/V
tr/h
Qr]~lC
ipd]PA
(ipd/ipc)]% ip]I2Aat
--1.10
0.17 0.25 0.50 1.0 • 1.5 2.0
1500 2500 5500 11000 16000 20000
12 15 21 22 22 22
54 70 95 100 100 100
--1.20
0.25 0.50 1.0 1.5
3500 6000 14000 19000
7.5 9.5 10.5 11.0
--1.30
0.25 0.50 1.0 1.5
7000 14000 21000 26000
3.0 4.0 3.5 4.0
--0.90 V
([p/ipc)]%
4 1 0 0 0 0
18 5 0 0 0 0
34 43 48 50
3.0 4.0 3.0 --
14 18 14 --
14 18 16 18
5.0 3.5 5.0 3.0
23 16 23 14
Values o f ipd exceeding the initial peak current at --0.9 V were obtained when the oxidation of deposited cobalt was carried out by polarising the electrode to more positive potential (+0.3 V). For Qr = 15 and 50 mC, ipd were found to be respectively 27 and 37 pA. It may be assumed that the increase of ipd with respect to the original peak current (equal under our experimental conditions to 22 pA) results from the partial discharge of hydrogen ions on the modified electrode surface. Under such conditions the cathodic current at - 0 . 9 V was still equal to 6.5 pA, independently of Qr. The initial electrolysis was carried out at --1.10 V.
Dependence o f ipd on temperature With increase o f temperature from 25 to 95°C the potential of the additional current ipd changes from --0.6 V to --0.3 V. A similar shift of potential is observed for the current appearing at greater cathodic potential. The ratio of ipd/ipc increases with temperature (Fig. 4).
Dependence o f Z~d on the anodic potential and time o f electrolysis at this po ten tial The values of ipd is dependent on the anodic potential ( E a n ) to which the electrode was polarised in the anodic reverse scan and also on the time the electrode was kept at this potential (to). With the HMDE, ipd only slightly increased with the increase of Ean, in the limited range of useful anodic potentials. Some results of such experiments a r e g i v e n in Table 5. For GCE, reproducibility is considerably lower than in HMDE. This m a y be due to irreproducibility in the state of the electrode surface before recording
18
i~,/ipc [%]
-5O
ipd/pA 12-
-3O
8-
-20
4-
-10
2:7
3:0
3:3
io 3 T-TK-T
Fig. 4. The dependence of ipd on 1/T and ipd/ipc on lIT for 2 X 10 -3 M Co(II) solution in 1.0 M NaSCN, HMDE. The charge passed in time of electroreduction was 2 0 0 p C (curves 1 and 1'), 4 0 0 p C (curves 2 a n d 2') a n d 6 0 0 pC (curves 3 and 3').
each curve. Despite this, an increase o f Ean and to with that electrode also leads to an increase of ipd. Results o f macroelectrolysis carried o u t on a large mercury p o o l cathode Solutions were stirred during these electrolyses, and subsequently analysed using electrochemical methods and spectrophotometry in the visible region. Sulphide and cyanide ions were detected and determined by using ion-selective electrodes. Electrolyses were carried out under various conditions, the initial solutions being always 10 -2 M with respect to Co(II) in 1 M NaSCN. For electrolysis performed at --0.9 to --1.0 V, CoS and metallic cobalt were observed on the electrode surface and S 2- and CN- anions and cyanide complexes of Co(II) were found in the solution. The presence of the later species was established on the basis of spectrophotometric measurements, though spectra differed to some extent from those relative to solutions of Co(II) added with CN- and S 2-. The concentrations of sulphide and cyanide ions were dependent on the charge passed. If the initial solution was 10 -3 M in Co(II) and 10 -1 M SCNions the concentrations of produced S 2- and CN- ions were respectively equal TABLE 5
Dependence of t0/min
0 2 4
ipd/ipc
on Ean and t o a
(ipd/ipc)/% Ean=--0.05V
Ean=----0.10V
Ean=--0.15V
Ean=--0.20V
6.0 7.0 7.5
5.0 6.0 --
4.5 6.5 6.5
4.0 5.0 6.0
a Results given in Table 5 were obtained for a 2 X 10 -3 M Co(II) solution in 5 × 10 -2 M NaSCN and 1.0 M NaC104. Cobalt was deposited for 2 min at - - 1 . 1 0 V.
