Promotion of the Cupellation Method for Accurate Determination of Gold Alloy’s Karat Containing Platinum-Group Metals

The main standard method for gold karat determination is the cupellation method. However, this method is not sufficiently accurate to determine gold karat in the presence of insoluble Platinum–Group Metals (PGMs), such as Ir, Ru, and Rh. In this study, for the first time, a complementary method that can be used coupled with the reference cupellation method is presented for the highly accurate determination of gold karat containing PGMs. According to this method, gold metal was separated from the PGMs by dissolving parted cornets in aqua regia and then, its selective precipitation using an aqueous solution of SO2 gas as a reducing agent. The gold amount in alloys containing PGMs was determined using the suggested strategy with an excellent recovery, high accuracy (average relative error=0.12%) and precision (SD=0.6 for n=3). The optimized volume of aqueous SO2 solution was 35 mL that provides a gold recovery as high as 99.7% with enough big grain size and high purity (999.0‰). The proposed strategy was successfully used to determine the gold amount in secondary gold jewelry containing Ir, Rh, and Ru (a gold recovery of 99.9% with a relative error of 0.07 was obtained). The obtained karat was comparable with the other methods. Accordingly, the proposed method will be a promising simple and available assay for gold alloys containing PGMs, which could be used routinely in most cupellation labs all over the world.