KINETICS OF THE DISSOLUTION OF PURE SILVER AND SILVER-GOLD ALLOYS IN NITRIC-ACID SOLUTION

This article describes a kinetic study of the dissolution of silver an d silver-gold alloys in nitric acid. For pure Ag, the nitric acid reac tion order is two at low concentrations and one for concentrations abo ve 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 i s 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 con trolled 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 dif fusion of silver nitrate through the undissolved gold layer. The disso lution rate is affected by the composition of the alloy. When the mola r 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 dif ficulty. Between 0.55 and 0.76, the reaction rate decreases quickly wh en the gold content increases because the atoms make up chains into la ttice and this makes the dissolution difficult. When the molar fractio n is less than 0.54, the alloy does not dissolve since the gold atoms form continuous surfaces that impede the attack.