19 t o 10 -3 and 10 -2 mmol 1-1 after passing 50C. The considerably lower concen-
tration of sulphide with respect to cyanides is probably due to precipitation of CoS. Additional experiments were also carried out in which the electrode surface was activated by short deposition of CoS and Co at --1.0 V. Subsequently, the potential was changed to -0.7 V. It was found that for charges up to 9 C, the concentration of Co(If) remained practicallyunaffected, even for electrolysis times as long as 6 h. Simultaneously the concentration of sulphides and cyanides increased with the total charge. Again the concentration of sulphide was approximately five times lower than that of cyanide. DISCUSSION The electroreduction process which is the main object of this work starts at potentials a b o u t --0.50 V and the peak current potential is near to --0.65 V (for 0.2 M thiocynate). This means that this process occurs at potentials approximately 250 mV more positive that the main process of electroreduction of thiocyanate-Co(II) complexes which brings the formation of metallic cobalt with small admixture of CoS. In the latter process, electroreduction of SCNto S 2- and CN- with simultaneous formation of CoS is only a side reaction, with an efficiency (calculated with respect to the total electroreduction charge) of the order of 10%. The significant potential difference between this process [6--8], and the new electroreduction wave observed in multicyclic experiments suggests that, in the potential range - 0 . 5 0 to --0.65 V of this wave, the relevant process is no longer electroreduction of Co(II) ions. The current for this process (ipd) is observed only if the concentration of Co(SCN) ÷ in the bulk of the solution exceeds 10 -s M. This leads to the conclusion that Co(II)-thiocyanate complexes are involved. Interaction of hydrated Co(II) ions with adsorbed SCN- ions does n o t seem to be a major factor. The activated complex (Electrode)--Hg--SCN--Co would indeed n o t favour the formation of CoS, because adsorbed SCN- interacts with mercury through its sulphur end. Comparative experiments have been carried o u t with HMDE and GCE. The fact that currents at - 0 . 6 5 V are larger for GCE at which interaction with thiocyanate ions should be considerably weaker, independently confirms our suggestion. An additional p r o o f that Co(II)-thiocyanate complexes participate in the electrode reaction in the potential range - 0 . 5 0 to - 0 . 6 5 V is afforded by the following observations. The voltammetric curves recorded after prolonged reduction of 2 × 10 -3 M solution of Co(II) in 1 M NaSCN (1 h for GCE and 5 min for HMDE), followed b y the oxidation of the solution after addition of Co(II), showed the increase of the cathodic current at - 0 . 6 5 V. This increase was proportional to the concentration of added Co(II), e.g. to the concentration of Co(II)-thiocyanate complexes. The electroreduction o f Co(II)-SCN- complexes within the potential range - 0 . 5 0 to - 0 . 6 5 V does n o t result in formation of metallic cobalt. According to Itabashi [ 12,13 ] in that range, there is electroreduction of HgS, which is formed in the course of the anodic polarisation of the electrode,
20 as a result of the reaction: CoS + Hg --2 e -~ HgS + Co 2+
(1)
Contrary to this suggestion, we think that the process m a y be schematically described b y the following equation: Co(SCN)~ -n + 2 e -~ CoS + CN- + (n -- 1) SCN-
(2)
This suggestion is supported b y the following experimental facts: (1) There is no cobalt oxidation peak current which would correspond to reduction in the potential range considered. This may suggest either that cobalt is not reduced in the cathodic process or that its oxidation is strongly inhibited. The first of these possibilities seems to be the more probable. The height of the anodic peak for cobalt oxidation decreases, while on the multicyclic mode a significant current is recorded at --0.65 V. This indicated that the product formed in the peak is n o t metallic cobalt. (2) We have found that neither mercury nor its ions (including HgS) participate in this process, since cyclic curves recorded with both HMDE and GCE are identical in shape and position. (3) When the peak height at --0.65 V increases, a simultaneous decrease of the main peak at --0.97 V is observed. The potential of the latter peak shifts with each subsequent cycle to less negative values up to a limiting value of the order of --0.80 V. In contrast the potential of a less negative peak is practically independent o f the number o f cycles. The Sum of the heights of b o t h peaks is very nearly constant and equal to the primary reduction peak --0.97 V. Since the maximum amplitude of the peak at - 0 . 6 5 V is equal to that of the primary cathodic peak, one may deduce that its values are also controlled by the diffusion of the Co(II)-thiocyanate complexes. Electroreduction at - 0 . 6 5 V requires that an a m o u n t of CoS has to be present on the electrode surface. While the primary electroreduction peak current at --0.97 V is dependent on the cathode material (mercury, glassy carbon) the peak potential at - 0 . 6 5 V is the same for both types of electrodes (in 0.2--1.0 M NaSCN). This may be explained b y the fact that both electrodes have acquired similar surface properties after deposition of Co + CoS. It seems that the presence of metallic cobalt is not a prerequisite. In fact, the additional process is favoured b y its oxidation. However, this conclusion may n o t be well founded, because the anodic current of the cobalt oxidation is large and interferes with the relatively small cathodic current ipd. One may deduce that CoS, possibly with some additional Co, acts as a semiconducting catalyst for reaction (2). The formation of CoS and CN- at the potential - 0 . 6 5 V is confirmed b y prolonged electrolysis. For cathodic charges <4 C the efficiency of the CNformation is equal, a b o u t 50%. For charges >7 C, the efficiency of this reaction drops to 25%. This decrease is probably due to some losses of cyanides at the longer time. Efficiencies <100% may also partly result from the oxidation of Co(II) cyanide complexes to Co(III). Since the latter complex is very stable, the determination of these fractions o f cyanide b o u n d to Co(III) may be difficult.
