High-resolution determination of H~+ by ion chromatography.Application to the simultaneous determination of H~+,Na~+,NH_4~+ and K~+ in acid rain

摘要

An ion hromatographic (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 H_2SO_4 as the eluent.After separation,H~+ was detected by direct conductimetric measurement.An h~+ detection limit of better than 8.2X10~(-6) M was otained from the analysis of standard aqueous H_2SO_4 solutions.Other monovalent cations could also be separated with this method,giving detection limits of 7.4X10~(-5),4.3X10~(-5) and 4.2X10~(-5) M for Na~+,NH_4~+ and K~+,respectively.The method was applied to the simultaneous determination of H~+,Na~+,NH_4~+ 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.