The Silver Sulfide Reference Electrode for Use in Alkaline Sulfide Solutions

The high-temperature behavior of the Ag-Ag₂S electrode has been investigated in aqueous solutions of NaOH + Na₂S, the compositions of which were of particular relevance to the kraft pulping process. Potentials were measured with respect to an external reference electrode and corrected for liquid junction potential (LJP) effects in the salt bridge. The corrected potentials were compared with thermodynamically calculated potentials based on equilibrium between the Ag-Ag₂S electrode and bisulfide ion (HS^–). Under some circumstances, reasonable agreement was obtained between the thermodynamically predicted potentials and the observed behavior. In other situations, anomalous effects were observed, indicating unpredictable and irreversible behavior of the electrode. The anomalous behavior was attributed to the formation of soluble silver complexes. It was concluded that the Ag-Ag₂S electrode is unsuitable for use as a reliable internal reference electrode in hot alkaline sulfide solutions, but it may find useful application as an external reference electrode operating at 25 C.