21
After this complex was decomposed by heating of alkalised solution for 1 h, the efficiency of cyanide formation rose to 75%. These efficiencies are similar for both GCE and HMDE. One m a y add that similar behaviour and conclusions have also been f o u n d for Ni(II) in thiocyanate medium [9,10]. The ratio of peak current at --0.65 V to the original peak at more negative values is dependent on the concentration of SCN- anions in the solution, as follows from data given in Table 3. Such a dependence, calculated with respect to the mean coordination number is presented in Fig. 5. The efficiency of S2- and CN- production increases to a mean coordination number ~2.5 and later decreases. This may suggest that complexes with coordination numbers 2 and 3 with respect to SCN- ions are responsible for this additional current at less negative potentials. It is n o t clear h o w the electrode reaction proceeds. It is known that in the Co(II)-thiocyanate complex, the Co--N bond predominates. It may be postulated that the reaction proceeds through an intermediate form (SCN),-Co-SCNCo(SCN)m. There is some evidence for the formation of a similar species in case of the Ni(II)-thiocyanate system. It is possible that an intermediate complex of Co(I) or Co(0) could be formed. For such complexes, the tendency to form a
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on the average c o o r d i n a t i o n number. 10 -3 M Co([[).
Fig. 6. M u l t i e y c l i c v o l t a m m e t r i e curves o f 2 x 10 -3 M Co(H) in 6.0 M NaSCN, recorded w i t h H M D E and GCE. The sequence o f curves is indicated by numbers and the scale o f current given by arrows.
i .
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22
Co--S bond, and in consequence, cobalt sulphide and cyanide ions should be favoured. It seems that it is only in the case of concentrated thiocyanate solutions (6 M) that mercury ions participate in the reaction. Typical multicycle voltammetric curves for 2 × 10 -3 M Co(II) in 6 M NaSCN recorded with GCE and HMDE are shown in Fig. 6. With HMDE, very sharp peaks are observed at --0.6 V, only when the electrode is polarised to potentials close to 0 V during the anodic run. The charges corresponding to these peaks do not exceed the charges of a monolayer. Curves of similar shape with charge corresponding to one monolayer were also observed when HMDE is polarized in sulphide solutions. The electroreduction potential of these peaks corresponds to the peak potential of reduction of HgS observed in the Ni(II)-NCS- system [9]. At potentials more negative than these sharp peaks, the additional step current (ipd) discussed earlier in this work is observed. REFERENCES
1 2 3 4 5 6 7 8 9 10 11 12 13
S.A. Perone and W.F. Gutknecht, Anal. Chem., 39 ( 1 9 6 7 ) 8 9 2 . E. E r i k s r u d , J. E l e c t r o a n a l . Chem., 36 ( 1 9 7 2 ) 1 8 9 . O.E. Ruvinskij, J.I. Turyan and LA. Sidorenko, E l e k t r o k h i m i y a , 10 ( 1 9 7 4 ) 4 6 7 . G.N. Babkin, N.V. K o p y t o v a a n d A . F . O m a r o v a , E l e c t r o k h i m i y a , 9 ( 1 9 7 3 ) 4 4 3 . G.N. Babkin, K.I. G e y n r i k s , N.V. K o p y t o v a a n d V.B. Lebiediev, Trans. Inst. K h i m . N a u k . Kaz. S.S.R., 1 4 0 ( 1 9 6 1 ) 1 3 6 8 . L. Janiszewska and Z. Galus, Chem. Anal., 17 ( 1 9 7 2 ) 6 9 1 . L. Janiszewska and Z. Galus, R o c z . C h e m . , 49 ( 1 9 7 5 ) 2 4 9 . L. Janiszewska and Z. Galus, R o c z . C h e m . , 51 ( 1 9 7 7 ) 1 8 3 3 . T. K r o g u l e c , A. BaraSski a n d Z. Galus, J. E l e c t r o a n a l . C h e m . , 57 ( 1 9 7 4 ) 63. T. K r o g u l e c , Doctoral Thesis, University of Warsaw, 1 9 7 7 . A. BaraSski a n d Z. Galus, J. E l e c t r o a n a l . C h e m . , 75 ( 1 9 7 7 ) 6 1 3 . E. Itabashi and M. Tan, J. E l e c t r o a n a l . C h e m . , 6 0 ( 1 9 7 5 ) 2 9 9 . E. I t a b a s h i , J. E l e c t r o a n a l . Chem., 88 ( 1 9 7 8 ) 2 0 5 .
11
ON THE NATURE OF A PRE-STEP OBSERVED IN THE ELECTROREDUCTION OF COBALT(II)-THIOCYANATE COMPLEXES
L A U R A JANISZEWSKA and ZBIGNIEW GALUS
Laboratory of Electroanalytical Chemistry, Institute of Fundamental Problems of Chemistry, University, 02093 Warsaw (Poland) (Received 6th April 1979; in revised form 9th July 1979)
ABSTRACT Using the method of stationary electrode voltammetry with hanging mercury drop and glassy carbon electrodes, the electroreduction of Co(II)-thiocyanate complexes occurring at smaller cathodic potentials than the formal potential of the Co(II)/Co(Hg) couple was studied. This process results in the formation of CoS and CN- ions, and occurs only on electrode surfaces modified by the presence of CoS with an admixture of metallic cobalt. The maximum efficiency of this reaction, calculated with respect to the charge passed through the cell, was found to be 75%, on the basis of macroelectrolysis carried out at a mercury pool cathode. The influence of various factors on the electroreduction of SCN- was also studied.
