Determination of metal (Bi)sulfide stability constants of Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+ by voltammetric methods

The stoichiometry as well as the conditional and thermodynamic stability constants for the (bi)sulfide complexes of the +2 cations of Mn, Fe, Ni, Co, Cu, and Zn have been determined by voltammetric methods in seawater and chloride solutions of varying ionic strength. Acid−base titrations allowed for the determination of the proton stoichiometry of the complexes. Mn, Fe, Ni, and Co form bisulfide, HS-, complexes of stoichiometry MSH+, M2(SH)3+, and M3(SH)5+, which are labile under diffusion control conditions, in seawater at pH values > 7. These complexes dissociate below pH = 7, releasing H2S from solution. Evidence for sulfide-rich complexes of form M(SH)2 was not found. Cu and Zn form sulfide, S2-, complexes of stoichiometry MS and M2S32-, which are inert (nonlabile) under diffusion control conditions, in seawater at pH values > 7. There is no evidence for metal-rich sulfide complexes of stoichiometry [MxS]2x-2 from the titration data. The M2S32- complexes are tetrameric structures (M4S6) with tetrahedr...