Differential-pulse polarographic determination of nitrate as 3-nitrobenzoic acid and other nitro derivatives

Abstract

The reaction of nitrate with more than 15 organic compounds has been studied polarographically in sulphuric acid medium in an initial search for a suitable reagent for nitrate determination in hydroponic fluids by means of a flow injection amperometric sensor incorporating a glassy carbon working electrode. Benzoic acid was found to be the most suitable compound: a single differential-pulse polarographic peak was obtained at a low negative potential (–130 mV vs. Ag-AgCl) in the sulphuric acid medium used for measurement. The calibration graphs were rectilinear over the range 1 × 10^–5–5 × 10^–3M in the sample solution and the coefficients of variation were low (<2% at the higher nitrate levels). Comparison with polarograms of 3-nitrobenzoic acid, obtained under identical conditions, proved that complete reaction of nitrate was occuring.

Salicylic acid and resorcinol, with peaks at –140 and –150 mV, respectively, were shown to be satisfactory reagents in many respects, but the response in each instance was no longer rectilinear above 1 × 10^–3M. Thiophene-2- and thiophene-3-carboxylic acids were nitrated rapidly but gave more than one peak.