Estimation of the Concentration of Fe 3+ and the (Fe 3+ )(OH ‐ ) 3 Ion Product from Equilibria of EDTA in Soil

Equilibria of EDTA (ethylenediaminetetraacetic acid) chelates were used to measure the concentration of ferric ion in near‐neutral soils. Concentrations of Fe 3+ and the (Fe 3+ )(OH ‐ ) 3 activity product were defined quantitatively by simultaneous equilibria involving EDTA 4‐ , Fe 3+ , Ca 2+ , 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 Fe 3+ 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 (Fe 3+ )(OH ‐ ) 3 ion product had a mean value of 39.3 with a standard deviation of approximately 0.2. The solubility of Fe 3+ in equilibrium with soil was stable for at least 6 weeks.