Determination of arsenate and phosphate in rainwater: Preconcentration bt membrane filtration

A simple and rapid method for the determining arsenate and phosphate in rainwater is proposed. These compounds form molybdoarsenate– and molybdophosphate–Malachite Green aggregates with MoMG reagent (MoMG = a mixture of ammouium molybdate and Malachite Green). The aggregates were first collected on a nitrocellulose membrane filter (pore size 3μm) and then dissolved with the filter in a small amount of 2‐methoxyethanol. Absorbance (627 nm), denoted as A(As + P), was found to be proportional to the sum of the arsenate and phosphate concentrations with a molar absorptivity of 2.8 × 10 5 (mol dm −3 ) −1 cm −1 . Following the addition of thiosulphate, an agent which reduces arsenate, the absorbance A (P) was measured as described above. A (P) corresponded to the concentration of phosphate alone and A (As + P)– A (P), to arsenate concentration only. This method is shown to be a rapid and highly accurate means for determining arsenate and phosphate levels within the 1–150 nmol dm −3 concentration range.