Evaluation of a molecular recognition ligand for performing the extraction of palladium, platinum and rhodium from ion-charged solutions and its application to geochemical exploration techniques using electrothermal atomic absorption spectrometry

Abstract

In order to develop a simple analytical procedure for determining the content of Pd, Pt and Rh in samples of stream and lake sediment, the ability of a molecular recognition ligand (Superlig 202: SL-202) to extract quantitatively these three elements from solutions loaded with different ions was assessed. The evaluation was based on (1) the use of a 0.1 mol dm^–3 HCl solution containing 50 ng of Pd, 250 ng of Pt and 50 ng of Rh, to which 100 µg of each of the following six metals had been added: Co, Cu, Cr, Ni, Pb and Zn; and (2) a solution resulting from an aqua regia[HCl–HNO₃(3 + 1)] attack on a standard sample of stream sediment the Pd, Pt and Rh content of which had been previously determined by the lead fire assay method. The results indicate complete recovery of Pd, Pt and Rh from the 0.1 mol dm^–3 synthetic solution, with no particular interference by the six added metals. For the solution resulting from the aqua regia attack, the data sets for Pd and Pt are indistinguishable from the concentrations measured by lead fire assay, when taking analytical errors into consideration. No Rh was detected in this solution. In order to show that the loss of Rh was not caused by problems related to the process of fixing and washing the ligand, but rather to the solubility of Rh during the aqua regia attack, standard additions of Pd, Pt and Rh (10 µg kg^–1 of Pd, 50 µg kg^–1 of Pt and 10 µg kg^–1 of Rh) were made directly to the portion of standard sample to be attacked by aqua regia. The total recovery of Pd and Pt and the 80% recovery of Rh implies that the effectiveness of the aqua regia attack is the determining factor in the final quantitative determination of Pt and Rh. An analytical approach in three steps for the quantitative determination of Pd, Pt and Rh in stream and lake sediment samples by electrothermal atomic absorption spectrometry is suggested.