โ Scribed by D.R. Canterford; A.S. Buchanan
No coin nor oath required. For personal study only.
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 Kiselev 4 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 S z-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-
๐ SIMILAR VOLUMES
Recently 1, differential pulse polarography has been shown to be suitable for the determination of small concentrations of anions (e.9., sulphide, halides) which give rise to anodic polarographic waves corresponding to mercury compound formation. The present communication reports on the application