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Polarographic behaviour of sulphide ion
Electroanalytical Chemistry and Interfacial Electrochemistry, 45 (1973) 193-203
193
© Elsevier Sequoia S.~., Lausanne - Printed in The Netherlands
POLAROGRAPHIC BEHAVIOUR OF SULPHIDE ION
D. R. CANTERFORD and A. S. BUCHANAN
Department of Physical Chemistry, University of Melbourne, Parkville, 3052, Victoria (Australia) (Received 19th December 1972; in revised form 15th January 1973)
The polarographic behaviour of sulphide ion has been investigated by a number of workers, particularly using the conventional direct current (d.c.) method 1- 5. Revenda 1 showed that an anodic wave appeared in the presence of sulphide ion, corresponding to the oxidation of mercury and the formation of the insoluble salt HgS. To explain the shape of the sulphide wave Kolthoff and Miller 2 suggested that the mercuric sulphide remained in solution as the complex ion HgS~- at the beginning of the wave, but at larger currents it deposited on the surface of the electrode causing the wave to deviate from its theoretical slope. The presence of a second wave was shown by Trifonov 3, who postulated that the first wave was an adsorption pre-wave (analogous to Brdicka's adsorption prewave for the reduction of methylene blue6), the limiting current of which was attained when the electrode was covered by a monomolecular layer of HgS. Trifonov suggested that the second wave, which was limited by diffusion of sulphide, corresponded to the formation of non-adsorbed HgS. Zhdanov and Kiselev4 reported the formation of three anodic waves for sulphide in 0.1 M KC1. According to them the first wave was an adsorption pre-wave (as proposed by Trifonov) and the other two were diffusion controlled. By observing the effect of changing the pH of the supporting electrolyte they concluded that HSand Sz- ions participated in the formation of the two diffusion waves. Julien and Bernard 5 also reported the presence of three d.c. waves for sulphide in 0.1 M NaOH. However, their results differed significantly from those of Zhdanov and Kiselev. Using the autoinhibition theory 7, Julien and Bernard explained their observations in terms of the deposition of successive layers of HgS on the electrode surface. The first wave was assumed to be the beginning of a normal wave inhibited by a monomolecular film of adsorbed HgS on the electrode surface, with the second wave appearing at a potential sufficiently positive for the inhibited electrochemical reaction to proceed. The third wave, which only appeared at high sulphide concentrations, was thought to be due to the formation of a second layer of HgS on the electrode. Alternating current (a.c.) polarography of sulphide ion has received little attention. Breyer and Hacobian 8 reported the presence of an a.c. wave for sulphide in 1 M NaOH, but gave few details of the characteristics of this wave. Because of the above conflicting results and interpretations 4,5, and because of the limited a.c. polarographic data in the literature, a reinvestigation of the polarographic behaviour of sulphide was undertaken using the a.c. and d.c. techniques. Previous authors have usually attempted to explain the polarographic be-
194
D.R. CANTERFORD, A.S. BUCHANAN
haviour by assuming that the electrode process is a simple two-electron oxidation resulting in the formation of mercuric sulphide according to : Hg ~ Hg z+ +2e
(1)
Hg z+ +S 2- ~ HgS
(2)
i.e., overall
Hg + S2- ~ HgS + 2e
(3)
However, as shown by Armstrong and co-workers in their recent detailed investigation of the sulphide-mercury interface 9-11, the electrode process is far more complicated. For example, specific adsorption 9 of Se-, dissolution of mercury 10 as the soluble complex HgS 2-, and electrocrystallization 11 of rigS have to be taken into account. EXPERIMENTAL
Analytical-reagent grade chemicals were used throughout without further purification. Concentrated stock solutions of sodium sulphide were prepared using de-oxygenated triply distilled water and stored under argon. To minimize oxidation of sulphide, while polarographic measurements were being made, small aliquots of the concentrated stock solution were transferred by microsyringe to a known volume of supporting electrolyte which had previously been thoroughly deaerated with oxygenfree argon and thermostatted at 25.0°C. Polarograms were recorded with a PAR (Princeton Applied Research) Model 170 Electrochemistry System using a three electrode cell. All potentials reported are relative to a silver/silver chloride (1 M NaC1) reference electrode, connected to the polarographic test solution by a salt bridge containing 1 M NaC10 4. Platinum wire was used as the third (auxiliary) electrode. A.c. measurements were made at an applied alternating voltage of 10 mV (peak-to-peak) at a frequency of 100 Hz unless otherwise stated. RESULTS
Previous investigations of the polarographic behaviour of sulphide ion have usually been carried out in a single supporting electrolyte only, and possible effects of varying the hydroxide ion concentration have not been considered (an exception being the work of Zhdanov and Kiselev4). However, a preliminary study indicated that the pH of the supporting electrolyte affected not only the position and shape of the sulphide waves but also, for the a.c. technique, the number of waves observed. To illustrate these effects results are presented for two supporting electrolytes of widely different pH, viz. 1 M NaC10 4 (pH between 9 and 10.4, depending on sulphide concentration), and 1 M NaOH. Results obtained in other supporting electrolytes will be presented where appropriate. Polarographic behaviour in 1 M N a C l O 4
With increasing sulphide ion concentration up to four d.c. waves were observed in 1 M NaC10 4. Typical d.c. polarograms are shown in Figs. la, 2a and 3a. Below 2 x 10 .4 M a single d.c. wave (wave 1) was observed (Fig. la), the limiting current of
195
POLAROGRAPHIC BEHAVIOUR OF SULPHIDE 1ON (a) (a)
.I i
(b) (b)
'iA
iI i
Ij\ -08 VOLT
-05 vs Ag/AgCL
-04
-0'8
-0 6 VOLT vs
Fig. 1. Polarograms of 1 x 10 -4 M sulphide in 1 M NaC10¢. (a) d.c.; (b) a.c. Fig. 2. Polarograms of 4 x 10 -4 M sulphide in 1 M NaC10¢. (a) d.c.; (b) a.c.
