The kinetic study of photoreduction of tetrachloroaurate acid by methanol in acidic media

• Kinetic studies of gold(III) chloride complex ions reduction by methanol.
• Influence of methanol and gold(III) initial concentration was investigated.
• Reduction reaction occur only in the presence of light.
• Metallic gold was formed after reaction due to disproportionation.

The aim of this study was the spectroscopic investigation of the mechanism of the reduction of gold(III) chloride complex ions in an aqueous methanol solution, illuminated at 405 nm. It has been shown that the gold(III) chlorate complex ions are reduced mainly to the gold(I) chloride complex ions with methanol only in the presence of light. The illumination must be within the range of the AuCl4− absorption. Kinetic studies have indicated that the reduction is first order with respect to both methanol initial concentration and light intensity, whereas the influence of gold(III) chloride complex ions initial concentration is more complicated. It has also been shown that the precipitation of a solid phase occurs mainly via the disproportionation of the AuCl2− ions. Moreover, illumination at 405 nm during the disproportionation process, results in the oscillation of the gold(III) chloride complex ions concentration.