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 +, M 2 (SH) 3+, and M 3 (SH) 5+, which are labile under diffusion control conditions, in seawater at pH values > 7. These complexes dissociate below pH = 7, releasing H 2 S from solution. Evidence for sulfide-rich complexes of form M(SH) 2 was not found. Cu and Zn form sulfide, S 2 -, complexes of stoichiometry MS and M 2 S 3 2 -, 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 [M x S] 2 x - 2 from the titration data. The M 2 S 3 2 - complexes are tetrameric structures (M 4 S 6 ) with tetrahedral metal coordination based on known thiolate complex and mineral geometries. The Zn complexes dissociate below pH = 6.7, releasing H 2 S from solution. However, the Cu complex does not fully dissociate below pH = 2 because of Cu(II) reduction and production of polysulfide, which do not appear to be quantitative. At seawater pH, both Zn and Cu sulfide complexes can be deposited on a mercury drop, indicating that these metal complexes are likely responsible for the presence of nanomolar levels of sulfide in oxic seawater.