Determination of homogentisic acid in urine.

A relatively simple method for the quantitative determination of homogentisic acid in urine is presented. Homogentisic acid (2,5-dihydroxyphenylacetic acid) is oxidized by atmospheric oxygen in mild alkali to form 1,4-benzoquinone-2-acetic acid. The latter compound is then conjugated with 2,4-dinitrophenylhydrazine. The resulting hydrazone, in the presence of alcoholic sodium hydroxide, produces a characteristic lavender color with an absorption maximum at 570-580 m1. A sample blank is run without oxidation to compensate for the carbonyls that may be present in the sample. The absorption curve produced is characteristic of homogentisic acid and may thus be used for its identification.