A tristimulus colorimetric study of the acid–base properties of 6,7-dihydroxybenzopyrylium chlorides in solutions

The acid–base equilibria of 6,7-dihydroxybenzopyrylium chloride derivatives in solutions are studied by tristimulus colorimetry. The ionization constants of their functional groups are determined. Possible schemes of the equilibria and the distribution diagrams of the ion-molecular forms of the reagents are proposed depending on the pH of the medium. It is shown that tristimulus colorimetry can be used to determine the protonation and hydroxylation constants of the dyes. The relation between these constants and the charges on the oxygen atoms and 2-C atoms of the benzopyrylium ring is established.