CYANIDE OXIDATION AT NICKEL ANODES .1. THERMODYNAMICS OF CN-H2O AND NI-CN-H2O SYSTEMS AT 298-K

The thermodynamics of CN-H2O, Ni-H2O, and Ni-CN-H2O systems are summarized in the form of potential-pH and activity-pH diagrams calculated from published standard Gibbs energies of formation of the various species considered. However, uncertainty in the Gibbs energies of formation of Ni(CN)2(c) and Ni(CN)4(2-) ions precludes reliable prediction of passivation or nickel dissolution at intermediate potentials. At higher potentials Ni(OH)3(c) is predicted to form from Ni(CN)2(c) or Ni(OH)2(c), and Ni2+ or Ni(CN)4(2-) ions, depending on the conditions, causing passivation, and should also be capable of oxidizing cyanide under open-circuit conditions. In addition, Ni(OH)3(c) on Ni may behave as a suitable conducting substrate for anodic cyanide oxidation (and oxygen evolution). These calculations provide predictions for the behavior of nickel anodes in alkaline electrolytes containing cyanide, the oxidation of which is described in a subsequent paper.