High-resolution determination of H+ by ion chromatography. Application to the simultaneous determination of H+, Na+, NH4+ and K+ in acid rain

Abstract

An ion chromatographic (IC) method was developed for the high-resolution determination of a sample’s free hydrogen ion concentration (H⁺). Highly purified lithium dodecyl sulfate was used as the stationary phase, a slightly acidified aqueous LiCl solution was used as the mobile phase and conductivity was used for analyte detection. An electrical double layer (EDL) containing H⁺ was established on the stationary phase by using a slightly acidified electrolyte solution as the eluent. H⁺ in the EDL protonated any weak acid groups (i.e., silanols) on the stationary phase so that H⁺ from the sample could be retained/separated purely by dodecyl sulfate. The optimum molar ratio of H⁺∶Li⁺ in the EDL for this IC system was obtained by using an aqueous solution containing 40.0 mM LiCl and 0.07 mM H₂SO₄ as the eluent. After separation, H⁺ was detected by direct conductimetric measurement. An H⁺ detection limit of better than 8.2 × 10⁻⁶ M was obtained from the analysis of standard aqueous H₂SO₄ solutions. Other monovalent cations could also be separated with this method, giving detection limits of 7.4 × 10⁻⁵, 4.3 × 10⁻⁵ and 4.2 × 10⁻⁵ M for Na⁺, NH₄⁺ and K⁺, respectively. The method was applied to the simultaneous determination of H⁺, Na⁺, NH₄⁺ and K⁺ in acid rain. The results obtained showed a significant improvement in reproducibility when compared with those from a conventional pH-meter. Acid rain samples with a pH <5 could be analyzed with this IC system.