Rapid determination of trace amounts of phosphate and arsenate in water by spectrophotometric detection of their heteropoly acid-malachite green aggregates following pre-concentration by membrane filtration

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Abstract
A simple and rapid method for the determination of trace amounts of phosphate and arsenate in water is proposed. Molybdophosphate-and molybdoarsenate-Malachite Green aggregates, formed by reaction of a reagent consisting of a mixed solution of ammonium molybdate and Malachite Green, were selectively collected on a nitrocellulose membrane filter (pore size 3 µm) and dissolved in methylcellosolve together with the membrane filter. The absorbance (λ= 627 nm), denoted A(P + As), was proportional to the sum of the concentrations of phosphate and arsenate with a molar absorptivity of 2.7 × 105 l mol–1 cm–1. Thiosulphate, a reducing agent for arsenate, was added to water samples, and the absorbance of the solution, A(P), was measured as described above. The absorbance A(P) corresponds to the concentration of phosphate alone. The difference, A(P + As)–A(P), then corresponds to the arsenate concentration. The proposed method makes it possible to determine phosphate and arsenate at levels ranging from 0.3 to 150 p.p.b.

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