Spectrophotometric determination of phosphorus and arsenic in steel by solvent extraction of their heteropolyacids with ethyl violet

Abstract

Under the same conditions, orthophosphate and orthoarsenate react with molybdate to form molybdophosphate and arsenomolybdate, which are extracted into a cyclohexane - 4-methylpentan-2-one mixture (1 + 3) with ethyl violet. The absorption spectrum of each ion pair, extracted into the organic phase, is almost the same in the visible region and the molar absorptivity of each ion pair in the organic phase is 2.8 × 10⁵ l mol^–1 cm^–1 at 602 nm. In determining phosphate, arsenate can be masked with thiosulphate and hydroxylamine. The arsenate concentration was obtained by subtracting the phosphate concentration from the total concentration of phosphate and arsenate. Steel samples (less than 0.25 g in mass) were dissolved in aqua regia and the solution was diluted to 1 l with distilled water. In the determination of phosphorus (about 0.003%) and arsenic (about 0.007%) in steel, the relative standard deviations were 2.5 and 2.1%, respectively. Fifteen standard steel samples were analysed, and the results obtained for phosphorus and arsenic were in good agreement with their certified values. The results for the recovery test were also good. The limit of detection for both phosphorus and arsenic is about 0.001% in steel.