Catalytic current of nickel(II) in acidic thiocyanate solutions at dropping and stationary mercury electrodes

Electroanalytical Chemistry and lnterfacial Electrochemistry, 60 (1975) 285 297

285

('~ ElsevierSequoia S.A.,Lausanne Printedin The Netherlands

CATALYTIC CURRENT OF NICKEL(II} IN ACIDIC THIOCYANATE SOLUTIONS AT DROPPING AND STATIONARY MERCURY ELECTRODES

EIKI ITABASHI Department of Chemistry, Miyagi University of Education, Aoba, Sendai (Japan)

(Received 24th September 1974; in revisedform 2nd January 1975)

Nickel(II) in aqueous I as well as in nonaqueous nitrile 2"3 solutions containing thiocyanate and tetraalkylammonium ions gives a catalytic polarographic wave having the shape of a maximum. This catalytic wave which resembles the profile of a Brdi6ka catalytic wave, occurs in the potential region where tetraalkylammonium ions are strongly adsorbed at the mercury electrode. It was proposed in a previous paper 1 that the electrode reaction mechanism of the catalytic wave was not due to the catalytic hydrogen evolution, but to the catalytic reduction of thiocyanate ion with the electroreduced active metallic nickel. The catalytic polarographic wave behavior of the nickel(II)-thiocYanate system is complicated by the presence of two maxima and by the coadsorption of thiocyanate and tetraalkylammonium ions at the mercury electrode. In addition, nickel(II) generated by the oxidation of metallic nickel with thiocyanate ion or diffusing to the electrode may react with the reduction products, cyanide and sulphide ions. Both cyanide and sulphide ions are inactive in the catalytic process. In order to get further information on the catalytic current of nickel(II) in thiocyanate solution, experiments were carried out in acidic thiocyanate solution in the absence of tetraalkylammonium ions. It was indeed found that nickel(II) in acidic thiocyanate solution exhibited a catalytic polarographic wave in the potential region between -0.6 and -1.0 V vs. SCE. This paper describes the characteristics of the catalytic current of the above-mentioned system. Because surface phenomena were responsible for the observed polarographic behavior, a.c. polarographic and cyclic voltammetric techniques were applied to further characterize the catalytic current. In addition to electroanalytical investigations, an attempt was made to obtain visual evidence for the chemical reaction of thiocyanate ion with metallic nickel by employing Raney nickel instead of electrodeposited metallic nickel. EXPERIMENTAL The d.c. polarographic and cyclic voltammetric experiments were performed with a conventional potentiostat constructed with operational amplifiers. The actual circuits used were similar to some of those described in the literature 4. Phaseselective a.c. polarograms were obtained with a Princeton Applied Research model 174 polarographic analyzer, used in connection with a PAR model 129A lock-in

286

E. ITABASHI

amplifier. Controlled drop times for a.c. and some d.c. polarographic experimen, were obtained with a PAR model 174/70 mechanical drop timer. The currentvoltage and the current-time (i-t) curves were recorded either with a XY recorder or an oscilloscope equipped with a Polaroid camera attachment. In the following section and in the Figures, "current" means the maximum current observed just before the fall of a mercury drop. A.c. polarograms were recorded with the applied a.c. voltage of 10 mV (peak to peak) and the frequency of 100 Hz. Nickel(II) perchlorate and tetraalkylammonium perchlorate salts were prepared by procedures previously reported 5. All the other chemicals were of analytical reagent grade. Redistilled water was used and all solutions were deaerated with high-purity nitrogen. The dropping mercury electrode (DME) had a flow rate of 1.496 m g s -1 and a drop time of 5.42 s in a deaerated 0.5 M NaC1 solution at -0.65 V vs. SCE for a mercury height of 55 cm. A Metrohm hanging mercury drop electrode (HMDE) with the surface area of 0.0485 cm 2 was employed as a working electrode for cyclic voltammetry. The counter electrode was a platinum spiral electrode which was connected to the electrolytic solutions through a glass sinter. The potential of the working electrode was measured against a saturated calomel electrode. Measurements were made at 25_+ 0.1 °C, unless otherwise stated. RESULTS

