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Arsenic Interference in the Determination of Inorganic Phosphate in Lake Sediments¹

ABSTRACT

The interference from As in the colorimetric determination of inorganic P by the method of Murphy and Riley was evaluated in water and sediment extract systems. Whereas up to 20 µg As/ml as arsenite (AsIII) did not interfere, arsenate (AsV) showed a positive interference and the extent of interference was independent of the level of inorganic P in both systems. On a weight basis, 1.0 µg of As is equivalent to 0.33 µg of "apparent inorganic P." The amounts of As extractable by 3N HCl from nine surficial Wisconsin lake sediments ranged from < 0.1 to 45 µg/g. The overestimation of inorganic P in 3N HCl extracts due to the positive interference from As was less than 1% in all cases. Calculations indicate that the maximum possible interference from As in a variety of extracts of these sediments is less than 2%, even if all of the As exists as arsenate, which is unlikely in reduced sediments. Arsenic was concentrated in the most recent sediments in a core from a Wisconsin lake, indicating that As interference in the determination of inorganic P in the older sediments is also not significant in view of the very low levels of As relative to inorganic P.

Key Words: arsenate • arsenite • arsenic accumulation

¹ Contribution from the Department of Soil Science and the Water Chemistry Laboratory, Univ. of Wisconsin, Madison 53706. Research supported by the College of Agr. and Life Sci., and the Engineering Exp. Sta., Univ. of Wisconsin, Madison, and by Federal Water Qual. Adm. Project No. WQ-014700-01 administered through the Univ. of Wisconsin Water Res. Center.

² Graduate Research Assistant, Associate Professor, and Associate Professor, respectively.

Received for publication October 14, 1971.