Potentiometric-determination of heavy metal sulphide solubilities using a pH2S (glass|Ag°, Ag2S) electrode cell

Abstract

PH₂S values (–log [H₂S]) were calculated from the electrochemical reaction of a glass|Ag⁰, Ag₂S electrode cell in equilibrium with metal sulphide precipitates at pH ⩽ 5 and compared with pH₂S values predicted by the law of mass action. The results obtained indicated that the pH₂S electrode cell has a Nernstian response at very low total sulphide concentrations. Measurements of the ion activity products in these solutions verified literature values. However, they also indicated that sulphide solubilities are influenced by the adsorption of hydrolysed metal species, leading to deviations from the theoretically predicted solubilities. As a general result, a detection limit for total sulphide activities corresponding to pH₂S 18.2 was obtained. The electrode cell proved to be a suitable tool for the determination of the ion activity products of metal sulphides in difficult media (marine or estuarine waters) with a precision of ±0.3 logarithmic unit.