SIMULTANEOUS DETERMINATION OF ARSENATE AND PHOSPHATE BY USE OF THE KINETIC WAVELENGTH-PAIR METHOD

Application of the kinetic wavelength-pair method was extended to the fixed-time kinetic variant with the simultaneous determination of arse nate and phosphate based on the formation of a heteropoly acid with mo lybdate and subsequent reduction with ascorbic acid to molybdenum blue compounds with strongly overlapping spectra. Resolution of the mixtur e by using discrete wavelengths was impossible. Selectivity in the res olution was achieved by measuring the absorbance difference at the wav elength pair 820-740 nm and a fixed time of 100 s, where the contribut ion of phosphate was greatly decreased. Arsenate and phosphate were si multaneously determined in weight ratios between 5:1 and 0.3:1 at conc entrations from 0.5 to 6 mug ml-1, with relative standard deviations b etween 3% and 5% for arsenate and between 1.5% and 2.5% for phosphate. The theoretical basis adapted to first-order kinetic-based determinat ions was establisbed.