Thermodynamics of solid solutions (Ag, Na)2MoO4 and silver molybdate complexation in molten (K, Na)NO3. Potentiometric and solubility study

Abstract

The complexes AgMoO^–₄, Ag₂MoO₄ and Ag(MoO₄)^3–₂ have been shown to exist in molten 1:1 (K, Na)NO₃ and the corresponding stability constants, β_mn, have been determined by potentiometry and solubility measurements. The complexes are weak and e.m.f. measurements give by far the best estimate of β₁₁ for AgMoO^–₄. Changes in β₁₁ in the interval 524 < T/K < 622 are analysed in terms of ΔH^°₁ and ΔS^°₁ for the association process. ΔH^°₁ and –TΔS^°₁ contribute to about the same extent to ΔG^°₁. This thermodynamic pattern is rather similar to that of AgCrO^–₄ formation in the same solvent, but the present results indicate the possibility of slightly stronger Ag—O bonding in the MoO^2–₄ system. A continuous range of solid solutions (Ag, Na)₂MoO₄ with the Ag¹ fraction ranging from 1 to 0.4 has been observed at 553 K. An analysis of distribution equilibria between solids and melts shows that the (Ag, Na)₂MoO₄ solids can be treated as regular cationic substitutional solutions with an interaction parameter A= 9.6 ± 0.5 kJ mol^–1.