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Estimation of the Concentration of Fe³⁺ and the (Fe³⁺)(OH^-)³ Ion Product from Equilibria of EDTA in Soil¹

ABSTRACT

Equilibria of EDTA (ethylenediaminetetraacetic acid) chelates were used to measure the concentration of ferric ion in near-neutral soils. Concentrations of Fe³⁺ and the (Fe³⁺)(OH^-)³ activity product were defined quantitatively by simultaneous equilibria involving EDTA^4-, Fe³⁺, Ca²⁺, and OH^-. Equilibrium between soil Fe and chelated Fe was established by varying the fraction of EDTA associated with Fe or by slightly altering the pH of the soil. Corrections were made to account for the differential sorption of various EDTA chelate species onto soils and for the formation of EDTA chelates of Zn and Cu.

Concentrations of Fe³⁺ in the range of 10^–16 to 10^–18 M were measured successfully in the presence of much higher concentrations of iron present as hydrolysis species and as unknown natural complexes. In 10 experiments using three near-neutral soils, the negative logarithm of the (Fe³⁺)(OH^-)³ ion product had a mean value of 39.3 with a standard deviation of approximately 0.2. The solubility of Fe³⁺ in equilibrium with soil was stable for at least 6 weeks.

¹ Contribution from the Dep. of Agronomy, Colorado State Univ., Fort Collins, and the Connecticut Agric. Exp. Stn., New Haven. Published with the approval of the Director of the Colorado State University Agric. Exp. Stn. as Scientific Series Paper no. 2162. The first Author gratefully acknowledges the support of a N.S.F. Graduate Fellowship during part of this study.

² Associate Soil Chemist, Connecticut Agric. Exp. Stn., and Professor of Agronomy, Colorado State Univ., respectively. Senior Author is now Soil Scientist, U.S. Plant, Soil and Nutrition Laboratory, Tower Road, Ithaca, NY 14853.

Received for publication July 8, 1981. Accepted for publication April 2, 1982.

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