Ion-exchange separation and spectrophotometric determination of thorium in silicate rocks with arsenazo III

A combined chromatographic-spectrophotometric method has been developed for the determination of thorium in silicate rocks. About 0.5 g of rock sample in a platinum dish is decomposed with a mixture of perchloric and hydrofluoric acids by heating. Heating to dryness with perchloric acid is repeated again, and the residue is taken in 6 M hydrochloric acid. This solution is passed down a column of Amberlite CG 400 (Cl-) to get rid of accompanying iron and other metals forming chlorocomplexes. The effluent is evaporated to dryness and taken in 6.6 M nitric acid. Anion-exchange from this solution allows thorium to be retained quantitatively on a column of Amberlite CG 400 (NO3-). Thorium is then stripped by elution with 30 ml of 0.1 M nitric acid and subsequently determined spectrophotometrically with arsenazo III in 9M hydrochloric acid solution. The detection limit and the coefficient of variation of this procedure were 0.1 ppm and 2.5%, respectively. Thorium in standard rocks of U.S. Geological Survey and Geological Survey of Japan were determined with the proposed procedure. The values of G-2, GSP-1, AGV-1, and BCR-1 of U.S.G.S. were 22.7, 11.8, 7.61 and 5.47 ppm, and of JG-1 and JB-1 13.2 and 8.89 ppm, respectively. (author)