DETERMINATION OF TOTAL NITROGEN IN WATER SAMPLES BY MEANS OF HIGH-PRESSURE BOMBS AND ION CHROMATOGRAPHY

The standard method for the determination of organic nitrogen in water samples in the past has been based on the Kjeldahl digestion technique that converts organic nitrogen into ammonia, which can then be determined along with any ammonia originally present. However, this method is time consuming and the accuracy is variable because the effect of interferences causes unreliable results, especially in complex matrices. An alternative method for the determination of organic nitrogen is the alkaline peroxodisulphate digestion technique. This oxidizes all nitrogen in the sample using potassium peroxodisulphate in a strongly alkaline environment under high pressure and temperature. Nitrate is the sole product and can easily be determined by ion chromatography or by other methods (e.g., cadmium reduction method). A method for total nitrogen determination was developed using 23-ml high-pressure bombs with potassium peroxodisulphate and sodium hydroxide to oxidize the organic nitrogen to nitrate. Urea and ammonium chloride were used as nitrogen compounds for calibration. The method was checked with the Kjeldahl method, showing good agreement (R.S.D. = 4.62%). Studies of digestion time were carried out to determine the optimum time in the pressure vessel. The results were checked with the cadmium reduction method (R.S.D. = 3.62%) for natural water samples. The recoveries with urea and ammonium chloride reagents were higher than 90%.