A simple method based on IC-ICP-MS to determine conditional stability constants

Abstract

A method was developed for the determination of conditional stability constants (K′_f) and it is demonstrated here using a simple system, i.e., the complexation of Zn or Co by ethylenediaminetetraacetic acid (EDTA) in a 1∶1 ratio. The method involves separation of the free metal and its EDTA complex by ion-exchange chromatography (IC) with on-line detection by inductively coupled plasma mass spectrometry (ICP-MS). An anion-exchange guard column (Dionex AG-7, 4 × 50 mm), which also exhibits some cation-exchange properties, provided adequate separation using a mobile phase of 0.1 M NH₄NO₃ at pH 2.0 and 2.5 for Zn and Co, respectively. Since the peak area response of ICP-MS was independent of the metal species, K′_f could readily be obtained by injecting a series of solutions with different metal and EDTA concentrations and then plotting log(peak area of complex/peak area of free metal) versus log(total free EDTA concentration). The resulting straight line had a slope equal to the chelation number and an intercept equal to log(K′_f). Values of 3.10 ± 0.42 and 4.40 ± 0.24 were obtained for log K′_f of Zn–EDTA²⁻ (pH 2.0) and Co–EDTA²⁻ (pH 2.5), respectively, in good agreement with expected values. The approach is more robust than the Scatchard approach, which yielded erroneous results as a result of drift in the ICP-MS response. In contrast, this drift, which was not significant during a chromatogram, was effectively compensated through ratioing the peak areas.