Determination of stability constants of metal complexes with IC-ICP-MS

Abstract

A method was recently developed to determine the conditional stability constant, Kf′, using ion chromatography (IC) hyphenated to inductively coupled plasma mass spectrometry (ICP-MS) (C. Huang and D. Beauchemin, J. Anal. At. Spectrom., 2006, 21, 317–320 and C. Huang, D. Beauchemin, J. Anal. At. Spectrom., 2006, 21, 1419–1422).1,2 It involves the separation of a metal complex from the free metal ion by ion exchange with on-line detection of eluting metal-containing species by ICP-MS. The relative amounts of the complexed and free metals were simply obtained from the area of the corresponding peaks in the time-resolved chromatograms. A plot of the logarithm of the peak area ratio of the complex over the free metal as a function of the logarithm of the analytical concentration of the ligand gave log Kf′ as the intercept and the chelation number, n, as the slope. Although this method was demonstrated to allow the accurate determination of the conditional stability constant of a simple system (EDTA complexes of Co and Zn),1,2 the corresponding chelation numbers were significantly off from the expected unity value. A close scrutiny of the method revealed the main source of the discrepancy: it assumed that the ligand was in excess, which was not the case. Furthermore, it neglected other complexes than that with EDTA4−, such as those with HEDTA3−, H2EDTA2−, etc. In this work, no asuumption is made and a linear regression of the logarithm of the peak area ratio of the complex over the free metal versus the logarithm of the concentration of the uncomplexed chelating reagent is shown to accurately give both log Kf′ as the intercept and the chelation number as the slope. This revised method was successfully applied to Co-EDTA and Zn-EDTA complex systems with 0.1 M NH4NO3 as the supporting electrolyte.