(a)
wave4
wave2~
wave1
(b) 15uA
-08
-{}6 VOLT
vs
-0-4 Ag/AgCL
-0-2
Fig. 3. Polarograms of 1.5 x 10 -3 M sulphide in 1 M NaC10¢. (a) d.c.; (b) a.c.
-04 Ag/AgQ
196
D. R. CANTERFORD, A. S. BUCHANAN
/[total
•J
L. 20 t.U
/
~ " " ' ~ • " ~ ' ~ / [I+2÷3
.J'j.J.
i,0 7" / "
•
' " - - ILl
/,7 ° g
10 [3 2-] × I o '~ / M
15
20
Fig. 4. Limiting current of d.c. waves v s . sulphide concentration. Supporting electrolyte, l M NaC10 4.
which was proportional to sulphide concentration. Above 2 x 10 - 4 M a second wave (wave 2) appeared at a more positive potential (Fig.2a). After the appearance of wave 2 the limiting current of wave 1 (i11)became almost independent of concentration (see Fig. 4). Wave 3, which became apparent above 7 x 10-4M, was difficult to distinguish from wave 2 (Fig. 3a). Above 1.1 × 10-3M another wave (wave 4) appeared at a considerably more positive potential. This wave, although drawn out, was well defined. The concentration dependence of the combined heights of waves 1 and 2 (ill+ 2) and waves 1, 2 and 3 (i11+2+3) is shown in Fig. 4. This diagram also shows that the total limiting current remained a linear function of concentration over the entire range studied, irrespective of the number of waves present. A.c. polarograms corresponding to the above d.c. polarograms are shown in Figs. lb, 2b and 3b. For concentrations where only one d.c. wave was present two a.c. waves were observed (Fig. lb). The more negative a.c. wave obviously corresponds to the d.c. wave but for the very sharp a.c. wave at a more positive potential there was no corresponding d.c. wave (apart from a small kink in the diffusion current plateau). The latter a.c. wave was present at all sulphide concentrations studied and although it became smaller and less sharp as the concentration increased, its peak potential remained constant. Wave 2 in the d.c. was accompanied by a well defined a.c. wave (Fig. 2b), but after the appearance of wave 3 in the d.c. the second a.c. wave became very broad as shown in Fig. 3b. Wave 4 in the d.c. was not accompanied by an a.c. wave. A feature of the d.c. polarograms of sulphide in 1 M NaC10 ~ was the sharpness of the start of anodic discharge (i.e. the foot of wave 1). Examination of current-time curves for individual drops indicated that a potential change as small as 1 mV was sufficient to shift from a region of no discharge to one where appreciable faradaic current flows. Figure 5 shows current-time curves recorded in the vicinity of the start of anodic discharge. At - 0.783 V (Fig. 5b) the current decreased continuously during the life of each drop, which is typical of a charging or capacitive current (i.e., i~ oc t- li3).
198
D. R. CANTERFORD, A. S. B U C H A N A N
(a)
(a)
L
A
2O~IA
-0'6 -0;4 VOLT vs Ag/AgCt
-08
-Oe -04 VOLT vs Ag/AgC[
Fig. 6. Polarograms of 1 x 10 -4 M sulphide in 1 M NaOH. (a) d.c.; (b) a.c. Fig. 7. Polarograms of 4 x 10 -4 M sulphide in 1 M NaOH. (a) d.c.; (b) a.c.
,/
30-
, ~ ' / ~ ' /
20"
°
"
/ * ~
lltotat
/[1+2.3
I[i.2
/" /"
/" 10
15
20
IS2-1 x 10 4 / M
Fig. 8. Limiting current of d.c. waves vs. sulphide concentration. Supporting electrolyte, 1 M NaOH.
wave. As shown in Fig. 9, varying the applied alternating voltage (AE) affected the first and second a.c. waves differently. Although the first wave decreased in height with decreasing AE its half-width remained approximately constant and thus the wave was well defined for small AE. However, for small AE the second wave disappeared almost completely (Fig. 9c). With increasing concentration the second a.c. wave became less sharp and decreased in height, and for concentrations where two or more d.c. waves were observed it again disappeared (Fig. 7b). Table 1 shows the peak potentials of the three a.c. waves for 1 x 10 -4 M sulphide in supporting electrolytes of varying pH. As the pH is decreased the separation between the first two waves decreases (e.g., the waves are 47 mV apart in 2 M N a O H but only 25 mV apart in 0.5 M NaOH). In 0.1 M NaOH the two waves could
P O L A R O G R A P H I C B E H A V I O U R OF S U L P H I D E I O N
197
ca~
rtt_
ll..IA
2S
D
time Fig. 5. Current-time curves (d.c.) for 1.5 x 10 -3 M sulphide in 1 M NaCIO4. (a) -0.784; (b) -0.783; (c) -0.782; (d) -0.781 v (Ag/AgCl).
However, at a potential only 1 mV more positive (Fig. 5c) the current increased rapidly at the beginning of each drop indicating that faradaic current was present at this potential (i.e., if oc tl/6). In other words, d.c. current-voltage curves in 1 M NaC104 start discontinuously. Accurate measurement of the peak potential of the first a.c. wave, using an applied alternating voltage of 0.1 mV (peak-to-peak) at 100 Hz, gave a value of -0.783+0.001 V, which coincided with the starting potential of anodic discharge.