INTRODUCTION
The mechanism of the electroreduction of Co(II), and also of the subsequent oxidation of the cobalt deposits produced in thiocyanate solutions is complicated by the formation of CoS, which adsorbs on the electrode surfaces. The papers published up to now have not paid much attention to the role of CoS, which may be produced as a result of the electroreduction of SCN- ions in the course of the both cathodic and anodic processes [1--5]. Our earlier studies [6--8] have stressed the influence of CoS on amalgam formation, on crystal formation of cobalt at the electrode surfaces and on the rate of Co(II) electroreduction. Similarly, the electroreduction of Ni(II) in the presence of thiocyanates also proceeds with the formation of NiS and CN- ions [9]. At higher temperatures, direct electroreduction of SCN- ions was observed, at potentials less cathodic than the potential of electroreduction of the Ni(II)-thiocyanate complexes [10]. Barafski and Galus [11] have proposed two possible routes for the electroreduction of sulphur-containing ligands formed during the electroreduction of cations of transition metals. Recently Itabashi and coworkers [12,13] have suggested a mechanism of reduction of Co(II) in the presence of thiocyanates which involves the participation of cobalt sulphide. They explained the current observed at --0.6 V on cyclic voltammetric curves as due to electroreduction of HgS, which results
12 from transformation of CoS during the anodic polarisation of the electrode, which was initially kept at negative potentials. Our own explanation, described in the present paper, is different: the appearance of this current is thought to be related to the modification of the electrode caused b y the presence of metallic cobalt and cobalt sulphide on its surface. EXPERIMENTAL
Reagents N a S C N (produced by POCh) and NaCIO4 (produced by Merck) used as background electrolytes were p.a. reagents. Co(CIO4)2 (produced by Ventrom) was used for the Co(II) stock solution. All solutions were prepared with triply distilledwater. The second distillation of water was carried out from an alkaline solution of permanganate and the third from an all-quartz still. Mercury was twice distilledunder reduced pressure and chemically purified by prolonged shaking with a solution of Hg2(NO3)2 acidified with HNO3. All the solutions were carefully deoxygenated by bubbling electrolytically generated hydrogen, which was freed from traces of oxygen by passage through a palladium catalyst bed.
Apparatus Voltammetric, chronoamperometric and various microelectrolyses were carried o u t with a hanging mercury drop electrode (HMDE; surface area 1.5 × 10 -2 cm 2) or with glassy carbon electrodes (GCE; 7 × 10 -2 cm2). Before each experiment the GCE surface was chemically cleaned with a mixture of HC1 and HNO3, followed b y rubbing with c o t t o n wool. Macroelectrolysis were carried o u t on a large mercury pool cathode. In all experiments, a NaCl-saturated calomel electrode was used as reference electrode. A platinum wire, separated from the solution b y a sintered glass was used as an auxiliary electrode. The electrolysis cell equipped with a water jacket was kept at a constant temperature (if n o t stated otherwise at 25 + 0.2 ° C). Macroelectrolyses were performed at r o o m temperatures of the order of 20 ° C. Voltammetric curves were recorded on a Radelkis polarograph type OH-102. For macroelectrolysis, TE-1C Or Radelkis type OH-404/A potentiostats fed to an O H 404/C integrator were used. Ion-selective electrodes were used for the analytical determination of sulphide and cyanide ions. Spectra in the visible region were recorded with a Carl Zeiss U V Vis spectrophotometer. RESULTS The cyclic polarisation of the electrode in solutions of Co(II) in thiocyanate medium at 25°C results in the appearence of a prestep in the potential region --0.60 V to - 0 . 6 5 V, during the second and following cathodic runs. This additional current (ipd) is observed with b o t h HMDE and GCE, and does n o t appear
13
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0:2 ' 0'.4 ' 016 '
121JA
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1:0 '
II2:E/v
Fig. 1. M u l t i c y c l i c v o l t a m m e t r i c c u r v e s o f 2 X 10 -3 M C o ( I I ) in 1.0 M N a S C N : (a) G C E , a n d ( b ) H M D E . T h e s e q u e n c e o f c u r v e s is i n d i c a t e d b y n u m b e r s : B e f o r e r e c o r d i n g c u r v e 20 t h e e l e c t r o d e was k e p t a t - - 0 . 1 0 V f o r 5 m i n .
during the first cathodic run. The potential of this pre-step is approximately 250 mV more positive than the potential of electroreduction of the Co(II)thiocyanate complexes at the HMDE. Deposition of CoS + Co is observed during the first cycle. There is no distinct anodic current which could correspond to these processes except for a small anodic step at potentials less negative than --0.6 V. Typical cyclic curves obtained with the use of the HMDE are shown in Fig. 1. In order to observe the pre-step, the cathodically deposited cobalt should be polarised anodically, at least for a short time. Moreover, the magnitude of this pre-step depends on several parameters, such as the potential and the duration of cobalt deposition (the charge passed during the cathodic process), the concentrations of Co(II) and thiocyanate, the potential and duration of the anodic oxidation o f deposited cobalt. These factors will be examined shortly.
