Simultaneous determination of two conditional stability constants by IC-ICP-MS

Abstract

The simultaneous capability of a method for the determination of conditional stability constants (K_f′) by ion chromatography (IC) coupled to inductively coupled plasma mass spectrometry (ICP-MS) is demonstrated through its application to the complexation of Zn and Co by ethylenediaminetetraacetic acid (EDTA). The approach simply involved separation of the free metal ions and their EDTA complexes by an anion-exchange guard column (Dionex AG-7, 4 × 50 mm), using a 0.1 M NH₄NO₃ mobile phase at pH 3.81. This column exhibited cation-exchange properties at this pH, i.e. the complexes eluted first, followed by the free metals. Because ICP-MS was used for detection of the metal species, only a separation of each metal cation from its anionic complex was required, the different elements being detected separately by ICP-MS. Hence, a guard column was sufficient, which kept the analysis time short and minimised instrument drift from the continuous aspiration of 0.1 M (or 0.8%) NH₄NO₃. A plot of log(peak area of complex/peak area of free metal) versus log(total free EDTA concentration), for a series of solutions containing Co, Zn and a range of EDTA concentrations, yielded straight lines. The resulting intercepts, which corresponded to log K_f′, were 7.77 ± 0.39 and 7.47 ± 0.25 for Zn–EDTA²⁻ and Co–EDTA²⁻, respectively, in good agreement with the expected values.