Simultaneous determination of anions and divalent cations using ion chromatography with ethylenediaminetetraacetic acid as eluent

Abstract

A method is described for the simultaneous determination of Cl^–, NO₃^–, SO₄^2–, Mg²⁺ and Ca²⁺ ions using ion chromatography with 0.0015 mol l^–1 ethylenediaminetetraacetic acid as eluent at pH 4.8, with ultraviolet (UV)(200 nm) and conductivity detectors in series. Detection limits for these ions [2s(n= 15) for a 1 + 99 dilution of the standard solution] are 6, 0.8, 4, 20 and 10 ng, respectively; a 200 µl aliquot was used for all determinations. In general, greater sensitivity is obtained for Cl^–, SO₄^2– and Mg²⁺ using conductivity detection, whereas for NO₃^– and Ca²⁺, UV detection affords a lower detection limit. The greater sensitivity for NO₃^– and Ca²⁺ compared with literature methods is due to a shorter wavelength for UV detection being accessible because of the lower absorbance of the eluent at pH 4.8. For Mg²⁺, partial protonation of the complex at this pH is thought to increase its equivalent conductance and hence sensitivity to conductivity detection. Analyses were made of four reference waters, which were within the 95% confidence interval for all analytes and for both detection modes. The calibration graphs were linear (r > 0.99) over at least three orders of magnitude of concentration from the detection limit, except for the UV detection of NO₃^–(r= 0.967). The method is directly applicable to both fresh- and sea-water samples, and was tested using a number of samples from natural and artificial water bodies near Jabiru, Northern Territory, Australia. Reproducible analyses for all analytes were obtained for these freshwater samples, with alkalinities in the range 1–180 mg l^–1 CaCO₃, after storage at 4 °C for 1 month. All peaks are well separated and no common ions interfere with any analyte.