Conventional polaroyraphic study Nickel(II) in acidic thiocyanate solution gave a round maximum wave in the potential region where the reduction wave of nickel(II) in neutral thiocyanate solution reaches its limiting plateau, as shown in Fig. 1. The increase in current at potentials more negative than - 1 . 0 V is caused by the direct reduction of hydrogen ions. The i-t curves of an individual drop at various potentials for the maximum wave were investigated to determine the nature of the current. Figure 2 shows the dependence of the exponent of the i-t curves on the electrode potential. The i-t curves exhibit a well-defined shape without any irregularities. At low concentration of HC104, the exponent of the i-t curve reaches a maximum at the peak potential of the maximum wave and is close to the value of 0.67 expected for a pure kinetic-controlled electrode mechanism. On further addition of HC104 above 0.04 M, the exponent of the i-t curve observed at the positive branch of the maximum wave is well beyond the value of 0.67. The effect of the rate of mercury flow, m, on the maximum wave was also investigated. When the drop time was controlled mechanically at 4.95 s, m values were between 0.7% and 1.49o mg s -1. The slope of the log /-log m plot obtained at -0.91 V in 0.5 M thiocyanate solution containing 0.2 mM nickel(II) and 7.1 mM HC104 was found to be 0.67. Measurements of the maximum wave were carried out at 0.5, 10, 25 and 35 °C, using the solution containing 0.5 m M nickel(II), 2.8 mM HC104 and 0.5 M NaSCN. The resultant temperature coefficient of the peak current was 2.9~o per degree. Under conditions where the maximum wave is large compared to the nickel(II) reduction wave obtained in neutral thiocyanate solution, these results suggest that the reaction mechanism for the maximum wave is kinetic in nature

287

Ni(II) CATALYTIC CURRENT IN NCS- SOLUTIONS

15

b

1,0

<10 21

'T "6 E

b 5

o

0,5

X W

0

-o18

-,;o

-0.6

E/Vvs.SCE

-018 -I.0 E/Vvs,SCE

Fig. l. Effect of concn, of HC104 on the polarograms of 0.5 mM nickel(II) in 0.5 M NaSCN. [HCIO4]: (a) 0, (b) 0.7, (c) 1.4, (d) 2.8 mM. Fig. 2. The dependence of the current ( ) and the exponent of i-t curve ( . . . . . ) for the reduction waves of 0.2 mM nickel(II) in 0.5 M NaSCN. [HCIO4]: (a, a') 7 mM, (b, b') 0.11 M.

80

c

4O

b (3

O0

0.05 0,10 [HCIO4]/M

0.15

Fig. 3. Variation of peak current with concn, of HCIO,~ in 0.5 M NaSCN solns, containing various concns, of nickel(II): (a) 0.1, (b) 0.2, (c) 0.3, (d) 0.5 mM.

288

E. ITABASHI

and also involves a chemical reaction which follows the electron transfer. In addition, the catalytic current is proportional to the electrode surface. Figure 3 presents the relationships between the peak current, ic, of the maximum wave and the hydrogen ion concentration in 0.5 M NaSCN solution containing four different concentrations of nickel(II). The increment of current, ic, is defined as ic= it-i~, where it is the observed total height of the maximum wave and id the diffusion current obtained in the absence of HC104. The peak current increases with hydrogen ion concentration to a limit. The plots of ic against the nickel(II) concentration are shown in Fig. 4. At low concentration of HC104, the height of the peak current with increase in concentration of nickel(II) is greater than linear. When the concentration of HC104 exceeds 0.04 M, the peak current increases somewhat more slowly than the concentration of nickel(II). A similar catalytic maximum wave of nickel(lI) was observed in thiocyanate solution containing acetate buffer instead of HC104. The pH of the solution was changed from 3.55 to 5.41 and the total acetate concentration was kept constant at 0.1 M. In the presence of acetate buffer, the peak current increased with decreasing pH. The peak current was found to be higher in the presence of acetate buffer than of perchloric acid at the same pH.

d

60 < ..2

<=.4O 40 b

cl

2C o o

i

0.'2 0.4 [Ni(ll)J/mM

0

o

'

015 ' [SCN-]/M

I.'0

Fig. 4. Variation of peak current with concn, of nickel(II) in 0.5 M NaSCN solns, containing various concns, of HCIO4: (a) 2.8, (b) 14, (c) 42, (d) 98 mM. Fig. 5. Variation of peak current with concn, of NaSCN in 0.07 M HC104 solns, containing various concns, of nickel(II): (a) 0.1, (b) 0.2, (c) 0.3 mM.