Polarographic behaviour in 1 M NaOH The d.c. polarographic behaviour of sulphide ion in 1 M N a O H was in many respects similar to that in 1 M NaC104 (typical d.c. polarograms are shown in Figs. 6a and 7a). For concentrations below 2 x 10 -4 M a single d.c. wave was observed (Fig. 6a) and as the concentration was increased three more waves appeared at more positive potentials. Again it was difficult to distinguish between waves 2 and 3. At high sulphide concentrations the limiting heights of waves 1 and 2 were very difficult to measure accurately because of maxima. The dependence of the height of the waves on sulphide concentration is shown in Fig. 8. As in 1 M NaC10¢, the limiting current of wave 1 became almost independent of concentration after the appearance of wave 2, and the total limiting current was again a linear function of concentration over the entire range studied. One of the main differences in the d.c. polarographic behaviour in the two supporting electrolytes was that in 1 M N a O H the current-voltage curves did not start discontinuously, as they did in I M NaC104 (compare Figs. la and 6a). The a.c. polarographic behaviour of sulphide in 1 M N a O H was significantly different to that in 1 M NaC104. For example, below 2 x 10 -4 M three distinct a.c. waves were recorded in 1 M N a O H (Fig. 6b) although only one d.c. wave was observed. The peak potential of the first a.c. wave corresponded closely to the half-wave potential of the d.c. wave. The second a.c. wave occurred near the beginning of the limiting current plateau of the d.c. wave and was much narrower than a normal a.c.
199
P O L A R O G R A P H I C B E H A V I O U R OF S U L P H I D E I O N
1 (a) -fig
(b -0:7
(
-0 9
VOLT
-0.'7
vs
c
-09
)
~ -07
Ag/AgCL
Fig. 9 A.c. polarograms of 1 x 10 - 4 M sulphide in 1 M N a O H at various applied alternating voltages (AE): (a) 10; (b) 5.0; (c) 1.0 inV.
TABLE 1 P E A K P O T E N T I A L OF A.C. WAVES F O R 1 x 10 - 4 M S U L P H I D E IN S U P P O R T I N G E L E C T R O L Y T E S OF D E C R E A S I N G p H
Supporting electrolyte
Peak potential/V (vs. Ag/AgCI) First wave
2 M NaOH 1 M NaOH 0.5 M N a O H
-0.775 -0.785 - 0.785
Second wave -0.728 -0.748 - 0.760
Third wave -0.326 -0.340 - 0.348
not be resolved. Thus the conditions necessary for observation of the second a.c. wave are low sulphide concentrations, high pH, and large AE. The third a.c. wave, at a much more positive potential, was similar to the sharp a.c. wave observed in 1 M NaC104. As in 1 M NaC104 this a.c. wave became smaller and less sharp with increasing concentration while its peak potential remained constant. However, Table i shows that the peak potential shifted to less negative values with increasing pH (also compare Figs. la and 6a). DISCUSSION
Apart from an extra wave being observed in the present work the results for d.c. polarography were similar to those reported by Julien and Bernard 5. Comparison of the results suggests that their second wave corresponds to waves 2 and 3 while their third wave corresponds to wave 4. As stated previously, it was difficult to distinguish
200
D. R. CANTERFORD, A. S. BUCHANAN
between waves 2 and 3. This was particularly so if damping was used on the recorder. Julien and Bernard reported difficultyin measuring the limiting current of their third wave at high sulphide concentrations. They also observed that after the appearance of the third wave the total limiting current increased with increasing concentration at a smaller rate than it did before the appearance of the third wave. That is, the total limiting current was not a linear function of concentration over the range studied. In the present work, however, the total limiting current plateau was very well defined at high sulphide concentrations (e.g., Fig. 3a), with reproducible measurement of the current being possible. Furthermore, the total limiting current varied linearly with sulphide concentration irrespective of the number of waves present (see Figs. 4 and 8). The fact that a three-electrode potentiostat was used whereas presumably only a two-electrode cell was used by Jutien and Bernard may account for these differences in results. Attempts to reproduce the results of Zhdanov and Kiselev4 were not successful. In 0.1 M KC1 the d:c. polarographic behaviour was similar to that in 1 M NaC104 and no evidence could be found for two diffusion controlled waves. The pH of a 10- 3M sulphide -0.1 M KC1 solution was 10.1 At this pH sulphide exists in solution almost exclusively as the HS- species. Thus it would seem most unlikely that two diffusion controlled waves (due to HS- and S2-) could be observed. Armstrong and co-workers have recently investigated the sulphide-mercury interface using impedance and potentiostatic techniques. They found that in solutions buffered at pH 9.5 sulphide was very strongly adsorbed on mercury over a short potential range, leading to high double-layer capacities 9. They also reported 1° that in highly alkaline sulphide solutions mercury passed into solution as the soluble complex HgS2z-, with the overall process being: Hg+2S z- ~ HgS~- +2e
(4)
Double-layer capacity measurements indicated the absence of any strong specific anion adsorption prior to the dissolution process. At low pH this reaction is considerably reduced because of the low concentration of "free" S2-. An investigation of the growth of thin anodic films of mercuric sulphide on a mercury electrode showed that in the initial stages of film growth mercuric sulphide was deposited on the electrode in the form of two successive monomolecular layers 11. The presence of each layer severely inhibited the rate at which mercury dissolved as HgS 2-. At high sulphide concentrations the presence of four d.c. waves suggests the formation of another layer of HgS apart from the two monolayers postulated by Armstrong et al. Current-time curves for individual drops were recorded for a number of sulphide concentrations in an attempt to confirm this suggestion. Figure 10 shows current-time curves at various potentials for 1.5 x 10 -3 M sulphide in 1 M NaC104. The position of the curves on the corresponding d.c. polarogram can be found by reference to Fig. 3a. (Current-time curves recorded in 1 M NaOH were similar to those in 1 M NaC104.) The three distinct maxima on these curves are consistent with the formation of three layers of HgS on the electrode surface, although this evidence is by no means conclusive. At low sulphide concentrations the overall electrode process is controlled
POLAROGRAPHIC
BEHAVIOUR
OF SULPHIDE
201
ION
by diffusion of sulphide (S 2- or HS-) to the electrode surface and a single d.c. wave is observed. The appearance of wave 2 indicates that the sulphide concentration has. become sufficient for the surface of the mercury drop to become completely covered by a monolayer of HgS. The limiting current of wave 1 (i11)then becomes independent (or almost so) of concentration because dissolution of mercury is no longer controlled by diffusion of sulphide to the electrode but by transport of "mercury" through the HgS film. The charge transfer process therefore proceeds with a considerable overvoltage in the presence of this film, and wave 2 appears at a more positive potential. Similarly with increasing sulphide concentration the appearance of waves 3 and 4 at still more positive potentials indicate that the electrode has become covered by a second and thirdlayer of HgS.
