Determination of p K a constants of hypericin in aqueous solution of the anti-allergic hydrotropic drug Cromolyn disodium salt

In this work we established three from altogether six proton dissociation constants (p K a ) of hydroxyl groups of hypericin in its monomeric form. The monomeric state of hypericin (5.0 × 10 −6 mol·L −1 ) in aqueous solution was stabilised by the presence of hydrotropic drug Cromolyn disodium salt (6.0 × 10 −2 mol·L −1 ). Data show that one acid-base transition occurs with the p K a of 7.8 and the other two are characterised by the apparent single p K a of 11.5. The spectral changes of hypericin above pH 13 indicate that the last two hydroxyls are deporotonized at this high pH values.