DETERMINATION OF DISSOLVED ORGANIC NITROGEN IN SEAWATER USING KJELDAHL DIGESTION AFTER INORGANIC NITROGEN REMOVAL

An update of the Kjeldahl method is presented for the direct determina tion of dissolved organic nitrogen (DON) in seawater. Dissolved inorga nic nitrogen is previously removed: ammonium as NH3 with NaOH at pH 9. 4; and, subsequently, nitrate and nitrite as nitric oxide with FeSO4 i n acid medium. The sample is then mineralized to ammonium, which is me asured with a Technicon autoanalyzer by the indophenol blue method. Th e range of recovery for tested standard compounds is similar to those obtained by high temperature oxidation (HTO) techniques. Direct determ ination of DON by the method described in this work marks an improveme nt in precision, in comparison with other methods; the standard deviat ion obtained for samples of seawater is +/-0.2 mu mol.l(-1). The preci sion of DON measurements is not dependent on dissolved inorganic nitro gen analysis. For several stations in the Northeast Atlantic Ocean, DO N values ranged between 3 and 10 mu mol.l(-1).