A novel ion chromatographic method based on cation-exchange and acid–base interactions for the simultaneous determination of total alkalinity and monovalent cations in samples of µl volume

Abstract

An ion chromatographic (IC) method based on the use of titrant (strong acid) as the stationary phase was developed for simultaneous determination of total alkalinity (TA) and monovalent cations. The titrant used in this study was obtained by initially loading lithium dodecylsulfate (Li-DS) onto a reversed-phase material and then conditioning the column with a slightly acidified aqueous LiCl solution (a mixture of 50.0 mM LiCl and 0.1 mM H₂SO₄). When a small amount of a basic sample was injected onto a column prepared in this way, the basic species (Bⁿ⁻) reacted predominantly with H⁺ on the stationary phase and the reaction with the eluent phase was negligible due to the very low concentration of eluent H⁺ (in the eluent, a molar ratio of [Li⁺]/[H⁺] = 250 ∶ 1 applied). The stationary phase H⁺ consumed in the acid–base reaction was then re-supplied by H⁺ from the eluent. By monitoring the conductance of the eluent using conductivity, an induced peak resulting from the basic species was observed. Calibration graphs of peak areas vs. molar concentration of the basic species for OH⁻, HCO₃⁻ and H₂PO₄⁻ were found to be identical. CO₃²⁻, HPO₄²⁻, and B₄O₇²⁻ also gave identical calibration curves but their slope values were twice those for HCO₃⁻. The detection limit for HCO₃⁻ was less than 3.2 µM and the calibration curve was linear up to 12.3 mM (injection volume, 100 µL). Seawater was directly analyzed and its total alkalinity was found to be 2.87 mM (RSD 0.53%, n = 5), which was in good agreement with the result of 2.88 mM (RSD 3.2%, n = 5) obtained using auto-potentiometric titration. Na⁺ and K⁺ were determined simultaneously and the concentrations were 481.6 and 10.6 mM, respectively.