SIMULTANEOUS SPECTROPHOTOMETRIC DETERMINATION OF NITRITE NITRATE BY FLOW-INJECTION ANALYSIS

An automatic direct spectrophotometric method for the simultaneous det ermination of nitrite and nitrate by flow-injection analysis has been developed. Nitrite reacts with 3-nitroaniline in the presence of hydro chloric acid (0.96-1.8 M HCl or pH 0.5-0.7) to form a diazonium cation , which is subsequently coupled with N-(1-naphthyl)ethylenediamine dih ydrochloride to form a stable purple azo dye, the absorbance of which is measured at 535 nm. Nitrate is reduced on-line to nitrite in a copp er-coated cadmium column which is then treated with azo dye reagent an d the absorbance due to the sum of nitrite and nitrate is measured; ni trate is determined from the difference in absorbance values. A copper column incorporated into the reaction manifold before the copperised cadmium column not only improves the long-term accuracy, but also exte nds the life time of the copperised cadmium column. Various analytical parameters, such as effect of acidity (pH), flow rate, sample size, d ispersion coefficient, time, temperature, reagent concentration and in terfering species, were studied. The calibration graphs were rectiline ar for 0.1-3.5 mu g ml(-1) of NO3- and 10 ng ml(-1)-2.2 mu g ml(-1) of NO2-. The method is successfully applied to some food samples (meat, flour and cheese), environmental waters (inland and surface), beer and soil samples. Up to 30 samples can be analysed per hour with a relati ve precision of approximate to 0.1-2%.