| I st rnonolai,er 2 2ndmonolayer 3 ~'d[ayer
(a) 2 (b)
2
3
A
C L_ L_ U (f) ~
li'~
(d) time Fig. 10. C u r r e n t - t i m e curves (d.c.) for 1.5 × 10 -3 M sulphide in 1 M NaC10~. (a) - 0 . 7 3 0 ; (b) - 0 . 6 4 0 ; (c) - 0.550; (d) - 0.420; (e) - 0.280; (f) - 0.120 V (Ag/AgC1). 1 --- 1 st monolayer, 2 = 2nd m o n o t a y e r , 3 = 3rd layer.
Current-time curves on the plateau of wave 1 (Fig. 10a) pass through a maximum and then rapidly decay. The sudden decrease in current can be ascribed to full coverage of the electrode surface by a layer of HgS. Curves of similar shape result when a species is adsorbed on the electrode surfacelZ.At more positive potentials (Figs. 10b-e) current-time curves become complicated due to the presence of two more layers of HgS which further inhibit the dissolution of mercury. For a sulphide concentration of 1.5 x 10-a M a very large overvoltage must be applied before the overall process becomes controlled by diffusion of sulphide to the electrode surface and currenttime curves have their normal i o c t 1/6 shape (Fig. 100. In 1 M NaCI04 (low pH) the coincidence of the peak potential of the first a.c. wave with the sharp start of the anodic discharge indicates that the negative (cathodic) branch of this a.c. wave lies in a potential region where no charge transfer across the electrode-solution interface occurs and, therefore, this branch involves non-faradaic
202
D. R. CANTERFORD, A. S. BUCHANAN
alternating currents. Similar results were obtained by Biegler 13 in his investigation of the polarographic behaviour of chloride ion. By analogy with Biegler's conclusions for chloride it is suggested that the negative branch of the first a.c. wave is due to the high differential capacity of the double-layer following upon specific adsorption of sulphide ion, a conclusion which is in agreement with the results of Armstrong et al. 9. In 1 M N a O H (high pH) the first a.c. wave probably corresponds to the dissolution process given by eqn. (4). Presumably this is the a.c. wave observed by Breyer and Hacobian 8. It is interesting to note that although Kolthoff and Miller 2 suggested that mercury dissolved as HgS 2-, subsequent authors ignored this possibility attempting to interpret the polarographic behaviour of sulphide. At low sulphide concentrations the appearance of a second a.c. wave at a slightly more positive potential (Fig. 6b) is more difficult to explain. The sharpness of this wave and the absence of a corresponding d.c. wave suggests that it is tensammetric 14 in nature. That is, it is due to a sharp change in surface charge density brought about by an adsorption/ desorption process. In all supporting electrolytes studied an a.c. wave was observed at a much more positive potential than the a.c. waves already discussed. The sharpness of this wave, particularly at low concentrations, and the absence of a corresponding d.c. wave suggest that it is also tensammetric in nature. Biegler 15 observed a similar wave for iodide which he attributed to desorption of a film of mercurous iodide or at least a severe change in its structure. It seems reasonable to propose that this previously unreported wave for sulphide arises from a similar process. SUMMARY The polarographic behaviour of sulphide ion has been investigated in several supporting electrolytes with the a.c. and d.c. techniques. Four distinct d.c. waves have been observed at high sulphide concentrations although only three waves have previously been reported. The appearance of four waves can be explained by deposition on the electrode surface of three successive layers of HgS which inhibit dissolution of mercury as HgS 2-. The a.c. polarographic behaviour depends markedly on pH, as well as sulphide concentration, with up to three waves being observed. A very narrow a.c. wave which was observed at all sulphide concentrations in all supporting electrolytes studied is probably due to desorption of HgS from the electrode surface.
REFERENCES i J. Revenda, Collect. Czech. Chem. Commun., 6 (1934)453. 2 I. M. Kolthoff and C. S. Miller, J. Amer. Chem. Soc., 63 (1941) 1405. 3 A. Trifonov,Izv. Khim. lnst. Bulg. Akad. Nauk., 4 (1956)21. 4 S. Zhdanov and B. Kiselevin G. J. Hills (Ed.), Polarography, 1964, Vol. 1, Macmillan, London, 1966 p. 473. 5 L. Julien and M. L. Bernard, Rev. Chim. Min., 5 (1968)521. 6 R. Brdicka, Z. Elektrochem., 48 (1942) 278. 7 E. Laviron and C. Degrand in G. J. Hills (Ed.),Polarography, 1964, Vol. l, Macmillan, London, 1966, p. 337. 8 B. Breyer and S. Hacobian, Aust. J. Sci. Res., A4 (1951)610. 9 R. D. Armstrong, D. F. Porter and H. R. Thirsk, J. Electroanal. Chem., 16 (1968)219.
POLAROGRAPHIC BEHAVIOUR OF SULPHIDE ION
203
R. D. Armstrong, D. F. Porter and H. R. Thirsk, J. Electroanal. Chem., 14 (1967) 17. R. D. Armstrong, D. F. Porter and H. R. Thirsk, J. Phys. Chem., 72 (1968) 2300. R. W. Schmid and C. N. Reilley, J. Amer. Chem. Soc., 80 (1958) 2087. T. Biegler, J. Electroanal. Chem., 6 (1963) 357. B. Breyer and H. H. Bauer, Chemical Analysis, Vol. XIII, Alternating Current Polarography and Tensammetry, Interscience, New York/London, 1963. 15 T. Biegler, J. Electroanal. Chem., 6 (1963) 373.
