Assessment of on-line nitration reactions as a means of determining nitrate by reverse flow injection with reductive amperometric detection at a glassy carbon electrode

Abstract

Five compounds were investigated for use as on-line reagents in concentrated sulphuric acid for the reductive reverse flow injection amperometric determination of nitrate as a nitro derivative at a glassy carbon electrode. The sulphuric acid was diluted rapidly on injecting the reagent solution into a sample carrier stream and therefore the nitration reaction, which generally only takes place at sulphuric acid concentrations of greater than about 70%, had to be rapid. Thiophene-2-carboxylic acid was found to be the most suitable reagent of those studied. The nitration reaction was sufficiently rapid and the first of two reduction steps was at –0.19 V versus SCE (as indicated by linear sweep voltammetry); hence the determination was free from interference by dissolved oxygen. Problems associated with contamination of the electrode surface with reduction product, which caused loss of signal after making repeated injections over an extended period, remained.