Preconcentration of traces of gold, silver and palladium on activated carbon and its determination in geological samples by flame AAS after wet ashing

A simple, rapid and cost effective preconcentration method is described for the determination of traces of gold (Au), silver (Ag) and palladium (Pd) in geological samples by flame atomic absorption spectrophotometry (AAS). The method is based on sorption of analytes (Au, Ag and Pd) on powdered activated carbon (AC) at pH 1 in hydrochloric acid (HCl) medium. The samples are decomposed by aqua regia — HCl treatment. The quantitative sorption (better than 92%) of analytes is obtained on AC (in absence of complexing agent), by simply manipulating optimal conditions. The unique feature of the method is, the analytes are recovered quantitatively from AC by oxidizing and completely solubilising the carbon using concentrated nitric acid (HNO₃) and perchloric acid (HClO₄) i.e. by wet ashing. The method of wet ashing has several advantages over conventional dry ashing. The accuracy of the method is evaluated by analysing, five Canada centre for mineral and energy technology (CANMET) standards; MA-3, MA-1b, FER-1, SU-1A, and CPB-1. In addition, ASK-3 and one inhouse standard, kolar gold field (KGF) samples was also analysed. As no standard for Pd is available, its accuracy was evaluated by standard addition method. The method was applied on numerous geological samples for the determination of Au, Ag and Pd down to 0.1 ppm (based on 10 g sample) within ±10% R.S.D. (n=5). The method could easily be adopted by any laboratory as the inputs are minimal (AC), inexpensive and easily available.