10 11 12 13 14
193
© Elsevier Sequoia S.~., Lausanne - Printed in The Netherlands
POLAROGRAPHIC BEHAVIOUR OF SULPHIDE ION
D. R. CANTERFORD and A. S. BUCHANAN
Department of Physical Chemistry, University of Melbourne, Parkville, 3052, Victoria (Australia) (Received 19th December 1972; in revised form 15th January 1973)
The polarographic behaviour of sulphide ion has been investigated by a number of workers, particularly using the conventional direct current (d.c.) method 1- 5. Revenda 1 showed that an anodic wave appeared in the presence of sulphide ion, corresponding to the oxidation of mercury and the formation of the insoluble salt HgS. To explain the shape of the sulphide wave Kolthoff and Miller 2 suggested that the mercuric sulphide remained in solution as the complex ion HgS~- at the beginning of the wave, but at larger currents it deposited on the surface of the electrode causing the wave to deviate from its theoretical slope. The presence of a second wave was shown by Trifonov 3, who postulated that the first wave was an adsorption pre-wave (analogous to Brdicka's adsorption prewave for the reduction of methylene blue6), the limiting current of which was attained when the electrode was covered by a monomolecular layer of HgS. Trifonov suggested that the second wave, which was limited by diffusion of sulphide, corresponded to the formation of non-adsorbed HgS. Zhdanov and Kiselev4 reported the formation of three anodic waves for sulphide in 0.1 M KC1. According to them the first wave was an adsorption pre-wave (as proposed by Trifonov) and the other two were diffusion controlled. By observing the effect of changing the pH of the supporting electrolyte they concluded that HSand Sz- ions participated in the formation of the two diffusion waves. Julien and Bernard 5 also reported the presence of three d.c. waves for sulphide in 0.1 M NaOH. However, their results differed significantly from those of Zhdanov and Kiselev. Using the autoinhibition theory 7, Julien and Bernard explained their observations in terms of the deposition of successive layers of HgS on the electrode surface. The first wave was assumed to be the beginning of a normal wave inhibited by a monomolecular film of adsorbed HgS on the electrode surface, with the second wave appearing at a potential sufficiently positive for the inhibited electrochemical reaction to proceed. The third wave, which only appeared at high sulphide concentrations, was thought to be due to the formation of a second layer of HgS on the electrode. Alternating current (a.c.) polarography of sulphide ion has received little attention. Breyer and Hacobian 8 reported the presence of an a.c. wave for sulphide in 1 M NaOH, but gave few details of the characteristics of this wave. Because of the above conflicting results and interpretations 4,5, and because of the limited a.c. polarographic data in the literature, a reinvestigation of the polarographic behaviour of sulphide was undertaken using the a.c. and d.c. techniques. Previous authors have usually attempted to explain the polarographic be-
194
D.R. CANTERFORD, A.S. BUCHANAN
haviour by assuming that the electrode process is a simple two-electron oxidation resulting in the formation of mercuric sulphide according to : Hg ~ Hg z+ +2e
(1)
Hg z+ +S 2- ~ HgS
(2)
i.e., overall
Hg + S2- ~ HgS + 2e
(3)
However, as shown by Armstrong and co-workers in their recent detailed investigation of the sulphide-mercury interface 9-11, the electrode process is far more complicated. For example, specific adsorption 9 of Se-, dissolution of mercury 10 as the soluble complex HgS 2-, and electrocrystallization 11 of rigS have to be taken into account. EXPERIMENTAL
Analytical-reagent grade chemicals were used throughout without further purification. Concentrated stock solutions of sodium sulphide were prepared using de-oxygenated triply distilled water and stored under argon. To minimize oxidation of sulphide, while polarographic measurements were being made, small aliquots of the concentrated stock solution were transferred by microsyringe to a known volume of supporting electrolyte which had previously been thoroughly deaerated with oxygenfree argon and thermostatted at 25.0°C. Polarograms were recorded with a PAR (Princeton Applied Research) Model 170 Electrochemistry System using a three electrode cell. All potentials reported are relative to a silver/silver chloride (1 M NaC1) reference electrode, connected to the polarographic test solution by a salt bridge containing 1 M NaC10 4. Platinum wire was used as the third (auxiliary) electrode. A.c. measurements were made at an applied alternating voltage of 10 mV (peak-to-peak) at a frequency of 100 Hz unless otherwise stated. RESULTS
Previous investigations of the polarographic behaviour of sulphide ion have usually been carried out in a single supporting electrolyte only, and possible effects of varying the hydroxide ion concentration have not been considered (an exception being the work of Zhdanov and Kiselev4). However, a preliminary study indicated that the pH of the supporting electrolyte affected not only the position and shape of the sulphide waves but also, for the a.c. technique, the number of waves observed. To illustrate these effects results are presented for two supporting electrolytes of widely different pH, viz. 1 M NaC10 4 (pH between 9 and 10.4, depending on sulphide concentration), and 1 M NaOH. Results obtained in other supporting electrolytes will be presented where appropriate. Polarographic behaviour in 1 M N a C l O 4
With increasing sulphide ion concentration up to four d.c. waves were observed in 1 M NaC10 4. Typical d.c. polarograms are shown in Figs. la, 2a and 3a. Below 2 x 10 .4 M a single d.c. wave (wave 1) was observed (Fig. la), the limiting current of
195
POLAROGRAPHIC BEHAVIOUR OF SULPHIDE 1ON (a) (a)
.I i
(b) (b)
'iA
iI i
Ij\ -08 VOLT
-05 vs Ag/AgCL
-04
-0'8
-0 6 VOLT vs
Fig. 1. Polarograms of 1 x 10 -4 M sulphide in 1 M NaC10¢. (a) d.c.; (b) a.c. Fig. 2. Polarograms of 4 x 10 -4 M sulphide in 1 M NaC10¢. (a) d.c.; (b) a.c.
