THE CLASSIFICATION OF DYES 173 



The tree is commonly felled at the age of about ten years. The 

 bark and sap-wood are chipped off and the heart-wood exported as 

 'logwood' in pieces three feet long. These logs are reduced to 

 chips for the extraction of the substance, haematoxylin, from which 

 the dye originates. Haematoxylin itself is a colourless solid, but if 



CH, 



O 



Haematein 



damp becomes oxidized by atmospheric oxygen to the reddish 

 dye, haematein. It is partial oxidation that gives colour to logwood. 

 Products in which various proportions of the parent substance 

 have been oxidized to the dye are available for use in the textile 

 industry. The biological significance of the presence of haema- 

 toxylin in the wood is unknown. 



The second e in the word haematein (four syllables) dis- 

 tinguishes this dye from an entirely unrelated coloured substance, 

 haematin, the non-protein component of haemoglobin. The word 

 hematine is used in the textile industry for partly or wholly 

 oxidized haematoxylin."^ 



Haematoxylin is the leuco-counterpart of haematein. The ring 

 represented in the formula as quinonoid in haematein is non- 

 quinonoid in haematoxylin. The oxygen attached to the ring that 

 is shown as quinonoid in haematein is replaced by -OH in haema- 

 toxylin. The carbon atom attached by a double bond to the upper 

 end of this ring in haematein has one valency free in haematoxylin, 

 and this is satisfied by hydrogen. In all other respects the formulae 

 for the two substances are the same. Haematoxylin crystallizes 

 with three molecules of water. ^^^ 



Many different ways of oxidizing haematoxylin in the laboratory 

 have been suggested.^^^'"^^^' 2"^' ^-^ Sodium iodate is as convenient 

 an oxidizer as any. It requires 0-187 g of this to oxidize i g of 

 haematoxylin crystals fully. ^^^ 



One might think it best ahvays to buy haematein and make one's 

 solutions from this, but in fact haematein solutions lose their 

 strength rather quickly by flocculation of the products of further 



