C16H14O6, 3H2O = 356.16.
Haematoxylin, C16H14O6, 3H2O, may be obtained by mixing powdered extract of logwood with sand, repeatedly extracting the mixture with aqueous ether, recovering the latter, treating the syrupy residue with water, and crystallising, the crystals being washed with cold water, and finally recrystallised from boiling water containing a little sulphurous acid or soluble sulphite. Haematoxylin occurs in the form of colourless or yellowish to yellowish-brown, glistening, transparent crystals, containing one molecule or three molecules of water of crystallisation, according to the conditions of crystallisation. It has a very sweet taste, but no astringency, and becomes red on exposure to the air. Its solution is dextrorotatory, and reduces Fehling's solution and also silver nitrate. The ammoniacal solution absorbs oxygen from the air and forms haematein-ammonia, which, on the addition of acetic acid, deposits haematein, C16H12O6, a blackish-violet crystalline powder, having the beetle-green metallic lustre seen on the surface of fermented logwood chips. This body may also be obtained by oxidising haematoxylin in ethereal solution by nitric acid. Reducing agents such as sulphurous acid reconvert it into haematoxylin. The aqueous solution of haematoxylin produces with solution of baryta, lead acetate, or copper sulphate, white or greyish precipitates, which quickly become blue. Fused with potassium hydroxide, haematoxylin yields pyrogallol.
Sparingly soluble in water, freely soluble in hot water, alcohol, or ether. Soluble also in ammonia, the caustic alkalies and their carbonates, the solutions being purple in colour.
Uses.—An alcoholic solution of haematoxylin (0.2 percent.) is used as an indicator. It is yellow to orange in acid solution and purple in alkaline solution. Solutions of haematoxylin are much used in microscopy to stain tissues in sections or in bulk. Freshly prepared solutions have no staining powers; on keeping, however, the haematoxylin is oxidised to haematein, which is the actual colouring agent. Haematoxylin solutions for this purpose are prepared with ammonia alum, which appears to hasten the process of "ripening." The tissues, after being stained red with haematoxylin, are washed in tap water, to change the colour to blue.