Determination of urea in natural water by an improved Newell method

Abstract

The method for determination of urea in sea water proposed by Newell et al. was re-examined in detail and partly modified. The following method is recommended.To 25 mlof a sample solution are added 3 g of sodium chloride for fresh water or 2 g for sea water (Fig. 1), 3 mlof concentrated sulfuric acid and 1 mlof the mixed reagent (4.0 g of diacetyl monoxime, 0.05 g of semicarbazide hydrochloride and 15 g of MnCl24H2O in 200 mlof water). The solution is allowed to stand for 90 min at 70°C (Fig. 2). After cooling in running water, the absorbance is measured at 520 mμ against water using a 5-cm cell. The blank is measured by the same procedure with distilled water for fresh water or artificial sea water for sea water. The common ions in natural water and nitrogen-containing compounds do not interfere as seen in Table I. Although citrulline, which is produced by decomposition of arginine, gives a positive error, it is unlikely that such amount of citrulline exsists in natural water. The molar extinction coefficient for urea is 22000. The relative standard deviations were 3 and 5% at the concentration of 86 and 43 μg urea/l, respectively.