Simultaneous kinetic determination of phospates and silicates in the form of vanadomolibdoheteropoly acids

The rates of colour development for oxidized forms of vanadomolybdoheteropoly acids of silicon and phosphorus are different enough to determine them in mixtures by the kinetic method without separation: at 0.0010-0.0035 M H2SO4 phosphate does not interfere with determination of silicate; at 0.2-0.5 M H2SO4 it is the other way round. Calibration curves are linear in the intervals: (1.1-14.0) ?g/mL P and (1.4-14.0) ?g/mL Si. Relative errors are found to be 1 % and 4 % respectively, at 1 % repeatability. Selectivity factors are determined for a number of cations and anions common in drinking water. The method has been applied for kinetic analysis of phosphates and silicates in tap water.