Determination of oxonium ion in strongly ionisable inorganic acids and determination of substituted acetic acids using flow injection and chemiluminescence detection

Abstract

A method is described for the determination of oxonium ion in strongly ionisable inorganic acids such as sulphuric (0.015–0.16 M), hydrochloric (0.03–0.18 M), perchloric (0.03–0.14 M), nitric (0.03–0.16 M) and phosphoric (0.02–0.09 M) acids. In addition, halogeno-substituted acetic acids such as chloroacetic (0.02–0.08 M), trichloroacetic (0.03–0.16 M) and trifluoroacetic (0.03–0.14 M) acids can be determined. Formic acid can be detected at the 0.1 M level but acetic acid could not be detected up to 0.2 M. The method is based on the in situ generation of bromine from the BrO₃^–-Br^–-H₃O⁺ reaction; the bromine is then reacted with H₂O₂ to liberate oxygen for the oxidation of luminol. A sample size of 60 µl was used with a flow-rate of 2 ml min^–1. The reproducibility of the determination at all the concentrations studied was close to zero. Linear regression of the log-log plot of concentration (M)versus chemiluminescence intensity [peak height (mV)] gave a correlation coefficient of 0.98–0.99 for all the acids studied, except for phosphoric and sulphuric acids which gave a correlation coefficient of 0.93 in the concentration range studied. Various parameters were optimised such as the concentration of the reactants. Electronegativity can be demonstrated using this method.