Potentiometric investigation of oxide solubilities in molten KCl-NaCl eutectic. The effect of surface area of solid particles on the solubilities

摘要

The sequential additions method (SAM) was used to determine solubility and disassociation characteristics of metal oxides MeO (CaO, PbO, ZnO, CdO) and carbonates (CaCO_3, SrCO_3, BaCO_3) in molten KCl-NaCl eutectic at 700 ℃. A membrane oxidereversible electrode Pt(O_2)/YSZ) was used to detect oxide ion concentrations. The oxides (MeO) were found to be weak bases in unsaturated solutions, their dissociation constant (pK) were found to be 3.27 ± 0.2 (CaO), 3.50 ± 0.1 (PbO), 6.1 ± 0.2 (ZnO) 5.3 ± 0.2 (CdO). All the emf-(-log m_(MeO) plots in the vicinity of the saturation point include bends to the concentration axis. These were explained by the partial dissolution of MeO powders resulting in a decrease of their particle sizes and a corresponding rise of the molar surface areas (MSA). Calculations performed give surface energies of MeO_(solid)/KCl-NaCl systems of the order of 30-50 J m~(-2). This allowed us to estimate MSAs of the MeO particles formed in MeO solubility determinations in molten KCl-NaCl by isothermal saturation and potentiometric titration methods. The solubilities of the alkaline earth carbonates were determined in a CO_2 atmosphere. Under these conditions solubility products of the carbonates were achieved before oxide precipitation. The alkaline earth carbonates were found to be appreciably soluble in the KCl-NaCl eutectic at 700 ℃. Their solubility products determined by SAM were equal to pP_(BrCO_3) = 0.36 ± 0.3, pP_(SrCO_3) = 2.04 ± 0.2, pP_(CaCO_3) = 2.04 ± 0.2 (in a CO_2 atmosphere). Under these conditions the particle size effect was imperceptible.