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Determination of Orthophosphate in Aqueous Solutions Containing Labile Organic and Inorganic Phosphorus Compounds¹

ABSTRACT

A simple and precise colorimetric method of determining orthophosphate in aqueous solutions containing labile organic and inorganic P compounds is described. It involves a rapid formation of molybdenum blue color by the reaction of orthophosphate with molybdate ions in the presence of ascorbic acid-trichloroacetic acid and citrate-arsenite reagents and complexation of the excess molybdate ions to prevent further formation of blue color from the phosphate derived from hydrolysis of the acid-labile P compounds. The color is stable up to 24 hours. The method is sensitive and accurate, and it permits determination of microgram quantities of orthophosphate in samples containing large amounts of acid-labile P compounds.

Tests with a wide range of condensed phosphate and organic phosphate compounds showed that none of the P compounds studied interfered with this method. Results by this method are compared with those obtained by the method of Murphy and Riley.

Key Words: water quality • phosphorus fertilizers

¹ Journal Pap. J-8518 of the Iowa Agric. & Home Econ. Exp. Stn., Ames, Iowa. Projects 1868 and 2112.

² Graduate Research Assistant and Associate Professor, respectively, Dep. of Agron., Iowa State Univ., Arnes, IA 50011.

Received for publication June 11, 1976.

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