Interference from chromate, germanate, tungstate and vanadate when determining phosphate in aqueous solution by the phosphoantimonylmolybdenum blue method

Interference from chromate, germanate, tungstate and vanadate, when determining phosphate in aqueous solution by the phosphoantimonylmolybdenum blue method, with ascorbate as the reducing agent, was examined. Blue complexes of germanate and tungstate can be formed, albeit with law molar absorptivities and at fairly slow reaction rates, possibly yielding overestimation of the true phosphate concentration. High levels of germanate (greater than or equal to 1 mg Ge(OH)(4)-Ge l(-1)) and tungstate (greater than or equal to 10 mg WO4-W l(-1)) are, however, needed for the development of significant additional amounts of blue tint in a solution containing 50 mu g PO4-P l(-1). The interference effect of tungstate is double-faced, since levels of less than or equal to 5 mg WO4-W l(-1) reduce the colour intensity of the blue phosphoantimonylmolybdic acid, as do high concentrations of chromate (greater than or equal to 1 mg CrO4-Cr l(-1)) and vanadate (greater than or equal to 2mg VO3-V l(-1)); all easily resulting in under-estimated phosphate readings. Similarly, high concentrations of permanganate (greater than or equal to 3 mg MnO4-Mn l(-1)) result in reduced blue hue yields, which calls into question the addition of this ion in order to eliminate interference from nitrite and sulphide. The chemical mechanisms causing the interference effects found are discussed. The concentration levels needed for significant interference effects on phosphate determinations from the studied oxoanions, appear to be unusual in samples of natural water. (C) 1998 Elsevier Science B.V.