Thermodynamic modeling of metal-ligand interactions in high ionic strength NaCl solutions:: the Co2+-citrate and Ni2+-citrate systems

The complexation of Co(II) and Ni(II) by citrate in 0.3-5 m NaCl solutions has been investigated. The stability constants for CoCit(-) and NiCit(-) were measured as a function of NaCl concentration using a solvent extraction method with HDEHP in toluene. The stability constants for both complexes decreased with increasing NaCl concentrations up to 5 m. The data were modeled using the ion-interaction formalism developed by Pitzer. The dimensionless standard chemical potentials for the CoCit(-) and NiCit(-) complexes were calculated to be - 2.561 and - 0.697, respectively. Binary interaction parameters calculated in this work were beta(0) = - 0.2249 and C-Phi = 0.0539 for NiCit(-) and Na+ interactions and beta(0) = - 0.5407 and C-Phi = 0.1353 for CoCit(-) and Na+ interactions. Because of limited data, the B-(1) value for both CoCit(-) and NiCit(-) with Na+ was fixed at an average value of 0.29 which was obtained by averaging 1-1 interactions for other carboxylic acid salts. Higher order interaction parameters wen not required to model the data. (C) 2000 Elsevier Science S.A. All rights reserved.