Thermodynamics of the aqueous copper–cyanide system

A critical literature survey was conducted to examine the stability constants of copper cyanide species. The distribution and equilibrium potentials of copper cyanide species depend on the mole ratio of cyanide to copper, total cyanide concentration, pH and temperature. With increasing CN:Cu mole ratio, the distribution of copper cyanide species shifts more completely to the highly coordinated complex (Cu(CN)43−) at a high cyanide concentration than that at a low cyanide concentration. The equilibrium potential for Cu(I)/Cu decreases with increasing CN:Cu mole ratio. Increasing the pH is similar to increasing free cyanide concentration. Increasing temperature results in decreasing stability constants. Therefore, the distribution of copper cyanide shifts to the lowly coordinated complexes. The potential measurements have confirmed the validity of the calculated results. The Gibbs free energies have been derived using reliable stability constants of copper cyanide species. In the potential–pH diagram, CuCN, Cu(CN)2−, Cu(CN)32− and Cu(CN)43− predominate in the different pH regions, depending on the concentrations of copper cyanide species and free cyanide.