Polyphosphate speciation for soil and fertilizer analysis

Abstract

Ammonium polyphosphate fertilizers provide an analytical challenge because they contain mixed phosphorus (P) species in solution as orthophosphate, pyrophosphate, and tripolyphosphate species. The conventional technique for the determination of polyphosphate concentration in solution is measuring the difference between total digested P and initial orthophosphate with colorimetry. Online colorimetry was compared with ion chromatography as a method for the speciation and quantification of the chemical species of P supplied in polyphosphate fertilizers. Ion chromatography was able to speciate all of the P species supplied in polyphosphate fertilizer, whereas colorimetry detected only P in solution as orthophosphate and, by the difference between the measurement of digested and undigested samples, total condensed P species. Ion chromatography had a detection limit of 0.02 mg P L-1 for orthophosphate, 0.03 mg P L-1 for pyrophosphate, and 0.05 mg P L-1 for tripolyphosphate. The detection limit for orthophosphate measured by colorimetry was the same as that measured by chromatography, but the working range of concentrations was considerably greater for chromatography - from 0.02 to 200 mg P L-1 compared with 0.02 to 2 mg P L-1. Copyright © Taylor & Francis Group, LLC.