The effect of the thiocyanate concentration over a range 0.05-1.0 M on the peak current is shown in Fig. 5. With increase in thiocyanate concentration, the peak current increases in accordance with a curve similar in the form to the Langmuir adsorption isotherm. Under conditions where the nickel(II) concentration

289

Ni(lI) CATALYTIC CURRENT IN NCS- SOLUTIONS

was above 0.5 raM, and furthermore, the thiocyanate concentration was below 0.1 M, the maximum wave was split into two maxima. The effect of the ionic strength of the solution on the peak current was investigated by employing the solution containing 0.2 mM nickel(II), 2.8 mM HC104 and 0.I M NaSCN. The ionic strength was adjusted to 0.2 to 1.1 by adding NaC104. The peak current decreased in height with increase in ionic strength. TABLE 1 EFFECT O F T E T R A A L K Y L A M M O N I U M IONS ON THE MAXIMUM WAVE OF 5 × 10 5 M Ni(II) IN 2.2 mM HCIO 4 AND 0.5 M NaSCN SOLUTIONS R 4 N CIO 4

Peak potential / V vs. S C E

Peak current ~ /#A

xb in i = k t ~

none 2 mM 2 mM 2 mM 2 rnM

- 0.875 -0.880 -0.895 - 0.925 -0.950

0.98 1.02 1.72 6.03 2.16

0.39 0.39 0.60 0.67 0.98

(CH3)4N + (C2Hs),~N + (C3HT)~.N + (C4H9)4N +

" Total peak height. b Value at the peak potential.

By the addition of gelatin of the order of 0.005~o, no variation in the peak current could be detected. With further addition of gelatin above 0.01~o, the peak current decreased with increasing concentration of gelatin. When tetraalkylammonium ions were added to the solution, the peak current increased in height. Table l presents the peak potential, peak current and exponent of the i-t curve obtained in acidic thiocyanate solutions containing tetraalkylammonium ions. The maximum wave of nickel(II) in the presence of(n-C4H9)4NC104 (n-Bu4NC104) showed some interesting features. The addition of a low concentration of n-Bu4NC104 depressed the maximum wave. At concentrations of n-Bu4NC104 higher than 0.5 raM, the maximum wave was improved, and i-t curves exhibited a well-defined shape. Phase-selective a.c. polarography

Figure 6 shows the conductance component of the electrode admittance of nickel(II) obtained with the phase-selective a.c. polarograph. Nickel(II) in acidic thiocyanate solution gave two a.c. waves. The first wave corresponds to the reduction of nickel(II). The second wave corresponds to the maximum wave of the d.c. wave. The summit potential of the second wave was slightly less negative than the peak potential of the corresponding d.c. wave. The two a.c. waves increase in height with increasing hydrogen ion concentration. The relation of the peak conductance of the second wave to the peak current of the d.c. wave is shown in Fig. 7. The peak conductance of the second wave was approximately proportional to the peak current at low concentrations of nickel(II) and hydrogen ions. This means that the same mechanism for the catalytic current is involved in both d.c. and a.c. polarograms.

290

E. ITABASHI 0.6

i

O.L 0.4 "T ¢ \ 0.~ >-

>0.2

0 ' -0.6

'

' - 0'8, - I . '0 E/Vvs.SCE

'

0

'

2£)

~

40

Lc/pA

Fig. 6. Phase-selective a.c. polarograms of 0.5 mM nickel(II) in 0.5 M NaSCN solns, containing various concns, of HCIO4: (a) 0, (b) 2.8, (c) 7.1 raM. The dashed line indicates the same componenk measured in the absence of nickel(II) and HCIO4. Fig. 7. Relation between the peak conductance of the second a.c. wave and the peak current of d.c. wave in 0.5 M NaSCN solns, containing 7.1 mM HC104 and various concns, of nickel(II) (C)), and 0.5 rnM nickel(II) and various concns, of HC104 ( 0 ) .


