Chemical Behavior of the Components of the KCN / KAu ( CN ) 2 Electroplating System

Abstract

The chemical reactions occurring within aqueous solutions of potassium cyanide, potassium cyanide and metallic gold, potassium gold cyanide, and mixtures of these compounds with potassium hydroxide have been examined. It was determined that aqueous potassium cyanide solutions are unstable to water and air. The major reaction products formed were the formate and carbonate ions which increased in concentration with time. This increase, which was temperature dependent, was accompanied by a corresponding decrease in the cyanide ion concentration. Analyses confirmed that gold would dissolve, to any extent, only in those aqueous potassium cyanide solutions exposed to air. The reactions of potassium cyanide with water and oxygen within solutions of potassium cyanide and metallic gold produced similar products to those found in pure potassium cyanide solutions.The addition of potassium hydroxide in concentrations greater than ∼0.1M to aqueous potassium cyanide solutions depressed the rate of hydrolysis of potassium cyanide. However, concentrations far in excess of 1M potassium hydroxide would be necessary to prevent the formation of formate ion completely. Analyses confirmed that aqueous potassium gold cyanide solutions are very stable to heat, high concentrations of hydroxide ion and atmospheric oxygen, and show no detectable signs of decomposition. The solutions are likewise stable to acid at room temperature down to a pH of ∼4.5, below which a slow decomposition to insoluble gold cyanide occurs. At pH 1, aqueous potassium gold cyanide solutions remain stable indefinitely at a temperature of 1°C. In mixtures of potassium gold cyanide, potassium cyanide, and potassium hydroxide, which are typical electroplating solution components, the stability of potassium gold cyanide was apparently unaffected. The reactions of potassium cyanide with water and oxygen within these mixtures gave no other products but those detected in a pure potassium cyanide solution. Potassium hydroxide merely decreased the rate of hydrolysis of the potassium cyanide.Activated charcoal treatment of the cyanide solutions did not remove any of the ionic products of decomposition.