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 conv ersion of inorganic and organic nitrogen species to nitrate by photoox idation in an alkaline peroxodisulphate medium. Following on-line phot ooxidation, sodium metabisulphite is added to reduce excess peroxodisu lphate reagent, and a portion of the resultant solution is injected in to 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 det ection is by visible spectrophotometry, the method could be readily em ployed using either commercial or low-cost laboratory-built flow-injec tion analysis systems.