Spectrophotometric studies on purpurin and quinalizarin in aqueous solutions

The absorption spectra of purpurine and quinalizarin is studied in solutions of varying PH's. The change of spectra with pH indicates the existence of two equilibria between noncharged molecules, singly ionized and doubly ionized species. The pK values of the two equilibria are determined using three methods and are discussed in relation to molecular structure. The effect of pH on the electronic transition reveals that all transitions are influenced by intramolecular charge transfer.