Interference from arsenate when determining phosphate by the malachite green spectrophotometric method

The effect of arsenate on phosphate determination by the malachite green sp ectrophotometric method was investigated. The molar absorptivities of the m olybdophosphate and malachite green-molybdoarsenate species at 625 nm and a final acidity of 0.38 M were calculated as 10.4 +/- 0.13 x 10(4) and 7.2 /- 0.17 x 10(4) 1 mol(-1) cm(-1) respectively, indicating that arsenate cou ld interfere in phosphate measurement. Arsenate concentrations as low as 23 mug 1(-1) caused increase in colour development in phosphate solutions. Ho wever, the extent of colour development for both anions depended on the fin al acid concentration of the solution. An acidified sodium sulphite solutio n (0.83 M NaSO3, 0.83 M H2SO4) quantitatively prevented arsenate colour dev elopment up to 300 mug 1(-1) As(V). It was also demonstrated that the metho d removed As(V) interferences in mixed As/P solutions and therefore can be used to treat natural water samples with elevated arsenate concentrations b efore phosphate measurement. (C) 2001 Elsevier Science B.V. All rights rese rved.