Solubility product— from potentiometric titration curve

We have also investigated the solubilities of MgO and CaO in molten potassium halides at 800 °C [197] to elucidate the effect of anion composition of the halide melt on metal-oxide solubility. The MgO was found to be practically insoluble in chloride and bromide melts, and the iodide-melt could not be investigated owing to intense iodine evolution from strongly acidic solutions. In contrast, CaO solubility products were determined successfully in all the potassium halide melts at 800 °C, by the potentiometric titration method. The corresponding potentiometric titration curves are shown in Fig. 3.7.16. 

Chao and Cheng [76CHA/CHE] studied the determination of a number of anions, single or in mixtures, by a stepwise potentiometric titration with silver nitrate at pH = II. The silver ion activity was measured with a silver ion selective electrode based on silver sulphide. The data were also used to estimate the solubility products of the silver salts formed during a titration. The method is only sketched in the paper but appears to have proceeded along the following course. The potential of incipient precipitation ( prec) was estimated graphically from the shape of the titration curve. p,ec would thus be a measure of the silver ion activity at the nominal and known concentration of the anion in the presence of its silver salt. 

As is seen from Fig. 3.7.1 the pO values at the excess of the studied cation are sufficiently low, and owing to the enhanced acidic properties of the KCl-LiCl melt, the increase in the melt acidity results in a considerable increase in the oxide solubility, so that CoO, which is practically insoluble in the molten KCl-NaCl equimolar mixture, becomes appreciably soluble with a sharp pronounced section of the unsaturated solution (see Fig. 3.7.1, curve 3). The existence of the said section allows us to calculate the dissociation constant of CoO in the molten KCl-LiCl eutectic at 700 °C using the potentiometric data for three initial points of the calibration curve (the corresponding treatment results are collected in Table 3.7.2), and its average value is presented in Table 3.7.3. The fourth point of the titration curve is the boundary one between the saturated and unsaturated solution, and therefore, it is available for calculations of the values of both the dissociation constant and the solubility product. 

---