Validation of Diacetyl Monoxime Method for Quantification of Urea in Sea Water

Urea is widely used in agriculture and aquaculture as a major source of nitrogenous fertilizer. Therefore, urea exists in dissolved form in the marine/freshwater ecosystem. However, if the concentration in waterbodies is higher than a threshold limit, it can be toxic to fish and prawns. Therefore, accurate measurement of urea is a must even in marine water systems. Estimation of urea from freshwater samples using colorimetric method is well established. The well-known diacetyl monoxime method is widely used for accurate estimation of urea from fresh water as well as various biological samples. The presence of various interfering ions in sea water affects the estimation and limits the adaptability of any new methods directly into sea water. The accurate and precise estimation of urea are essential to optimize the nutrient addition. In this study, the diacetyl monoxime method was considered for urea estimation in sea water (3.5% salinity) and the method was validated for specificity, linearity, robustness, ruggedness and sensitivity as per International Council for Harmonization guidelines. During this study, urea showed maximum absorbance at 524 nm wavelength in both the samples. This method was found linear up to 100 ppm of urea in fresh and sea water samples. The variations of wavelength by 5-10%, reagent volumes by 2.5-5%, sample storage, analysis did not show any significant impact on the estimation of urea from both the samples. However, the method was found more sensitive and showed the limit of detection of 0.5 ppm in fresh water and 1.5 ppm in sea water. The overall results indicate that this method is accurate and robust for sea water samples and therefore can be used for estimation of urea in sea water samples.