If I solution is added to homogentisic acid in bicarbonate solution until starch indicator turns blue, the acid is oxidized to quinone acetic acid. If the solution is then acidified the reaction is reversed with liberation of the I used in the oxidation, which can then be titrated with thiosulphate. The reaction is as follows: [image] The authors have determined the conditions under which this reaction occurs quantitatively. This reaction may also be applied to the determination of homogentisic acid in urine, as the substances in normal urine which react with I do not liberate it again in acid solution. By careful use of this method the maximum error in the titration of 10 cc. of alcaptonuric urine is usually less than 0.5 mg. of homogentisic acid. So long as the urine is acid, no loss of homogentisic acid is demonstrable upon standing exposed to air.