2O

c t,_

o

~3 I0 o a~ t~ (3

0

A

-0.2

C

-0,6 -I,0 E/Vvs,SCE

Fig. 8. Cyclic voltammograms of 0.2 mM nickel(II) in 0.5 M NaSCN solns, containing various conchs, of HCIO4: (a) 0, (b) 1.4, (c) 2.8 raM. Sweep rate 50 mV s- i.

291

Ni(ll) CATALYTIC CURRENT IN NCS- SOLUTIONS

Cyclic voltammetry Cyclic voltammetry was carried out to characterize more clearly the characteristics of the catalytic current of nickel(II) in acidic thiocyanate solution. The effect of the concentration of HC104 on the cyclic voltammetric behavior of 0.2 mM nickel(II) in 0.5 M NaSCN solution is illustrated in Fig. 8. Nickel(II) in neutral thiocyanate solution exhibits cathodic~nodic waves indicated by letters I c and I a , which correspond to the reduction-oxidation of nickel 6. Additional peaks II c and II A can be observed in acidic thiocyanate solution. Wave II c corresponds to the catalytic maximum wave of the d.c. wave. The very small anodic wave IIA near - 0 . 3 0 V corresponds to the dissolution of mercury as Hg(CN)2, as will be stated later. With increasing concentration of HC104, the waves IIc and I I a increase in height. With hydrogen ion concentration above 7 mM, wave I c appeared as a shoulder on the wave II c. The effect of sweep rate on the peak current of the wave IIc is shown in Fig. 9. The sweep rate, v, was varied from 0.01 to 10 V s -1 in the potential region from -0.18 to - 1 . 3 0 V. The peak current ip for the wave IIc was determined by measuring to the extension of the wave I o which was obtained in neutral thiocyanate solution. The current function ip/v~ decreases with increasing sweep rate. With sweep rate above 5 V s-l, the catalytic reaction has little effect on the overall electrode process. Multi-sweep voltammograms corresponding to several consecutive triangular voltage sweeps at the same mercury drop are shown in Fig. 10. A marked increase in the magnitude of the second cathodic peak occurs on the second and sub120 4 300

80 u~

,-> z o o

.._-40

I00

C 4 0

0

i

0

i

i

I

V t/2/Vt'~S'I/2

i

2

-0,2

-0 6

- 1.0

E/Vvs.SCE

Fig. 9. ip/v ~ vs. v i plots for the second cathodic peak of 0.2 mM nickel(II) in 0.5 M NaSCN solns. containing various concns, of HC104: (a) 1.4, (b) 2.8, (c) 7.1 mm. Fig. 10. Multi-sweep voltammogram of 0.2 mM nickel(II) in 7.1 mM HCIO 4 and 0.5 M NaSCN. The electrode was cycled between - 0 . 2 0 and - 1 . 2 0 V. Initial scan from -0.40 V in a cathodic direction. Sweep rate 100 mV s 1.

292

E. ITABASHI

sequent cycles. In addition, a relatively small cathodic peak near -0.36 V is seen on the second and subsequent cathodic sweeps. This cathodic peak corresponds to the reduction of Hg(CN)2 formed at the electrode surface. Controlled potential electrolysis Controlled potential electrolysis was used in order to identify the reaction products for the electrode reaction of nickel(II) in acidic thiocyanate solution. The working electrode for the controlled potential electrolysis was a mercury pool of surface area approximately 1.5 cm 2. Electrolyses were performed with the solution containing 0.5 mM nickel(II), 7.1 mM HC104 and 0.5 M NaSCN. With the electrode potential controlled at -0.80 and -0.90 V vs. SCE, respectively, bubbles of gas appeared during the electrolysis. These bubbles which were introduced into 0.1 M NaOH solution exhibited a double anodic wave with the half-wave potentials of -0.76 and -0.31 V. The first and second waves correspond to the reactions (1) and (2), respectively.

