Studies on Haematoxylin and Haematein, the Colouring Principles of Logwood: I–Absorption Spectra of Pure Compounds in Various Solvents and a Spectrophotometric Method of Analysis for Haematoxylin and Haematein

Abstract

A modified method is described for the preparation from logwood extracts of pure haematoxylin. The criterion of purity is the ultraviolet absorption spectrum in methanol. In most solvents with low ultraviolet absorption, haematoxylin is either too insoluble or too susceptible to oxidation for suitable spectra to be obtained without special precautions. The salient features of the visible and ultraviolet absorption spectra of haematein (about 10 mg. / litre) in six solvents are given. In most of these haematein is eventually oxidised to give a product with a characteristically different spectrum. Solutions in dimethylformamide containing 1 % of a phosphate–citric acid buffer are sufficiently stable for the haematein content to be determined by reference to the peak at 440 mμ. instead of the previously used peak at 280 mμ., which is shown to be unsuitable. Examples of the application of the analytical method to commercial materials are given.