Potentiometric acid—base titrations in molten salts—III. The use of a metal/ metal-oxide electrode as an indicator electrode and the determination of acid—base equilibria in fused KNO3

Abstract

The feasibility of the Cu/CuO electrode for O2−-ion determination and for acid—base titrations in molten KNO3 at 350°C has been established. The potential of the electrode varies linearly with log [O2−] and the slope of the calibration curve is 2·303(RT/2F) at the working temperature. This allows the determination of the Cu/CuO/O2− standard potential. The CuO electrode is used for titrating the acid K2Cr2O7 with Na2O2 in molten KNO3. Sharp potential drops are recorded at the equivalence points. The equilibrium constant of the reaction Cr2O72− + O2− = 2CrO42− is calculated as ca 1·8 × 1012.The standard O2/O2− potential in fused KNO3 at 350°C has been calculated indirectly. With this value and the potentiometric titration curves obtained with the oxygen electrode, the first known acid—base equilibrium constants in fused salts are computed, as follows: ▪A correlation is drawn between the equilibrium constants calculated in the present study and the acidity numbers of the same oxy-anions previously proposed.