A method for concentrating and determining phosphate in natural waters, using poly(N-isopropylacrylamide)(PNIPAAm), a thermally reversible polymer, is described. Poly(N-isopropylacrylamide) is soluble in water below 31 °C but shrinks abruptly on heating above 31 °C, becoming insoluble. With phase separation of PNIPAAm from aqueous solution at 45 °C, a molybdophosphate–Malachite Green aggregate (P-Mo–MG), formed by reaction of a Mo–MG reagent, obtained by mixing ammonium molybdate and Malachite Green, was incorporated in PNIPAAm and the resulting solid stuck to the walls of the reaction vessel. After discarding the supernatant solution by decantation, the P-Mo–MG aggregate was dissolved in a small volume of methylcellosolve together with PNIPAAm. The absorbance (λmax= 627 nm) was proportional to the concentration of phosphate with an apparent molar absorptivity of 2.6 × 104 m2 mol–1. This method makes possible the rapid determination of trace amounts of phosphate in water using simple apparatus. The detection limit was 2 nmol dm–3 of phosphate. The method was successfully applied to the determination of phosphate in natural water samples such as tap water, mineral water and rain water.
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