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 low molar absorptivities and at fairly slow reaction rates, possibly yielding over-estimation of the true phosphate concentration. High levels of germanate (≥1 mg Ge(OH)4-Ge l−1) and tungstate (≥10 mg WO4-W l−1) are, however, needed for the development of significant additional amounts of blue tint in a solution containing 50 μg PO4-P l−1. The interference effect of tungstate is double-faced, since levels of ≤5 mg WO4-W l−1 reduce the colour intensity of the blue phosphoantimonylmolybdic acid, as do high concentrations of chromate (≥1 mg CrO4-Cr l−1) and vanadate (≥2 mg VO3-V l−1); all easily resulting in under-estimated phosphate readings. Similarly, high concentrations of permanganate (≥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.