Spectrophotometric determination of nitrite with composite reagents containing sulphanilamide, sulphanilic acid or 4-nitroaniline as the diazotisable aromatic amine and N-(1-naphthyl)ethylenediamine as the coupling agent
Composite diazotisation-coupling reagents containing sulphanilamide, sulphanilic acid and 4-nitroaniline as the diazotisable aromatic amine and N-(1-naphthyl)ethylenediamine (NED) as the coupling agent have been used for the spectrophotometric determination of nitrite. The proposed procedure consists in adding 10 ml of the composite reagent to the sample and diluting to 50 ml in a calibrated flask. The recommended composition of the composite reagents is equivalent to the following: 0.25% sulphanilamide, 0.0060% NED and 5.42% hydrochloric acid; 0.50% sulphanilic acid, 0.0050% NED and 0.29% hydrochloric acid; 0.25% 4-nitroaniline, 0.010% NED and 15.0% sulphuric acid. A minimum and maximum amount of NED and large excess of the aromatic amine must be present at an optimum acidity in order to avoid interfering side-reactions, especially the nitrosation of NED. The same results are obtained over the range 7.5–49.0 ml of sulphanilamide-NED or sulphanilic acid-NED reagent and 3.0–49.0 ml of 4-nitroaniline-NED reagent per 50 ml; this would indicate that the ratios of aromatic amine, NED and acid are just as important as the specific amounts of these substances. The sensitivity, accuracy and precision of the methods decrease in the order 4-nitroaniline-NED > sulphanilamide-NED > sulphanilic acid-NED.