Abstract :
The effect of arsenate on phosphate determination by the malachite green spectrophotometric method was investigated. The molar absorptivities of the molybdophosphate and malachite green-molybdoarsenate species at 625nm and a final acidity of 0.38M were calculated as 10.4±0.13×104 and 7.2±0.17×104lmol-1cm-1 respectively, indicating that arsenate could interfere in phosphate measurement. Arsenate concentrations as low as 23µgl-1 caused increase in colour development in phosphate solutions. However, the extent of colour development for both anions depended on the final acid concentration of the solution. An acidified sodium sulphite solution (0.83M NaSO3, 0.83M H2SO4) quantitatively prevented arsenate colour development up to 300µgl-1 As(V). It was also demonstrated that the method removed As(V) interferences in mixed As/P solutions and therefore can be used to treat natural water samples with elevated arsenate concentrations before phosphate measurement.
