Studies on Haematoxylin and Haematein, the Colouring Principles of Logwood

Abstract

A method is described for measuring the absorption spectra of both compounds in aqueous solutions under conditions which minimise the risk of oxidation. Data so obtained are presented for concentrations 10‐4 to 10‐5M. over the pH range up to 10. Despite the special precautions, oxidation effects are still observed with. both compounds above pH 9–10. Below pH 6 haematein undergoes an acid–catalysed reaction.There is no evidence of any ionisation process with haematoxylin solutions up to pH 9'6, above which reliable data could not be obtained. For haematein at 25°C. pKa(I) is 6. 86 ± 0. 05 and apparently applies to the hydroxyl in the quinonoid ring, and pKa(II) is 9. 9 ± 0. 1.Absorption and kinetic studies suggest that in the acid reaction of haematein, catalysed by hydrogen ions in a pseudo–first–order manner, a molecule of water is added to form dihydrohaemateinol, by a reversible process proceeding to about 65% completion at equilibrium under the present conditions. The reaction is thus in some respects analogous to the reverse of the previously reported dehydration of hydroxy–di– or –tri–phenylmethanols to give the corresponding p–methylenequinones.