Abstract
The determination of urea in sea water was performed by an automated method based on the reaction of urea with diacetyl-monoxime. The sensitivity, which has been greatly enhanced compared to previous methods, by using reagents containing thiosemicarbazide and iron III, is 0.044 absorbance units/μg-at urea-N/L with the 50 mm flow cell. The precision is ±0.01 μg-at urea-N/L and the limit of detection (2 SD) is 0.02 μg-at urea-N/L. The absorbance–concentration relationship is linear up to 15 μg-at urea-N/L. The analytical blanks have been strictly determined and the interferences studied. Citrulline is the main compound that interferes. However, in attempting to remove this compound from the samples by a chelating resin, we demonstrated its absence from the analyzed coastal waters. The color development with urea does not depend on the salt concentration. The problems of contamination and storage are discussed: great care must be taken in handling the samples; quickly freezing the samples is proved very suitable for storage during a long time. Some examples of application of the method to oceanic, coastal and estuarine waters show concentrations ranging from 0.00 to 3.0 μg-at urea-N/L.
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