(a)
wave4
wave2~
wave1
(b) 15uA
-08
-{}6 VOLT
vs
-0-4 Ag/AgCL
-0-2
Fig. 3. Polarograms of 1.5 x 10 -3 M sulphide in 1 M NaC10¢. (a) d.c.; (b) a.c.
-04 Ag/AgQ
196
D. R. CANTERFORD, A. S. BUCHANAN
/[total
•J
L. 20 t.U
/
~ " " ' ~ • " ~ ' ~ / [I+2÷3
.J'j.J.
i,0 7" / "
•
' " - - ILl
/,7 ° g
10 [3 2-] × I o '~ / M
15
20
Fig. 4. Limiting current of d.c. waves v s . sulphide concentration. Supporting electrolyte, l M NaC10 4.
which was proportional to sulphide concentration. Above 2 x 10 - 4 M a second wave (wave 2) appeared at a more positive potential (Fig.2a). After the appearance of wave 2 the limiting current of wave 1 (i11)became almost independent of concentration (see Fig. 4). Wave 3, which became apparent above 7 x 10-4M, was difficult to distinguish from wave 2 (Fig. 3a). Above 1.1 × 10-3M another wave (wave 4) appeared at a considerably more positive potential. This wave, although drawn out, was well defined. The concentration dependence of the combined heights of waves 1 and 2 (ill+ 2) and waves 1, 2 and 3 (i11+2+3) is shown in Fig. 4. This diagram also shows that the total limiting current remained a linear function of concentration over the entire range studied, irrespective of the number of waves present. A.c. polarograms corresponding to the above d.c. polarograms are shown in Figs. lb, 2b and 3b. For concentrations where only one d.c. wave was present two a.c. waves were observed (Fig. lb). The more negative a.c. wave obviously corresponds to the d.c. wave but for the very sharp a.c. wave at a more positive potential there was no corresponding d.c. wave (apart from a small kink in the diffusion current plateau). The latter a.c. wave was present at all sulphide concentrations studied and although it became smaller and less sharp as the concentration increased, its peak potential remained constant. Wave 2 in the d.c. was accompanied by a well defined a.c. wave (Fig. 2b), but after the appearance of wave 3 in the d.c. the second a.c. wave became very broad as shown in Fig. 3b. Wave 4 in the d.c. was not accompanied by an a.c. wave. A feature of the d.c. polarograms of sulphide in 1 M NaC10 ~ was the sharpness of the start of anodic discharge (i.e. the foot of wave 1). Examination of current-time curves for individual drops indicated that a potential change as small as 1 mV was sufficient to shift from a region of no discharge to one where appreciable faradaic current flows. Figure 5 shows current-time curves recorded in the vicinity of the start of anodic discharge. At - 0.783 V (Fig. 5b) the current decreased continuously during the life of each drop, which is typical of a charging or capacitive current (i.e., i~ oc t- li3).
198
D. R. CANTERFORD, A. S. B U C H A N A N
(a)
(a)
L
A
2O~IA
-0'6 -0;4 VOLT vs Ag/AgCt
-08
-Oe -04 VOLT vs Ag/AgC[
Fig. 6. Polarograms of 1 x 10 -4 M sulphide in 1 M NaOH. (a) d.c.; (b) a.c. Fig. 7. Polarograms of 4 x 10 -4 M sulphide in 1 M NaOH. (a) d.c.; (b) a.c.
,/
30-
, ~ ' / ~ ' /
20"
°
"
/ * ~
lltotat
/[1+2.3
I[i.2
/" /"
/" 10
15
20
IS2-1 x 10 4 / M
Fig. 8. Limiting current of d.c. waves vs. sulphide concentration. Supporting electrolyte, 1 M NaOH.
wave. As shown in Fig. 9, varying the applied alternating voltage (AE) affected the first and second a.c. waves differently. Although the first wave decreased in height with decreasing AE its half-width remained approximately constant and thus the wave was well defined for small AE. However, for small AE the second wave disappeared almost completely (Fig. 9c). With increasing concentration the second a.c. wave became less sharp and decreased in height, and for concentrations where two or more d.c. waves were observed it again disappeared (Fig. 7b). Table 1 shows the peak potentials of the three a.c. waves for 1 x 10 -4 M sulphide in supporting electrolytes of varying pH. As the pH is decreased the separation between the first two waves decreases (e.g., the waves are 47 mV apart in 2 M N a O H but only 25 mV apart in 0.5 M NaOH). In 0.1 M NaOH the two waves could
P O L A R O G R A P H I C B E H A V I O U R OF S U L P H I D E I O N
197
ca~
rtt_
ll..IA
2S
D
time Fig. 5. Current-time curves (d.c.) for 1.5 x 10 -3 M sulphide in 1 M NaCIO4. (a) -0.784; (b) -0.783; (c) -0.782; (d) -0.781 v (Ag/AgCl).
However, at a potential only 1 mV more positive (Fig. 5c) the current increased rapidly at the beginning of each drop indicating that faradaic current was present at this potential (i.e., if oc tl/6). In other words, d.c. current-voltage curves in 1 M NaC104 start discontinuously. Accurate measurement of the peak potential of the first a.c. wave, using an applied alternating voltage of 0.1 mV (peak-to-peak) at 100 Hz, gave a value of -0.783+0.001 V, which coincided with the starting potential of anodic discharge.
