ANALYSIS OF TOTAL DISSOLVED NITROGEN IN NATURAL-WATERS BY ONLINE PHOTOOXIDATION AND FLOW-INJECTION

A sensitive automated method for the determination of total dissolved nitrogen (TDN) is described. This method is based on the complete conversion of inorganic and organic nitrogen species to nitrate by photooxidation in an alkaline peroxodisulphate medium. Following on-line photooxidation, sodium metabisulphite is added to reduce excess peroxodisulphate reagent, and a portion of the resultant solution is injected into a flow-injection manifold for the measurement of nitrate by cadmium reduction to nitrite, diazotization, coupling and detection at 540 nm. The method is comparatively fast (ca. 25 determinations h(-1)), has a detection limit of 0.03 mg l(-1) N, and is linear up to 3 mg l(-1) N. The PTFE tubing photoreactor is simple to construct, and because detection is by visible spectrophotometry, the method could be readily employed using either commercial or low-cost laboratory-built flow-injection analysis systems.