Trace metal-ion catalysis of oxidation of aqueous hydrogen sulfide by outer-sphere oxidants

Stopped-flow kinetic studies have been conducted on the oxidation of aqueous H2S in acetate buffers (pH ∼5) at 25 °C. The oxidants studied include [IrCl6]2−, [Ru(NH3)5isn]3+, [Ru(NH3)5py]3+, and [Ru(NH3)4bpy]3+. In all cases, the reactions are highly sensitive to catalysis by Cu2+ at the 2 μM level. In the presence of chelating ligands such as oxalate, EDTA, phenanthroline, and bathophenanthrolinedisulfonate the rates are substantially inhibited relative to reactions with no added metal ions. It is clear that the rates obtained without added chelating ligands or metal ions are due to catalytically active amounts of metal ions unavoidably present as impurities. This sensitivity to adventitious catalysis is likely a general characteristic of outer-sphere oxidations of H2S in acetate buffers. On the other hand, with Na9P2W15Nb3O62 as the oxidant in borate buffers (pH ∼9) the reaction rates are insensitive to the presence of EDTA or 2 μM Cu2+; this difference may be either a pH effect of a reflection of an inner-sphere mechanism.