Sulphur and its species have a significant environmental role because of their complexation with many toxic agents and their primary contribution in acidification of the water systems. Consequently, the oxidation of sulphur aqueous species (H_2S, HS~-, S~2) is of considerable environmental and technological importance as well. It is known that vanadium (Ⅴ) salts are responsible for the oxidation of hydrosulphide, therefore the oxidising action of vanadium (Ⅴ) salts stimulated our interest, as reported here. The approach taken was to investigate the Ⅴ(Ⅴ)/Ⅴ(Ⅳ) redox couple using electrochemical techniques such as cyclic and pulse voltammetry. Products were identified using UV-visible and esr spectroscopy. The reduction of vanadium (Ⅴ) was found to be irreversible on a variety of electrode surfaces (mercury, carbon, gold; at 20-40°C: pH9.2), and led to the formation of solid oxide films (V_3O_5, V_2O_3 and VO) rather than to Ⅴ(Ⅳ) solution species. Irreversible behaviour is commonly observed when a large structural rearrangement is necessitated as the reactant is reduced. In the present case, HV_2O_7~3 is the probable Ⅴ(Ⅴ) species and V_(18)O_(42)~(12-) is the likely Ⅴ(Ⅳ) species. The fact that vanadium (Ⅴ) is an effective oxidising agent for hydrogen sulphide suggests that there is some specific chemical interaction between them that facilitates Ⅴ(Ⅴ) reduction, such as the formation of thiovanadate complexes. An attempt was made to investigate the redox chemistry of the thiovanadate ion VS_4~(3-), using cyclic voltammetry, but large background currents due to the oxidation of HS~- ions obscured any currents that might have been due to the reduction of VS_4~(3-) ions.A concluding remark is that the search for a more effective sulphide oxidation agent may be best focused on those transition metals which are also known to form thiometalates; for example molybdenum, tungsten, and rhenium.
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