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 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 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 H2S 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.