Hg+SZ-~HgS+2

e

Hg+2 CN-~Hg(CNh+2

(1)

e

(2)

This result suggests that the catalytic current in acidic thiocyanate solution in the presence of nickel(II) is related to the reduction of thiocyanate ions. Reduction of thiocyanate ions with Raney nickel An attempt was made to gain insight into the reduction of thiocyanate ions with metallic nickel by employing Raney nickel instead of the electrodeposited metallic nickel. A Raney nickel-aluminum alloy (Ni: 50 wt.~o) was developed using the procedures given in the literature 7. The resultant catalyst was washed with distilled water until the washing became neutral, and was stored in distilled water. The Raney nickel, ca. 0.2 g, was put into a glass reaction vessel containing 30 ml of 0.5 M NaSCN solution. The solution was occasionally shaken at room temperature for 2 h. The qualitative analysis of the solution was carried out polarographically. The thiocyanate solution treated with Raney nickel gave anodic and cathodic waves with the half-wave potentials of -0.75 and -1.38 V, respectively. The anodic and cathodic waves correspond to the reactions (1) and (3), respectively.

Ni(CN) 2- + 2 e ~ N i ( C N ) ~ -

(3)

After the completion of the reaction of thiocyanate ions with Raney nickel, a black compound was obtained. The black compound was identified as NiS. The analysis of NiS was carried out by the procedure previously reported 1. Nickel(II) in ammoniacal thiocyanate solution also exhibits a catalytic polarographic wave in the potential region between - 1 . 0 and -1.3 V vs. SCE a. With treatment ofammoniacal thiocyanate solution with Raney nickel, it was found that thiocyanate ions reacted with Raney nickel to produce Ni(CN) 2- and NiS. DISCUSSION

The results of the cyclic voltammetry with a H M D E and the controlled

Ni(II) CATALYTIC CURRENT IN NCS SOLUTIONS

293

potential electrolysis with a mercury pool electrode indicate that in the presence of nickel(II), thiocyanate ions are reduced to cyanide and sulphide ions. The peak morphology of the catalytic polarographic wave could be the result of a slight convective maximum or it could indicate that some adsorbed species is involved in the catalytic process. It is known that catalytic reactions involving adsorbed species may exhibit a peak current 8. Although the absence of the adsorption of nickel(II)-thiocyanate complexes at the mercury electrode has been inferred from the chronocoulometric measurements 9, the fact that the exponent of the i - t curves observed on the positive branch of the maximum wave is close to 7/6 suggests that the catalytic current of nickel(II) in acidic thiocyanate solution has a mixed adsorption-kinetic character. The rise in the instantaneous current at the DME proportional to f7/6 indicates that the process of the catalyst adsorption is limited by the diffusion 1°. A relatively small temperature coefficient of the catalytic current could be taken as an indication of the weak adsorption of nickel(II)-thiocyanate complexes. It is known 11 12 that the surface kinetic current often undergoes almost no change with rise in temperature, because an increase in the rate of the chemical reaction is compensated for by a decrease in the adsorption of the catalyst at the electrode surface. Inhibition of the catalytic reaction by the presence of gelatin seems to be due in part to the desorption of thiocyanate ions or nickel(II)-thiocyanate complexes. The catalytic current decreases with increasing ionic strength of the solution. The depression of the surface kinetic process with increase in ionic strength is a characteristic of other catalytic surface reactions 13-1s. At potentials more negative than the peak potential of the polarographic maximum wave, catalytic current decreases because of decreasing adsorption of thiocyanate ions and hence of decreasing concentration of the active metallic nickel at the electrode surface. In the potential region where the catalytic current of nickel(II) in acidic thiocyanate solution appears, a small part of the electrode is covered with the adsorbed thiocyanate ions16-18, and the surface coverage is less than 0.118. The experimental adsorption isotherm of thiocyanate ions at mercury electrode is found to be fitted to the Frumkin isotherm 18. When the degree of the surface coverage is low, the Frumkin isotherm is reduced to the Langmuir isotherm. If the catalytic reaction of nickel(II) in acidic thiocyanate solution proceeds at the electrode surface covered with adsorbed thiocyanate ions, the catalytic polarographic current ic should be proportional to the surface coverage and a plot c/i~ vs. c should be linear 19, where c is the bulk concentration of thiocyanate ions. The resultant plot was found to be linear, as shown in Fig. 11. It is impossible to explain completely at present the overall dependence of the limiting catalytic current on the hydrogen ion concentration. The peak current on the d.c. wave with increase in concentration of nickel(II) is greater than linear at low concentrations of HC10~, while the peak current increases somewhat more slowly than the nickel(II) concentration at higher concentrations of HC104 than 0.04 M. The exponent of i - t curves for the catalytic current also depends on the hydrogen ion concentration. At low concentration of HC104, the exponent of i - t curves is very close to the value of 0.67, while that obtained at higher concentrations of HC104 is well beyond the value of 0.67. These results could indicate that the reaction mechanism of the catalytic current changes slightly with the concentration

