Thermodynamics of Transfer and Solvent Transport of Silver(I) Sulphate, Bromate and Iodate in Ethanol-Pyridine Mixtures

Abstract

The Gibbs energies of transfer and the solvent transport numbers of silver(I) salts viz. silver sulphate, silver bromate and silver iodate have been determined in ethanolpyridine mixtures over the complete range of compositions at 30°C. The solubility of all the salts passes through a maximum near Xpy =0.6, Xpy =0.5 and Xpy =0.6 for silver sulphate, bromate and iodate respectively and decreases subsequently with further addition of pyridine. The standard Gibbs energies of transfer from ethanol to ethanolpyridine mixtures were calculated from the thermodynamic solubility products of the salts in these mixtures and found to decrease up to Xpy =0.3, Xpy =0.3 and Xpy =0.6 for silver sulphate, bromate and iodate respectively and then increase with the addition of pyridine. The ionic free energy of transfer of silver ion, determined on the basis of negligible liquid junction potential method, was found to be negative and decreases continuously while that of anions is positive and increases with the addition of pyridine. A large positive value of the solvent transport number, Δ is observed for all the salts in these mixtures. These results indicate that all the silver salts are heteroselectively solvated ie the silver ion is preferentially solvated by pyridine and anions by ethanol in this mixed solvent system.