A method is described for the direct colorimetric determination of urea in biological fluids. The method depends on the reaction (first described by Wheatley, 1948) between urea, diacetylmonoxime, and phenylanthranilic acid in the presence of controlled amounts of oxidant; chloride ions are included to sensitize the reaction; manganous ions stabilize the resultant colour; and phosphate enables reasonable reproducibility to be achieved.
The method is rapid and suitable for routine analytical purposes. Precision and accuracy are good; sensitivity is high for an activated acid reagent up to about one week old, and thereafter decreases.
A hypothetical mechanism for colour formation is presented.
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