DETERMINATION OF NITRATE IN SURFACE WATERS BY ION-EXCHANGE CHROMATOGRAPHY AFTER OXIDATION OF TOTAL ORGANIC NITROGEN TO NITRATE

The official method for the determination of organic nitrogen in surface water consists of a Kjeldahl distillation to reduce nitrogen to ammonia, followed by colorimetric or volumetric determination. The purpose of this work is to determine whether the Valderrama method can be used as an alternative procedure. The Valderrama method estimates total nitrogen, either organic or inorganic, after oxidation to nitrate. The first step is an oxidative digestion in an autoclave in the presence of potassium persulphate, boric acid and sodium hydroxide. Nitrate is then determined colorimetrically, after acidification by concentrated sulphuric acid, or by ion-exchange chromatography, without acidification. This last procedure takes advantage of some important features of ion-exchange chromatography: greater sensitivity than UV methods, rapid analysis, high selectivity and, thus, low matrix interferences.