On the basis of the coloration formed with molybdate and malachite green in aqueous solution, trace amounts of phosphate were determined. The molar absorptivity was 7.8 × 104 l mol–1 cm–1 at 650 nm. The absorbance of the reagent blank was about 0.02, and its relative standard deviation was less than 10%. The recommended concentration range of phosphorus was 0.1–5 µg and the limit of detection was 0.01 µg of phosphorus. The sample solution was acidified with sulphuric acid and heated in a water-bath above 90 °C for 40 min, and subsequently it was coloured with molybdate and malachite green. The colour was stabilised by adding poly(vinyl alcohol). The method was applied to the determination of parts per billion (109) amounts of phosphorus in river and tap waters; the relative standard deviation was less than 4% and the recovery was 95–101%.
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