Electrosorptive detection based on double-layer capacitance for selective anion monitoring in ion chromatography : Part 1. Mercury Electrodes

A novel scheme is presented for the detection of selected anions in ion chromatography, based on changes in differential double-layer capacitance, ΔC_d, for a metal electrode at a suitable electrode potential induced by specific anion adsorption. The detector in this liquid chromatography/double-layer capacitance (LC/DLC) arrangement is a hanging or dropping mercury electrode in a large-volume wall-jet configuration. The anions observed to be detected readily by this approach include chloride, bromide, iodide, azide, thiocyanate, and thiosulfate. The relationship between ΔC_d and the analyte concentration can be arranged to be approximately linear at least over the range 1–100 mg l^?1. An attractive feature of the technique is its insensitivity to ionic concentration gradients in the flowing stream. Another virtue of LC/DLC is its selectivity, although it is less sensitive for the detection of complexing anions than amperometric detection based on mercury oxidation. Dual detection schemes are also devised, involving monitoring double-layer capacitance at two potentials or combined with amperometric detection.