Acid−Base Equilibrium Constants for Ferric trans-1,2-Diaminocyclohexanetetraacetic Acid (Fe3+CDTA4-/Fe3+OH-CDTA4-) in NaCl, Na2SO4, and LiCl Aqueous Solutions at 298 K

Abstract

The dependence of the ferric chelate (trans-1,2-diaminocyclohexanetetraacetic acid) acid−base stoichiometric equilibrium constant on pH and on low-to-moderate ionic strength [(0 < I ≤ 2) mol kg-1] was systematically studied in sodium chloride, sodium sulfate, and lithium chloride aqueous solutions at 298 K. Activity coefficient models characterizing the ionic behavior of electrolytic solutions were applied in conjunction with the measured equilibrium constant (KC) in order to obtain the thermodynamic equilibrium constant (K) for the Fe3+CDTA4-/Fe3+OH-CDTA4- couple in slightly-to-moderately alkaline conditions (8 ≤ pH ≤ 10.5). On the basis of the Hückel, Bromley, Scatchard, and Pitzer models, the best fit over 150 KC measurements procured an optimal log(K) value of 4.288 ± 0.022 at (298 ± 1) K. A good fit of the KC measurements was achieved in the following ionic strength regions: Hückel (I < 0.25 mol kg-1), Bromley (I < 1 mol kg-1), Scatchard (I < 1.5 mol kg-1), Pitzer (I < 2 mol kg-1). In those instances, the predictions presented acceptable fitting capability well within the accepted limits (average absolute error on pKC < 0.05).