Chemical Place


Hematoxylin

Applications


Hematoxylin is a natural dye extracted by boiling the wood of the South American and West Indian logwood tree, and partly purified by re-crystallization. It is sold commercially as a crude mixture of hematoxylin and other, unidentified substances. It comes as a brownish tan powder which is poorly soluble in water and somewhat more soluble in ethyl alcohol (1 gm/100ml water and 30-40 gm/100 ml alcohol). The active dye is not hematoxylin itself, but its oxidized product, hematein. The latter can be bought, but it is expensive, and the same results can be had simply by oxidizing the crude hematoxylin.

​In the early days, oxidation was carried out by making up the hematoxylin solution, plugging the container with cotton, and leaving it exposed to light and air for 6 weeks to several months.

​However, it was soon found that oxidation (also called ripening) could be achieved much faster by adding a wide variety of oxidizing agents, the two most common being sodium iodate and mercuric oxide. With either agent, oxidation not only begins immediately, but apparently continues for some time, causing dye precipitates of uncertain composition, and eventually exhausting the stain.

​The addition of glycerin to several formulas is said to guard against over-oxidation and perhaps to retard fungal growth.