Polarographic behaviour in 1 M NaOH The d.c. polarographic behaviour of sulphide ion in 1 M N a O H was in many respects similar to that in 1 M NaC104 (typical d.c. polarograms are shown in Figs. 6a and 7a). For concentrations below 2 x 10 -4 M a single d.c. wave was observed (Fig. 6a) and as the concentration was increased three more waves appeared at more positive potentials. Again it was difficult to distinguish between waves 2 and 3. At high sulphide concentrations the limiting heights of waves 1 and 2 were very difficult to measure accurately because of maxima. The dependence of the height of the waves on sulphide concentration is shown in Fig. 8. As in 1 M NaC10¢, the limiting current of wave 1 became almost independent of concentration after the appearance of wave 2, and the total limiting current was again a linear function of concentration over the entire range studied. One of the main differences in the d.c. polarographic behaviour in the two supporting electrolytes was that in 1 M N a O H the current-voltage curves did not start discontinuously, as they did in I M NaC104 (compare Figs. la and 6a). The a.c. polarographic behaviour of sulphide in 1 M N a O H was significantly different to that in 1 M NaC104. For example, below 2 x 10 -4 M three distinct a.c. waves were recorded in 1 M N a O H (Fig. 6b) although only one d.c. wave was observed. The peak potential of the first a.c. wave corresponded closely to the half-wave potential of the d.c. wave. The second a.c. wave occurred near the beginning of the limiting current plateau of the d.c. wave and was much narrower than a normal a.c.
199
P O L A R O G R A P H I C B E H A V I O U R OF S U L P H I D E I O N
1 (a) -fig
(b -0:7
(
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c
-09
)
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Ag/AgCL
Fig. 9 A.c. polarograms of 1 x 10 - 4 M sulphide in 1 M N a O H at various applied alternating voltages (AE): (a) 10; (b) 5.0; (c) 1.0 inV.
TABLE 1 P E A K P O T E N T I A L OF A.C. WAVES F O R 1 x 10 - 4 M S U L P H I D E IN S U P P O R T I N G E L E C T R O L Y T E S OF D E C R E A S I N G p H
Supporting electrolyte
Peak potential/V (vs. Ag/AgCI) First wave
2 M NaOH 1 M NaOH 0.5 M N a O H
-0.775 -0.785 - 0.785
Second wave -0.728 -0.748 - 0.760
Third wave -0.326 -0.340 - 0.348
not be resolved. Thus the conditions necessary for observation of the second a.c. wave are low sulphide concentrations, high pH, and large AE. The third a.c. wave, at a much more positive potential, was similar to the sharp a.c. wave observed in 1 M NaC104. As in 1 M NaC104 this a.c. wave became smaller and less sharp with increasing concentration while its peak potential remained constant. However, Table i shows that the peak potential shifted to less negative values with increasing pH (also compare Figs. la and 6a). DISCUSSION
Apart from an extra wave being observed in the present work the results for d.c. polarography were similar to those reported by Julien and Bernard 5. Comparison of the results suggests that their second wave corresponds to waves 2 and 3 while their third wave corresponds to wave 4. As stated previously, it was difficult to distinguish
200
D. R. CANTERFORD, A. S. BUCHANAN
between waves 2 and 3. This was particularly so if damping was used on the recorder. Julien and Bernard reported difficultyin measuring the limiting current of their third wave at high sulphide concentrations. They also observed that after the appearance of the third wave the total limiting current increased with increasing concentration at a smaller rate than it did before the appearance of the third wave. That is, the total limiting current was not a linear function of concentration over the range studied. In the present work, however, the total limiting current plateau was very well defined at high sulphide concentrations (e.g., Fig. 3a), with reproducible measurement of the current being possible. Furthermore, the total limiting current varied linearly with sulphide concentration irrespective of the number of waves present (see Figs. 4 and 8). The fact that a three-electrode potentiostat was used whereas presumably only a two-electrode cell was used by Jutien and Bernard may account for these differences in results. Attempts to reproduce the results of Zhdanov and Kiselev4 were not successful. In 0.1 M KC1 the d:c. polarographic behaviour was similar to that in 1 M NaC104 and no evidence could be found for two diffusion controlled waves. The pH of a 10- 3M sulphide -0.1 M KC1 solution was 10.1 At this pH sulphide exists in solution almost exclusively as the HS- species. Thus it would seem most unlikely that two diffusion controlled waves (due to HS- and S2-) could be observed. Armstrong and co-workers have recently investigated the sulphide-mercury interface using impedance and potentiostatic techniques. They found that in solutions buffered at pH 9.5 sulphide was very strongly adsorbed on mercury over a short potential range, leading to high double-layer capacities 9. They also reported 1° that in highly alkaline sulphide solutions mercury passed into solution as the soluble complex HgS2z-, with the overall process being: Hg+2S z- ~ HgS~- +2e
(4)
Double-layer capacity measurements indicated the absence of any strong specific anion adsorption prior to the dissolution process. At low pH this reaction is considerably reduced because of the low concentration of "free" S2-. An investigation of the growth of thin anodic films of mercuric sulphide on a mercury electrode showed that in the initial stages of film growth mercuric sulphide was deposited on the electrode in the form of two successive monomolecular layers 11. The presence of each layer severely inhibited the rate at which mercury dissolved as HgS 2-. At high sulphide concentrations the presence of four d.c. waves suggests the formation of another layer of HgS apart from the two monolayers postulated by Armstrong et al. Current-time curves for individual drops were recorded for a number of sulphide concentrations in an attempt to confirm this suggestion. Figure 10 shows current-time curves at various potentials for 1.5 x 10 -3 M sulphide in 1 M NaC104. The position of the curves on the corresponding d.c. polarogram can be found by reference to Fig. 3a. (Current-time curves recorded in 1 M NaOH were similar to those in 1 M NaC104.) The three distinct maxima on these curves are consistent with the formation of three layers of HgS on the electrode surface, although this evidence is by no means conclusive. At low sulphide concentrations the overall electrode process is controlled
POLAROGRAPHIC
BEHAVIOUR
OF SULPHIDE
201
ION
by diffusion of sulphide (S 2- or HS-) to the electrode surface and a single d.c. wave is observed. The appearance of wave 2 indicates that the sulphide concentration has. become sufficient for the surface of the mercury drop to become completely covered by a monolayer of HgS. The limiting current of wave 1 (i11)then becomes independent (or almost so) of concentration because dissolution of mercury is no longer controlled by diffusion of sulphide to the electrode but by transport of "mercury" through the HgS film. The charge transfer process therefore proceeds with a considerable overvoltage in the presence of this film, and wave 2 appears at a more positive potential. Similarly with increasing sulphide concentration the appearance of waves 3 and 4 at still more positive potentials indicate that the electrode has become covered by a second and thirdlayer of HgS.