294

E. ITABASHI r

o

3

k \

o 2

_o f

°o

ols

,io

C/M

Fig. 11. Langmuiradsorption isotherm plots in NaSCN solns, containing0.07 M HC104 and various concns, of nickel(II):(a) 0.1, (b) 0.2, (c) 0.3 mM. of hydrogen ions. Qualitatively, the experimental results in the presence of small quantities of hydrogen ions can be accounted for by a mechanism of the type Ni(II) + NCS~s ~rapid [NiNCS]~s

(4)

[NiNCS]~s + 2 e ~

(5)

Ni(0)NCS-

slow

Ni(0)NCS-

" N i ( I I ) + C N - + S z-

(6)

fast .j H+ fast ) 2 H +

HCN

HzS

(7)

The cyclic regeneration of the electroactive nickel(II) accounts for the enhancement of the limiting current. Hydrogen ions act as a neutralizing agent of the reduction products of thiocyanate ions, cyanide and sulphide ions. This elucidation is similar to that for the catalytic current of nickel(II) in neutral aqueous 1 as well as in nonaqueous nitrile 2"3 solutions containing thiocyanate and tetraalkylammonium ions. The characteristics of the catalytic wave of the present system resemble those of the catalytic wave denoted as maximum A observed at less negative potentials 1-3. The catalytic polarographic wave in aqueous and in nitrile solutions containing thiocyanate and tetraalkylammonium ions usually separated into two maxima. With higher concentrations of HC104 present, the dissolution of electrodeposited metallic nickel and also the direct reduction of hydrogen ions at the surface of metallic nickel acting as the working electrode can not be ruled out as a possibility.

Ni(II) CATALYTIC

CURRENT

2 H ++Ni(0)

~

slow

IN NCS-

SOLUTIONS

(8)

Ni(II)+H 2

2 H + + 2 e (Ni(0)} ~

s low

295

H2+Ni(0)

(9)

The limiting current of the dip on the d.c. wave observed at approximately - 1.0 V increases slightly with increase in hydrogen ion concentration. This could indicate that reactions 8 and 9 are partly responsible for the total height of the catalytic current. However, the catalytic current of the present system could not be explained without taking into account the reduction of thiocyanate ions. The reactions 8 and 9 do not provide any evidence as to the evolution of hydrogen cyanide and hydrogen sulphide. The reduction of thiocyanate ions in the presence of nickel(II)could be explained byconsidering the direct reduction of the adsorbed thiocyanate ions at the surface of electrodeposited metallic nickel acting as the working electrode instead of reaction 6, as has been discussed in previous papers 1-3. NCSa~s+2 e (Ni(0))--CNfast

H~

HCN

+S 2- +Ni(0) fast

(10)

2H +

H,S (7)

