Kinetics of the dissolution of pure silver and silver-gold alloys in nitric acid solution

Abstract

This article describes a kinetic study of the dissolution of silver and silver-gold alloys in nitric acid. For pure Ag, the nitric acid reaction order is two at low concentrations and one for concentrations above 4 M. When attacking alloys, the reaction order is one and drops to zero when the alloy contains 51.1 pct of silver by weight or less and the nitric acid concentration is 6 M or above. The activation energy is 12.1 kcal/mol in both cases. When the molar fraction of silver is 0.70 or more, the rate of silver dissolution from its gold alloys is controlled by the chemical reaction on the solid surface. When the molar fraction of the silver is less than 0.65 and nitric acid activity and temperature are high, the dissolution is controlled by the outward diffusion of silver nitrate through the undissolved gold layer. The dissolution rate is affected by the composition of the alloy. When the molar fraction of silver is 0.76 or more, the gold atoms are found in the lattice isolated or in pairs and silver atoms may dissolve without difficulty. Between 0.55 and 0.76, the reaction rate decreases quickly when the gold content increases because the atoms make up chains into lattice and this makes the dissolution difficult. When the molar fraction is less than 0.54, the alloy does not dissolve since the gold atoms form continuous surfaces that impede the attack.