Influence of the electroreduction potential For HMDE as well as GCE ipd decreases if the initial potential of electroIysis of Co(II)-thiocyanate complexes is more negative than that of electroreduction of CoS. This effect, which is more pronounced for HMDE than for GCE, is quantitatively illustrated b y the values given in Table 1. HMDE with 2 X 10 -3 M Co(II) in 1 M NaSCN and GCE with 10 -3 M Co(II) in 1 M NaSCN were used in
14 TABLE 1 D e p e n d e n c e o f ipd a n d ipd/ipc for Co(II) in t h i o c y a n a t e s o n e l e c t r o r e d u c t i o n p o t e n t i a l , charges passed in e l e c t r o r e d u c t i o n a n d t y p e o f e l e c t r o d e used Ered]V
HMDE Qred/PC
GCE ipd]PA
(ipd]ipc)]%
--1.00
--
--
--
--1.10
200 700 200 700 --
1.0 3.0 1.1 3.1 --
17 50 18 51 --
200 700
0.2 0.25
2.5 3.0
--1.20 --1.30 --1.40
Qred]PC
ipd]PA
(ipd]ipc)]%
350 1150 700 2100 850 2500 1000 3000 1500 3800
2.1 4.6 2.4 4.5 2.6 4.8 2.4 4.2 1.5 2.8
15 34 18 33 19 35 15 30 11 21
these experiments. Table 1 also shows the dependence of ipd on the charge passed during the electroreduction (Qr~d), as well as the ratio of ip~ to the main peak current of Co(II)-thiocyanate, ip¢. The decrease of ipd observed when the initial electroreduction is carried o u t at negative potentials is probably due to the electroreduction of CoS at such potentials. The deposited cobalt is then wetted b y mercury (in case of the HMDE) and the electrode surface is n o t modified. With GCE, CoS is also unstable on the electrode surface b e y o n d --1.40 V. However, deposition of metallic cobalt modifies the electrode properties to some extent. Larger Qred at reduction potentials more negative than --1.3 V results from the participation of hydrogen ions in the cathodic process. This side reaction m a y also change the properties of the electrode surface to some extent. On the basis of these results, one may conclude that a pre-peak is observed on the voltammetric curves only when CoS is cathodically deposited on the electrode surfaces.
Influence o f the Co(II) concentration In the cyclic voltammetric experiments presented the HMDE was polarized at a scan rate of 1.5 V/min from --0.5 V to --1.1 V, kept at this potential for some time and then scanned back to --0.10 V; ipd was recorded on the next cathodic scan. The concentration of thiocyanate ion was 5 X 10 -2 M. The influence of the time of cathodic electrolysis at --1.10 V and that of the Co(II) concentration on the ratio ipd/ipc are given in Table 2. The increase o f Co(II) concentration results in some decrease of ipd/ipc. Such behaviour was also found in studies carried o u t at 95°C.
Influence of the thiocyanate concentration The experiments were carried o u t in a manner analogous to that described above. The concentration of Co(II) was 2 × 10 -3 M and the concentration of
15 TABLE 2 D e p e n d e n c e o f ipd/ipc o n t h e Co(II) c o n c e n t r a t i o n a n d d u r a t i o n o f c a t h o d i c electrolysis ( t e l ( m i n ) ) at --1.1 V 103 X c o n c e n t r a tion of Co(II)/M
1 2 5 50
(ipd/ipc)/% tel = 0
tel = 0.5
tel = 1
tel = 2
tel = 4
6.5 5.0 4.0 3.0
6.5 6.0 5.0 .
10 10 8
15 14 11
27 23 20
.
.
.
thiocyanate was changed from 10 -3 M to 6.0 M. As in the previous section, the electrode was maintained at --1.10 V during various times (see Table 3), before reversing the polarization. During the anodic scan, the electrode was again polarised to - 0 . 1 0 V (--0.20 V in the case of 6 M NaSCN). The results obtained are given in Table 3. The increase of the pre-peak with concentration of thiocyanate is evident. This is probably due to larger amounts of CoS on the electrode surface. At very large thiocyanate concentrations (6.0 M) the current of the pre-peak decreases. For this solution, an additional very narrow peak appears on cyclic voltammetric curves at potentials less cathodic than the potential relative to the normal pre-peak. Its width at half-height was approximately equal to only 10 mV. One may assume that this new peak is due to the electroreduction of HgS, since it was never observed when using GCE.
