A titrimetric method for the determination of phosphate

Abstract

Phosphate has been determined by precipitating it as zirconium phosphate with a known excess of zirconium, and back-titrating the excess with EDTA. Iron(III), titanium(IV), thorium(IV) and bismuth(III) do not interfere. Fluorine forms a strong complex with zirconium(IV) and its prior elimination is necessary when present in excess of 1·4 mg. The method has been applied to the rapid determination of phosphorus pentoxide in phosphatic fertilisers, and the time taken is less than 90 minutes.

The ratio of zirconium to phosphorus in the precipitate was found to be 1:1 and the compound is probably zirconyl phosphate, ZrO(HPO₄), as distinct from the usual compound, zirconium phosphate, Zr(HPO₄)₂, obtained by the addition of an excess of phosphate to a zirconium(IV) solution. X-ray diffraction patterns of the two compounds were also found to be different.