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Concept
Haematoxylin
Summary
Haematoxylin or hematoxylin (ˌhiːməˈtɒksᵻlᵻn), also called natural black 1 or C.I. 75290, is a compound extracted from heartwood of the logwood tree (Haematoxylum campechianum) with a chemical formula of C16H14O6. This naturally derived dye has been used as a histologic stain, as an ink and as a dye in the textile and leather industry. As a dye, haematoxylin has been called palo de Campeche, logwood extract, bluewood and blackwood. In histology, haematoxylin staining is commonly followed by counterstaining with eosin. When paired, this staining procedure is known as H&E staining and is one of the most commonly used combinations in histology. In addition to its use in the H&E stain, haematoxylin is also a component of the Papanicolaou stain (or Pap stain) which is widely used in the study of cytology specimens.
Although the stain is commonly called haematoxylin, the active colourant is the oxidized form haematein, which forms strongly coloured complexes with certain metal ions (commonly Fe(III) and Al(III) salts). In its pure form, haematoxylin is a colourless and crystalline solid, although commercial samples are typically light to dark brown based on the level of impurities present.
Haematoxylin has been synthesized, although never in commercially viable amounts. Historically the logwood was exported and the haematoxylin extracted in Europe. More recently extraction takes place closer to where the logwood is harvested. Extraction of haematoxylin from logwood on industrial scales has been accomplished in the 'French process' by boiling the wood chips or in the 'American process' with steam and pressure. Once extracted, the dye can be sold as a liquid concentrate or dried and sold in a crystalline form. Modern production methods use water, ether or alcohol as a solvent, at which point the extracts may be further refined to the level of purity needed.
The commercial product may vary from batch to batch and between manufacturers in both the level of impurities and in the ratio of haematoxylin to haematein.
Official source
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