Influence of the charge flowing during the electroreduction step (Q~) For 10 -3 M Co(II) solution in 1 M NaSCN, metallic cobalt and CoS were deposited at various potentials. The influence of the charge flowing at the electroreduction and d~position potential on the additional current (at a b o u t --0.6 V) is shown in Fig. 2. Voltammetric curves were recorded on GCE. The dependence o f ipd on the electroreduction charge is linear as long as the potential is n o t sufficient to reduce CoS. Since at --1.4 V CoS is n o t stable on
TABLE 3 D e p e n d e n c e o f ipd/ipc o n t h e c o n c e n t r a t i o n o f t h i o c y a n a t e and d u r a t i o n o f c a t h o d i c electrolysis, tel (rain) at - - 1 . 1 0 V Concentration of thiocyanate [SCN-]/M
(ipd/ipc)/% tel = 0
tel = 0.5
tel = 1.0
tel = 2.0
tel = 4.0
1 × 10 -3 5 × 10 -2 1 6
2.5 5.0 8.5 3.0
2.5 6.0 14 .
3.5 10 17
7 14 24
10 23 50
.
.
.
16 ipd/pA 2O16-
84-
260 4~50 660 ~o 1obo 12ooOr/pC Fig. 2. The dependence of ipd on the charge passed during electroreduction at different potentials: ~ --1.20 V, o --1.30 V and X--1.40 V. 10 -3 M Co(II) solution in 0.20 M NaSCN and 0.80 M NaCIO4; GCE. the electrode, after preliminary electroreduction at this potential ipd w a s relatively small and practically independent on time of electrolysis. Such behaviour is typical both for GCE and HMDE. The dependence of ipd on Qr (--1.1 V) for HMDE is given in Fig. 3. Similar measurements were also carried out at 60 and 95 ° C. Under these conditions, the ipd--Qr relationship was, in general, also linear. However, the deviations were larger. Inspection of all these results leads to the conclusion that the larger Qr the higher the additional current at --0.6 V. To test this conclusion for extended electroreduction times, measurements were carried out with 10 -3 M Co(II) in 1 M NaSCN at GCE with initial electrolysis times lasting up to 2 h. The oxidation of deposited cobalt was carried out by scanning the potential up - 0 . 1 0 V, where the electrode was kept for 5 min. The results obtained are shown in Table 4. Maximum values of/pd w e r e equal to the original cathodic current at - 0 . 9 0 V. When ipd was at its maximum, no current was observed at - 0 . 9 0 V nor any cobalt oxidation.
10-
a
86o
42-
46o s60 ~2bo ~6bo 2doo 2~0 28b0 3~00 3600 ~-l~,c Fig. 3. The d e p e n d e n c e o f ipd on the charge passed during e l e c t r o r e d u c t i o n at different potentials: L~ L~--1.0 V, O O --1.20 V, [] [] --1.40 V and × × results obtained f r o m multicyclic experiments. 2 × 10 -3 M Co(II) solution in 0.2 M NaSCN and 0.8 M NaC104; HMDE.
17 TABLE 4 T h e d e p e n d e n c e o f ipd, ip, tion
ipdlipc a n d ip/ipc o n
p o t e n t i a l , t i m e a n d charge o f e l e c t r o r e d u c -
Er/V
tr/h
Qr]~lC
ipd]PA
(ipd/ipc)]% ip]I2Aat
--1.10
0.17 0.25 0.50 1.0 • 1.5 2.0
1500 2500 5500 11000 16000 20000
12 15 21 22 22 22
54 70 95 100 100 100
--1.20
0.25 0.50 1.0 1.5
3500 6000 14000 19000
7.5 9.5 10.5 11.0
--1.30
0.25 0.50 1.0 1.5
7000 14000 21000 26000
3.0 4.0 3.5 4.0
--0.90 V
([p/ipc)]%
4 1 0 0 0 0
18 5 0 0 0 0
34 43 48 50
3.0 4.0 3.0 --
14 18 14 --
14 18 16 18
5.0 3.5 5.0 3.0
23 16 23 14
Values o f ipd exceeding the initial peak current at --0.9 V were obtained when the oxidation of deposited cobalt was carried out by polarising the electrode to more positive potential (+0.3 V). For Qr = 15 and 50 mC, ipd were found to be respectively 27 and 37 pA. It may be assumed that the increase of ipd with respect to the original peak current (equal under our experimental conditions to 22 pA) results from the partial discharge of hydrogen ions on the modified electrode surface. Under such conditions the cathodic current at - 0 . 9 V was still equal to 6.5 pA, independently of Qr. The initial electrolysis was carried out at --1.10 V.
Dependence o f ipd on temperature With increase o f temperature from 25 to 95°C the potential of the additional current ipd changes from --0.6 V to --0.3 V. A similar shift of potential is observed for the current appearing at greater cathodic potential. The ratio of ipd/ipc increases with temperature (Fig. 4).
