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Colorimetric Determination of Iron of Mixed Valency by Orthophenanthroline¹

ABSTRACT

A colorimetric method for the determination of Fe in a mixed Fe³⁺ and Fe²⁺ valency system has been proposed. Complete reduction of Fe³⁺ to Fe²⁺, which was essential for the formation of a stable colored complex with orthophenanthroline, was obtained when the iron solution, buffered at pH 3.5 with sodium acetate, was heated for 1 hr at 65C with 5% hydroxylamine hydrochloride. This reduction procedure was found to be suitable even for solutions at a higher pH where iron was present either as Fe³⁺ or as a hydroxylated complex. The organe color complex was found to be stable at pH values 2 to 9 for at least 20 hr and no interference of citrate ion was observed in the stability or the formation of the color.

¹ Contribution from the Department of Soil Science, University of Wisconsin, Madison. Supported in part by a Rockefeller Foundation Research Fellowship to the first author.

² Assistant Physicist, Indian Agr. Res. Inst., Delhi, India; Project Associate; and Professor of Soil Science, respectively, Dep. Soil Science, University of Wisconsin, Madison.

Received for publication April 25, 1966. Accepted for publication June 7, 1966.