A spectrophotometric method for determination of urea

If phenazone is substituted for perchloric acid or arsenic pentoxide in the diacetylmonoxime reaction for urea determination, a colored product is formed, apparently from a double condensation of phenazone with diacetylmonoxime and urea. The color has a maximum absorption at 460 mμ. Beer's law is followed for concentrations of urea ranging from 0.1 to 2 μg/ml in the final reaction mixture. The sensitivity is very high (E1 cm1% 5.900), the color is stable, and the reaction seems to be specific. It is very simple and is routinely performed on an amount of filtrate corresponding to 10 μl of blood.