The Second Dissociation Constant of H 2 S

A recent method demonstrated for glass p H electrode measurement in concentrated alkaline solutions is employed to measure the aqueous second acid dissociation constant, , of . At 25°C, we find to be <10 −17.3 . Values up to 5 orders of magnitude larger than this are commonly applied. Uncertainties arising in previous studies, from attempts to access small amounts of available free sulfide, are minimized by using recently characterized, extremely concentrated electrolytes. Measured p H values, for ( i ) and ( ii ) titrated by , are compared to simulated curves which are computed with measured water activities and cation selective electrode measured potassium ion activities. The concentration dependent dissociation parameter, , where imposes a unit ratio of the HS − to S = activity coefficients, is found to be , consistent with recent spectroscopic studies.