Dependence o f Z~d on the anodic potential and time o f electrolysis at this po ten tial The values of ipd is dependent on the anodic potential ( E a n ) to which the electrode was polarised in the anodic reverse scan and also on the time the electrode was kept at this potential (to). With the HMDE, ipd only slightly increased with the increase of Ean, in the limited range of useful anodic potentials. Some results of such experiments a r e g i v e n in Table 5. For GCE, reproducibility is considerably lower than in HMDE. This m a y be due to irreproducibility in the state of the electrode surface before recording
18
i~,/ipc [%]
-5O
ipd/pA 12-
-3O
8-
-20
4-
-10
2:7
3:0
3:3
io 3 T-TK-T
Fig. 4. The dependence of ipd on 1/T and ipd/ipc on lIT for 2 X 10 -3 M Co(II) solution in 1.0 M NaSCN, HMDE. The charge passed in time of electroreduction was 2 0 0 p C (curves 1 and 1'), 4 0 0 p C (curves 2 a n d 2') a n d 6 0 0 pC (curves 3 and 3').
each curve. Despite this, an increase o f Ean and to with that electrode also leads to an increase of ipd. Results o f macroelectrolysis carried o u t on a large mercury p o o l cathode Solutions were stirred during these electrolyses, and subsequently analysed using electrochemical methods and spectrophotometry in the visible region. Sulphide and cyanide ions were detected and determined by using ion-selective electrodes. Electrolyses were carried out under various conditions, the initial solutions being always 10 -2 M with respect to Co(II) in 1 M NaSCN. For electrolysis performed at --0.9 to --1.0 V, CoS and metallic cobalt were observed on the electrode surface and S 2- and CN- anions and cyanide complexes of Co(II) were found in the solution. The presence of the later species was established on the basis of spectrophotometric measurements, though spectra differed to some extent from those relative to solutions of Co(II) added with CN- and S 2-. The concentrations of sulphide and cyanide ions were dependent on the charge passed. If the initial solution was 10 -3 M in Co(II) and 10 -1 M SCNions the concentrations of produced S 2- and CN- ions were respectively equal TABLE 5
Dependence of t0/min
0 2 4
ipd/ipc
on Ean and t o a
(ipd/ipc)/% Ean=--0.05V
Ean=----0.10V
Ean=--0.15V
Ean=--0.20V
6.0 7.0 7.5
5.0 6.0 --
4.5 6.5 6.5
4.0 5.0 6.0
a Results given in Table 5 were obtained for a 2 X 10 -3 M Co(II) solution in 5 × 10 -2 M NaSCN and 1.0 M NaC104. Cobalt was deposited for 2 min at - - 1 . 1 0 V.
19 t o 10 -3 and 10 -2 mmol 1-1 after passing 50C. The considerably lower concen-
tration of sulphide with respect to cyanides is probably due to precipitation of CoS. Additional experiments were also carried out in which the electrode surface was activated by short deposition of CoS and Co at --1.0 V. Subsequently, the potential was changed to -0.7 V. It was found that for charges up to 9 C, the concentration of Co(If) remained practicallyunaffected, even for electrolysis times as long as 6 h. Simultaneously the concentration of sulphides and cyanides increased with the total charge. Again the concentration of sulphide was approximately five times lower than that of cyanide. DISCUSSION The electroreduction process which is the main object of this work starts at potentials a b o u t --0.50 V and the peak current potential is near to --0.65 V (for 0.2 M thiocynate). This means that this process occurs at potentials approximately 250 mV more positive that the main process of electroreduction of thiocyanate-Co(II) complexes which brings the formation of metallic cobalt with small admixture of CoS. In the latter process, electroreduction of SCNto S 2- and CN- with simultaneous formation of CoS is only a side reaction, with an efficiency (calculated with respect to the total electroreduction charge) of the order of 10%. The significant potential difference between this process [6--8], and the new electroreduction wave observed in multicyclic experiments suggests that, in the potential range - 0 . 5 0 to --0.65 V of this wave, the relevant process is no longer electroreduction of Co(II) ions. The current for this process (ipd) is observed only if the concentration of Co(SCN) ÷ in the bulk of the solution exceeds 10 -s M. This leads to the conclusion that Co(II)-thiocyanate complexes are involved. Interaction of hydrated Co(II) ions with adsorbed SCN- ions does n o t seem to be a major factor. The activated complex (Electrode)--Hg--SCN--Co would indeed n o t favour the formation of CoS, because adsorbed SCN- interacts with mercury through its sulphur end. Comparative experiments have been carried o u t with HMDE and GCE. The fact that currents at - 0 . 6 5 V are larger for GCE at which interaction with thiocyanate ions should be considerably weaker, independently confirms our suggestion. An additional p r o o f that Co(II)-thiocyanate complexes participate in the electrode reaction in the potential range - 0 . 5 0 to - 0 . 6 5 V is afforded by the following observations. The voltammetric curves recorded after prolonged reduction of 2 × 10 -3 M solution of Co(II) in 1 M NaSCN (1 h for GCE and 5 min for HMDE), followed b y the oxidation of the solution after addition of Co(II), showed the increase of the cathodic current at - 0 . 6 5 V. This increase was proportional to the concentration of added Co(II), e.g. to the concentration of Co(II)-thiocyanate complexes. The electroreduction o f Co(II)-SCN- complexes within the potential range - 0 . 5 0 to - 0 . 6 5 V does n o t result in formation of metallic cobalt. According to Itabashi [ 12,13 ] in that range, there is electroreduction of HgS, which is formed in the course of the anodic polarisation of the electrode,
20 as a result of the reaction: CoS + Hg --2 e -~ HgS + Co 2+
(1)
