Thermodynamic modeling of metal-ligand interactions in high ionic strength NaCl solutions: the Co2+-oxalate system

Abstract

First and second apparent stability constants for cobalt(II) with oxalate ion have been determined using solvent extraction. Data were collected in 0.3 m to 5.0 m NaCl aqueous solutions. The logarithms of first stability constant ranged from 3.30 ± 0.03 to 3.57 ± 0.03 and second stability constant ranged from 5.49 ± 0.05 to 6.02 ± 0.06. The data were modeled using the Pitzer equations. For the 1:1 complex, values of the μ0/RT = -20.3 and the single neutral-ion interaction parameter λ0CoOx-Cl-= -0.161 were determined. For the 1:2 complex, a value for the μ0/RT = -12.4 was calculated from literature data and the Pitzer parameters β0Co(Ox)22--Na+= -0.628; β1Co(Ox)22--Na+= 1.74 and cΦ= 0.251 were determined. Standard chemical potential values are based on a model for oxalate deprotonation reported in an earlier study. Stability constants determined in this work are compared to available values from the literature.