Insight into the chemistry of Folin’s test for uric acid

Folin developed an analytical technique for uric acid spectroscopic determination. It is based on the interaction of uric acid with sodium tungstate in phosphoric acid. This produces an intense blue colour after alkalinization, whose concentration is measured. However, the structure of the organic product obtained was not mentioned. The present communication gives the references related to each reaction step, as well as information about the reaction products obtained in uric acid oxidations carried-out in acidic or in alkaline mediums. The reaction route is as follows: addition of tungstic acid to the C-C double bond, acidolysis of the organo-metallic ester, producing tungstic subacid and an epoxide. Oxirane opening to vicinal diol after reaction with water. Two carbinolamines result, which are broken by protonation at the nitrogen; carbonyl groups are formed at C-4 and C-5 with concomitant elimination of a urea molecule. This way alloxan monohydrate is formed. The blue products obtained are described.