Contrary to this suggestion, we think that the process m a y be schematically described b y the following equation: Co(SCN)~ -n + 2 e -~ CoS + CN- + (n -- 1) SCN-
(2)
This suggestion is supported b y the following experimental facts: (1) There is no cobalt oxidation peak current which would correspond to reduction in the potential range considered. This may suggest either that cobalt is not reduced in the cathodic process or that its oxidation is strongly inhibited. The first of these possibilities seems to be the more probable. The height of the anodic peak for cobalt oxidation decreases, while on the multicyclic mode a significant current is recorded at --0.65 V. This indicated that the product formed in the peak is n o t metallic cobalt. (2) We have found that neither mercury nor its ions (including HgS) participate in this process, since cyclic curves recorded with both HMDE and GCE are identical in shape and position. (3) When the peak height at --0.65 V increases, a simultaneous decrease of the main peak at --0.97 V is observed. The potential of the latter peak shifts with each subsequent cycle to less negative values up to a limiting value of the order of --0.80 V. In contrast the potential of a less negative peak is practically independent o f the number o f cycles. The Sum of the heights of b o t h peaks is very nearly constant and equal to the primary reduction peak --0.97 V. Since the maximum amplitude of the peak at - 0 . 6 5 V is equal to that of the primary cathodic peak, one may deduce that its values are also controlled by the diffusion of the Co(II)-thiocyanate complexes. Electroreduction at - 0 . 6 5 V requires that an a m o u n t of CoS has to be present on the electrode surface. While the primary electroreduction peak current at --0.97 V is dependent on the cathode material (mercury, glassy carbon) the peak potential at - 0 . 6 5 V is the same for both types of electrodes (in 0.2--1.0 M NaSCN). This may be explained b y the fact that both electrodes have acquired similar surface properties after deposition of Co + CoS. It seems that the presence of metallic cobalt is not a prerequisite. In fact, the additional process is favoured b y its oxidation. However, this conclusion may n o t be well founded, because the anodic current of the cobalt oxidation is large and interferes with the relatively small cathodic current ipd. One may deduce that CoS, possibly with some additional Co, acts as a semiconducting catalyst for reaction (2). The formation of CoS and CN- at the potential - 0 . 6 5 V is confirmed b y prolonged electrolysis. For cathodic charges <4 C the efficiency of the CNformation is equal, a b o u t 50%. For charges >7 C, the efficiency of this reaction drops to 25%. This decrease is probably due to some losses of cyanides at the longer time. Efficiencies <100% may also partly result from the oxidation of Co(II) cyanide complexes to Co(III). Since the latter complex is very stable, the determination of these fractions o f cyanide b o u n d to Co(III) may be difficult.
21
After this complex was decomposed by heating of alkalised solution for 1 h, the efficiency of cyanide formation rose to 75%. These efficiencies are similar for both GCE and HMDE. One m a y add that similar behaviour and conclusions have also been f o u n d for Ni(II) in thiocyanate medium [9,10]. The ratio of peak current at --0.65 V to the original peak at more negative values is dependent on the concentration of SCN- anions in the solution, as follows from data given in Table 3. Such a dependence, calculated with respect to the mean coordination number is presented in Fig. 5. The efficiency of S2- and CN- production increases to a mean coordination number ~2.5 and later decreases. This may suggest that complexes with coordination numbers 2 and 3 with respect to SCN- ions are responsible for this additional current at less negative potentials. It is n o t clear h o w the electrode reaction proceeds. It is known that in the Co(II)-thiocyanate complex, the Co--N bond predominates. It may be postulated that the reaction proceeds through an intermediate form (SCN),-Co-SCNCo(SCN)m. There is some evidence for the formation of a similar species in case of the Ni(II)-thiocyanate system. It is possible that an intermediate complex of Co(I) or Co(0) could be formed. For such complexes, the tendency to form a
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on the average c o o r d i n a t i o n number. 10 -3 M Co([[).
Fig. 6. M u l t i e y c l i c v o l t a m m e t r i e curves o f 2 x 10 -3 M Co(H) in 6.0 M NaSCN, recorded w i t h H M D E and GCE. The sequence o f curves is indicated by numbers and the scale o f current given by arrows.
i .
1.2-E/v
22
Co--S bond, and in consequence, cobalt sulphide and cyanide ions should be favoured. It seems that it is only in the case of concentrated thiocyanate solutions (6 M) that mercury ions participate in the reaction. Typical multicycle voltammetric curves for 2 × 10 -3 M Co(II) in 6 M NaSCN recorded with GCE and HMDE are shown in Fig. 6. With HMDE, very sharp peaks are observed at --0.6 V, only when the electrode is polarised to potentials close to 0 V during the anodic run. The charges corresponding to these peaks do not exceed the charges of a monolayer. Curves of similar shape with charge corresponding to one monolayer were also observed when HMDE is polarized in sulphide solutions. The electroreduction potential of these peaks corresponds to the peak potential of reduction of HgS observed in the Ni(II)-NCS- system [9]. At potentials more negative than these sharp peaks, the additional step current (ipd) discussed earlier in this work is observed. REFERENCES
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