Characteristic constants of 2,2',4'-trihydroxyazobenzene-5-sulfonic acid, a reagent for spectrophotometric analysis

The dye 2,2',4'-trihydroxyazobenzene-5-sulfonic acid, has shown promise as a reagent for the determination of zirconium. As the literature contains very little information about this dye, basic data pertinent to its use as a reagent were determined. The sulfonic acid group and all three of the hydroxy groups show acidic characteristics. Apparent dissociation constants were determined for the three more labile protons and the approximate order of magnitude for the fourth constant was estimated. Absorption spectra for the different ionization species are given. A curve is also included which shows the fraction of dye in the different ionization forms at acidities from 10.35M hydrochloric acid to pH 11.9. A sixth dye species was found in 1.0 to 8.4M potassium hydroxide solutions, but its nature is unknown.