Effective separation of condensed phosphate by ion-exchange chromatography

Abstract

A conventional method of ion-exchange chromatography has been modified effectively for separating ortho- and pyro-phosphates, which were contained as impurities in recrystallized tripoly-phosphate, by gradient elution with 500 ml of 2×10-3 N HCl, to which 0.6 M KCl is added continuously and mixed. The separation can be brought about at an elution volume of 100ml for orthophosphate, of 200 ml for pyrophosphate and of 300 to 350 ml for tripolyphosphate. In the course of pretreatment with molybdic acid for colorimetry, a faint yellow coloration is developed due to phosphomolybdate, if 10 ml of the fractionated aliquot contains a large amount of phosphate. One tenth portion of the yellow solution is then diluted to 10 ml for the subsequent colorimetry by the molybdenum blue method, in order to obtain an appropriate range of optical density. The use of 25 cm length of resin column allows to take some hundred milligrams of sample tripolyphosphate for the determination of the phosphates as impurities as low as 0.01%. For the analysis of higher condensed polyphosphate, an eluent of more concentrated KCl solution is employed, for instance 1 M or 2 M KCl gives better separation similarly to that observed with diluted eluent. A recovery of total phosphorous is more than 95% by colorimetry.