Stability of silver–thiourea complexes in montmorillonite clay

Abstract

Ion exchange adsorption of a silver–thiourea complex into sodium-, calcium- and aluminum montmorillonite at 25°C has been studied. The reversible nature of the reaction is demonstrated and the standard free energy changes for the displacement of the three ions by the silver complex are calculated. The results are, in kJ equiv.^–1: –20 (Na–Ag), –20.7 (Ca–Ag) and –19 (Al–Ag). These values are consistent with values for Na–Ca and Ca–Al exchanges, found in the literature. The free energy loss for adsorption of the silver complex exceeds that for the aqueous ion by some 19 kJ equiv.^–1 The enhancement of the selectivity and the coordination values of adsorbed silver ions are consistent with the following free energy changes for the successive formation of the complexes: ΔG₁=–59.5, ΔG_1→2=–20.3 and ΔG_2→3=–7.4 kJ mol^–1. Relative to the free energies of formation of these complexes in solutions, these values correspond to stabilization of the complex containing one thiourea molecule and destabilization of that containing three.