Determination of phosphate, arsenate and arsenite in natural water by flotation-spectrophotometry and extraction-indirect atomic absorption spectrometry using Malachite Green as an ion-pair reagent

Abstract

Sensitive and accurate methods are described for the determination of phosphate, arsenate and arsenite in natural water using Malachite Green as an ion-pair reagent. The ion pair of Malachite Green with molybdophosphate is floated as a membrane-like substance at the phase boundary between the aqueous phase and a diethyl ether phase. After discarding the aqueous phase, methanol is added to dissolve the floated ion pair. The determination of phosphate is performed by measuring the absorbance of the organic phase. The apparent molar absorptivity of phosphate is 3.3 × 10⁵ l mol^–1 cm^–1 at 620 nm. Arsenate is also determined using the same procedure. An oxidative or a reductive reaction of arsenic with potassium dichromate or sodium thiosulphate is utilised for the determination of phosphate, arsenate and arsenite. An extraction-indirect atomic absorption spectrometric determination method was also developed in which the atomic absorption of molybdenum in the ion pair of Malachite Green with molybdophosphate is measured. The proposed methods were applied to the determination of phosphate, arsenate and arsenite in some natural water samples.