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Phosphate Determinations in Waters Using an Anion Exchange Resin¹

ABSTRACT

Amounts of water samples from 1 to 1000 ml were passed through 10-cc columns of 100- to 200-mesh Dowex 1-X8 anion-exchange resin. Orthophosphate was quantitatively removed from the water sample by the resin and could be eluted from the column with 25 ml of 1M KCl. Phosphate in the 1M KCl eluent was reacted with ammonium molybdate, extracted into isobutanol, reduced with SnCl₂, and the optical density of the blue complex measured. The working range of the method was between 1 and 8 µg of P.

Arsenic does not interfere in the phosphorus determination if the phosphomolybdate complex is extracted into isobutanol before reduction with SnCl₂. Pyrophosphate if present will be approximately 20% hydrolyzed and the hydrolyzed portion indicated as orthophosphate in the method described. An alternate elution procedure to separate and determine ortho-, pyro-, and tripoly-phosphate eliminated condensed phosphate interference in the orthophosphate determination. Glucose-1-phosphate, glucose-6-phosphate, and glycero-phosphate are removed from the column with the ortho-phosphate fraction but are not hydrolyzed and indicated as ortho-phosphate. Phytic acid phosphate and para nitrophenyl phosphate are not leached from the column with KCl and do not interfere.

Analysis of water samples indicated that in the 1 to 20 ppb P range an appreciable amount of the phosphate exists as pyro- and tripoly-phosphate. The resin method is not specific for dissolved orthophosphate in water, but may give a more valid estimate than direct reaction with the acid molybdate reagent.

Key Words: tripolyphosphate • pyrophosphate • polyphosphate • phosphate analysis

¹ Contribution from the Missouri Agr. Exp. Sta., Journal Series Number 6851. Approved by the Director. Supported by the Dep. of Interior, Office of Water Resources Research, Project B-051-Missouri. Administered through the Missouri Water Resources Center.

² Associate Professor of Agronomy and Research Assistant, respectively.

Received for publication February 19, 1974.