On the cyclic voltammetric experiment presented in Fig. 8, the anodic peak current IA corresponding to the oxidation of metallic nickel or nickel amalgam decreases in height with increase in hydrogen ion concentration. The catalytic current appears in the potential region where the reduction of nickel(II) complexes takes place already. If the reactions 5 and 10 proceed simultaneously at the electrode surface, electroreduced metallic nickel should remain on the surface of the mercury electrode and the magnitude of the anodic current corresponding to the oxidation of metallic nickel should not vary with the hydrogen ion concentration. This cyclic voltammetric evidence does suggest the chemical oxidation of electrodeposited metallic nickel with thiocyanate ion. At the same time, thiocyanate ions react rapidly with Raney nickel to produce divalent nickel compounds, NiS and Ni(CN)~-, although the detailed reaction mechanism is uncertain at present and also the thermodynamic data on the chemical reaction between metallic nickel andthiocyanate ions is not presently available in the literature. It was found that Raney nickel reacted also with selenocyanate and azide ions in aqueous solution 2°. If the chemical reactivity of the electrodeposited metallic nickel is the same as that of Raney nickel, the electrodeposited metallic nickel could react with thiocyanate ions to produce divalent nickel compounds. Hydrogen ions do not play any .essential role on the catalytic reduction of thiocyanate ions. It is known 1'21 that the reduction of nickel(II) in aqueous thiocyanate solution at mercury electrode gives a nickel sulphide, although the catalytic polarographic current having the shape of a maximum is not observed in neutral thiocyanate solution in the absence of tetraalkylammonium ions because of the formation of insoluble nickel sulphide and nickel(II) cyanide complex. The formation of nickel sulphide has been recently confirmed by Hurlen and Eriksrud 22, who postulated that the formation of nickel sulphide was due to the direct reduction of

296

E. ITABASHI

thiocyanate ions at the electrode. However, they have made no detailed discussion on the reaction mechanism. The formation of nickel sulphide inhibits the enhancement of the limiting current of nickel(II). At the same time, if Ni(CN)4z- ions are formed in the vicinity of the electrode in neutral thiocyanate solution, they are not reduced at potentials less negative than - 1.4 V vs. SCE. Nickel(II) in the presence of cyanide ions forms stable Ni(CN) 2-, which is the only complex even when the total concentration of cyanide ions is lower than its stoichiometric value 23'24. In acidic thiocyanate solution, the reduction products of thiocyanate ions are easily removed from the electrode surface. The catalytic current of nickel(II) in acidic thiocyanate solution is greatly enhanced by the presence of various kinds of tetraalkylammonium ions. This result suggests that the adsorbed tetraalkylammonium ions aid in stabilizing the metallic nickel deposited on the electrode surface and in inhibiting the amalgamation of the electrodeposited ,metallic nickel. It is reasonable to assume that the primary reduction product of nickel(II) in thiocyanate solution is metallic nickel or a zerovalent nickel compound, which is unstable and quickly decomposes to metallic nickel and thiocyanate ion. Metallic nickel deposited at the mercury electrode is also unstable and quickly amalgamates 25. The amalgamation of the electrodeposited iron-group metals in aqueous solution is known to be inhibited by the presence of some surface active substances z6'z~. Although this study has indicated the sequence of reactions that are probably involved in the mechanism of the catalytic current of nickel(II) in acidic thiocyanate solution, the detailed reaction mechanism seems to be more complex. The complete elucidation requires a greater body of experimental data. Such a catalytic current was also observed in acidic thiocyanate solution containing cobalt(II) 28. A systematic study of the catalytic current of nickel(II) and cobalt(II) in aqueous as well as in nonaqueous thiocyanate solutions is now in progress. ACKNOWLEDGEMENTS The author is indebted to Mr. Yasuo Toda for his help in d.c. polarographic measurements. Financial aid by the Ministry of Education is also acknowledged. SUMMARY Nickel(II) in acidic thiocyanate solution gave a catalytic polarographic wave which appears as a maximum in the potential region between - 0 . 6 and - 1.0 V vs. SCE. The characteristics of this catalytic current have been investigated by d.c. polarography, i - t curve analysis, cyclic voltammetry, a.c. polarography and controlled potential electrolysis. A mechanism which involves a rapid adsorption equilibrium between the bulk and adsorbed nickel(II)-thiocyanate complex and oxidation of electroreduced metallic nickel with thiocyanate ion has been presented and discussed. Hydrogen ions act as a neutralizing agent of the reduction products of thiocyanate ions, cyanide and sulphide ions. The oxidation of metallic nickel with thiocyanate ions has been estimated by the reaction of Raney nickel with thiocyanate ions.

Ni(II) CATALYTIC CURRENT IN NCS- SOLUTIONS

297

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