INTERFERENCE FROM ARSENATE, FLUORIDE AND SILICATE WHEN DETERMINING PHOSPHATE IN WATER BY THE PHOSPHOANTIMONYLMOLYBDENUM BLUE METHOD

In the range of 50-500 mug PO4-P L-1, analytical procedures according to Murphy and Riley, Koroleff2 and Parsons et al.3 all resulted in complete colour development of the phosphoantimonylmolybdenum blue complex within 5 min after reagent addition. Reducing the reaction temperature from +25-degrees to +15-degrees-C slowed the formation rate of the blue complex, but not enough to be of practical importance. Arsenate (0-1.0 mg AsO4-As L-1), fluoride (0-200 mg F L-1) and silicate (0-50 Mg SiO4-Si L-1) affect the phosphate determination in different ways. Arsenate forms a blue complex with molybdate, but with a slower reaction rate than for the phosphate complexation. At low arsenate concentrations this interference can be reduced by timing the photometric recording according to the phosphoantimonylmolybdenum formation rate. High concentrations of fluoride slow the colour development of the phosphoantimonylmolybdenum blue complex. Silicate may affect the analytical result in two ways. Alone, after prolonged standing, a slighly increased light absorption results from a blue complex formed with molybdate, but silicate can also reduce the inhibitory effect of fluoride. High fluoride concentrations counteract the development of the blue complex between silicate and molybdate. The interfering effects of arsenate, fluoride and silicate can all be reduced by dilution of the sample prior to reagent addition.