Chelatometric determination of phosphoric acid as zinc ammonium phosphate

Quantitative analaysis of phosphoric acid. VIII

Abstract

An indirect quantitative determination of phosphoric acid by precipitation of phosphoric acid as ZnNH₄PO₄ and titration of Zn in the precipitate by EDTA has been investigated. ZnNH₄PO₄ is soluble in an NH₄Cl-NH₄OH buffer solution of pH 10 and it was unnecessary to do back-titration as in the case of MgNH₄PO₄; the end point is clear even in the presence of PO₄^3-.
Conditions for precipitation of ZnNH₄PO₄ have been investigated. It was found that the most suitable condition was made by setting the pH 5.87.5, the final concentration of NH₄Cl to 23M, and 25 times as much of equivalent weight of ZnCl₂ against the PO₄^3-.
The effect of the presence of several coexisting substances has been investigated and it was found that ammonium molybdate gave positive error but it had no disturbing influence after reprecipitation, while the presence of citrate ion gave a considerable lower value.
The analyses of three samples of rock phosphates have been carried out by this method and the phosphorus could be determined within 0.2% relative error.