Trace determination of mercury, thallium and gold with crystal violet

Abstract

A spectrophotometric method for the determination of small amounts of mercury, thallium and gold is presented. The sample is wet ashed with a nitric acid-hydrochloric acid mixture, and interferences are eliminated by adding ethylene glycol monomethyl ether and EDTA. Thallium interference is eliminated by adding sodium metabisulphite for mercury and gold, whereas excess oxidant interference is eliminated by adding hydroxylamine for thallium and gold. Because gold interferes in the determination of thallium and mercury, it is determined separately after heating the sample to volatilise the mercury. Iodide forms complexes with mercury and gold, whereas bromide forms complexes with thallium and gold. Crystal violet produces a tolueneextractable compound with these complexes in an acidic medium. A single extraction step suffices to determine 0·1 µg of each element in a 1-cm cell with a standard spectrophotometer at 605 nm; Beer's law is followed up to absorbance of unity. The method has been tested with urine, air, vegetation, water, soil, rocks and sediments.