Environ. Sci. Technol. 1996, 30, 3638-3639
Comment on “Determination of Metal (Bi)Sulfide Stability Constants of Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+ by Voltammetric Methods” SIR: The paper by Luther et al. (1) illustrates how voltammetry can be used to determine stability constants and reports data for metal sulfides where there has been poor agreement previously (2-5). Square wave voltammetry was used to measure the concentration of labile sulfide species by electrochemically oxidizing the mercury electrode according to eq 1 where HgS is an insoluble substance:
HS- + Hg f HgS + H+ + 2e
(1)
Titration of sulfide with the +2 cations of Mn, Fe, Co, and Ni produced a shift in the sulfide peak potential, Ep. Luther et al. (1) assumed that a labile metal sulfide species was formed and that the Ep shift, ∆Ep, could be interpreted using Heath and Hefter’s (6) modification of the method of DeFord and Hume (7). Using this procedure, they obtained cumulative constants β1, β2, and β3 for the species MSH+, M2(SH)3+, and M3(SH)5+ where M2+ represents a divalent metal cation. Values of log β1, log β2, and log β3 measured in dilute seawater were remarkably similar (
