Total nitrogen determination by flow injection using on-line microwave-assisted digestion

A method for the determination of total nitrogen in wastewaters involving flow injection and on-line microwave-assisted digestion is described. The method is based on the oxidation of nitrogen-containing compounds to nitrate by means of peroxodisulfate using a microwave oven as a radiation source. Nitrate is determined by the Griess-Ilosvay reaction, after reduction to nitrite by hydrazine under alkaline conditions, using sulfanilamide and N-(1-naphthyl)ethylene diamine. A thorough study of the factors affecting the digestion process has been carried out using several nitrogen-containing compounds as test substances. Under the optimized conditions, the described method allows quantitative digestion of all the model compounds for concentrations up to 20 mg l−1 N. The method enables the analysis of 10 samples in triplicate per hour, the relative standard deviation (RSD) of 10 replicates being ca. 3%. The detection limit (3 × SD of blank) is 0.21 mg l−1 N. The system has been applied to the determination of total nitrogen in wastewaters and the results have been compared with those obtained by Kjeldahl digestion.