REDUCTIVE REVERSE FLOW-INJECTION AMPEROMETRIC DETERMINATION OF NITRATE AT A PLATINUM-ELECTRODE AFTER ONLINE REDUCTION TO NITROSYL CHLORIDE IN CONCENTRATED SULFURIC-ACID MEDIUM CONTAINING CHLORIDE

A simple method has been developed for the determination of nitrate using flow injection with amperometric detection. The nitrate sample, which is made 2 M in hydrochloric acid, is the carrier stream, and into this is injected a small volume (25 μl) of concentrated sulphuric acid. The nitrosyl chloride and chlorine formed are detected as a combined reduction signal at a platinum electrode held at +0.70 V versus Ag - AgCl. As formation of nitrosyl chloride only occurs in the initial stages of dispersion of the sulphuric acid into the sample solution (i.e., at high concentrations of sulphuric acid), incorporation of a 40-cm long single bead string reactor as the transmission tubing between the injection valve and the detector, which reduces this dispersion, increases the size of the signal and provides conditions in which gas (mainly hydrogen chloride) formed on-line, which otherwise results in a noisy base line, is re-dissolved before reaching the detector. Calibration graphs were rectilinear over the range from 9 x 10-5 M (the detection limit) to 5 x 10 -3 M. Coefficients of variation were typically less than 4%. Other common constituents of hydroponic fluids did not interfere.