| I st rnonolai,er 2 2ndmonolayer 3 ~'d[ayer
(a) 2 (b)
2
3
A
C L_ L_ U (f) ~
li'~
(d) time Fig. 10. C u r r e n t - t i m e curves (d.c.) for 1.5 × 10 -3 M sulphide in 1 M NaC10~. (a) - 0 . 7 3 0 ; (b) - 0 . 6 4 0 ; (c) - 0.550; (d) - 0.420; (e) - 0.280; (f) - 0.120 V (Ag/AgC1). 1 --- 1 st monolayer, 2 = 2nd m o n o t a y e r , 3 = 3rd layer.
Current-time curves on the plateau of wave 1 (Fig. 10a) pass through a maximum and then rapidly decay. The sudden decrease in current can be ascribed to full coverage of the electrode surface by a layer of HgS. Curves of similar shape result when a species is adsorbed on the electrode surfacelZ.At more positive potentials (Figs. 10b-e) current-time curves become complicated due to the presence of two more layers of HgS which further inhibit the dissolution of mercury. For a sulphide concentration of 1.5 x 10-a M a very large overvoltage must be applied before the overall process becomes controlled by diffusion of sulphide to the electrode surface and currenttime curves have their normal i o c t 1/6 shape (Fig. 100. In 1 M NaCI04 (low pH) the coincidence of the peak potential of the first a.c. wave with the sharp start of the anodic discharge indicates that the negative (cathodic) branch of this a.c. wave lies in a potential region where no charge transfer across the electrode-solution interface occurs and, therefore, this branch involves non-faradaic
202
D. R. CANTERFORD, A. S. BUCHANAN
alternating currents. Similar results were obtained by Biegler 13 in his investigation of the polarographic behaviour of chloride ion. By analogy with Biegler's conclusions for chloride it is suggested that the negative branch of the first a.c. wave is due to the high differential capacity of the double-layer following upon specific adsorption of sulphide ion, a conclusion which is in agreement with the results of Armstrong et al. 9. In 1 M N a O H (high pH) the first a.c. wave probably corresponds to the dissolution process given by eqn. (4). Presumably this is the a.c. wave observed by Breyer and Hacobian 8. It is interesting to note that although Kolthoff and Miller 2 suggested that mercury dissolved as HgS 2-, subsequent authors ignored this possibility attempting to interpret the polarographic behaviour of sulphide. At low sulphide concentrations the appearance of a second a.c. wave at a slightly more positive potential (Fig. 6b) is more difficult to explain. The sharpness of this wave and the absence of a corresponding d.c. wave suggests that it is tensammetric 14 in nature. That is, it is due to a sharp change in surface charge density brought about by an adsorption/ desorption process. In all supporting electrolytes studied an a.c. wave was observed at a much more positive potential than the a.c. waves already discussed. The sharpness of this wave, particularly at low concentrations, and the absence of a corresponding d.c. wave suggest that it is also tensammetric in nature. Biegler 15 observed a similar wave for iodide which he attributed to desorption of a film of mercurous iodide or at least a severe change in its structure. It seems reasonable to propose that this previously unreported wave for sulphide arises from a similar process. SUMMARY The polarographic behaviour of sulphide ion has been investigated in several supporting electrolytes with the a.c. and d.c. techniques. Four distinct d.c. waves have been observed at high sulphide concentrations although only three waves have previously been reported. The appearance of four waves can be explained by deposition on the electrode surface of three successive layers of HgS which inhibit dissolution of mercury as HgS 2-. The a.c. polarographic behaviour depends markedly on pH, as well as sulphide concentration, with up to three waves being observed. A very narrow a.c. wave which was observed at all sulphide concentrations in all supporting electrolytes studied is probably due to desorption of HgS from the electrode surface.
REFERENCES i J. Revenda, Collect. Czech. Chem. Commun., 6 (1934)453. 2 I. M. Kolthoff and C. S. Miller, J. Amer. Chem. Soc., 63 (1941) 1405. 3 A. Trifonov,Izv. Khim. lnst. Bulg. Akad. Nauk., 4 (1956)21. 4 S. Zhdanov and B. Kiselevin G. J. Hills (Ed.), Polarography, 1964, Vol. 1, Macmillan, London, 1966 p. 473. 5 L. Julien and M. L. Bernard, Rev. Chim. Min., 5 (1968)521. 6 R. Brdicka, Z. Elektrochem., 48 (1942) 278. 7 E. Laviron and C. Degrand in G. J. Hills (Ed.),Polarography, 1964, Vol. l, Macmillan, London, 1966, p. 337. 8 B. Breyer and S. Hacobian, Aust. J. Sci. Res., A4 (1951)610. 9 R. D. Armstrong, D. F. Porter and H. R. Thirsk, J. Electroanal. Chem., 16 (1968)219.
POLAROGRAPHIC BEHAVIOUR OF SULPHIDE ION
203
R. D. Armstrong, D. F. Porter and H. R. Thirsk, J. Electroanal. Chem., 14 (1967) 17. R. D. Armstrong, D. F. Porter and H. R. Thirsk, J. Phys. Chem., 72 (1968) 2300. R. W. Schmid and C. N. Reilley, J. Amer. Chem. Soc., 80 (1958) 2087. T. Biegler, J. Electroanal. Chem., 6 (1963) 357. B. Breyer and H. H. Bauer, Chemical Analysis, Vol. XIII, Alternating Current Polarography and Tensammetry, Interscience, New York/London, 1963. 15 T. Biegler, J. Electroanal. Chem., 6 (1963) 373.
10 11 12 13 14