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 de termining phosphate in aqueous solution by the phosphoantimonylmolybde num blue method, with ascorbate as the reducing agent, was examined. B lue complexes of germanate and tungstate can be formed, albeit with la w molar absorptivities and at fairly slow reaction rates, possibly yie lding overestimation of the true phosphate concentration. High levels of germanate (greater than or equal to 1 mg Ge(OH)(4)-Ge l(-1)) and tu ngstate (greater than or equal to 10 mg WO4-W l(-1)) are, however, nee ded 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 phosphoantimon ylmolybdic acid, as do high concentrations of chromate (greater than o r 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 re adings. Similarly, high concentrations of permanganate (greater than o r equal to 3 mg MnO4-Mn l(-1)) result in reduced blue hue yields, whic h calls into question the addition of this ion in order to eliminate i nterference from nitrite and sulphide. The chemical mechanisms causing the interference effects found are discussed. The concentration level s needed for significant interference effects on phosphate determinati ons from the studied oxoanions, appear to be unusual in samples of nat ural water. (C) 1998 Elsevier Science B.V.