174 DYEING 



oxidation, and it is therefore generally best to have some haema- 

 toxylin present in addition to the haematein, to replenish the dye 

 gradually through oxidation by atmospheric oxygen. Solutions of 

 haematoxylin are often allowed to 'ripen' gradually. Long ago 

 Unna ^^^ used to stabilize his solutions after a time by the addition 

 of sulphur, which was presumably changed in part to hydrogen 

 sulphide and thus acted as a reducing agent; when he wanted a 

 strong haematein solution, he fully oxidized a part of the stabilized 

 solution. A simpler plan is to oxidize a haematoxylin solution only 

 partly, by the use of less sodium iodate than would be necessary 

 for full oxidation. ^^ One half or one quarter of the full amount of 

 the oxidizer is suitable. The solution is ready for use directly it has 

 been made up, and maintains its strength for a long time by gradual 

 'ripening' of the part of the haematoxylin that was not oxidized by 

 iodate. It is desirable to start with wholly unoxidized (white) 

 haematoxylin. 



The products of further oxidation diffuse more slowly into 

 tissues than haematein, work less well with mordants, and tend to 

 precipitate.*^^ It is doubtful whether definite stages of further 

 oxidation can be distinguished, and such terms as Trioxy- 

 hdmatein ^^^ are not acceptable without more evidence than we 

 possess. 



If one makes a simple aqueous solution of haematein, one has 

 an acid dye of a dirty reddish colour. It will be guessed from the 

 chemical formula that this substance is in fact of weakly acidic 

 nature, like the related substance, catechol. Most acid dyes, as we 



HO 

 HO 



Catechol 



have seen, are salts, usually of sodium, but in haematoxylin and a 

 few others (carminic acid and picric acid among them) the metal is 

 replaced by hydrogen. For clear contrast with other colours one 

 requires a dye with rather a narrow absorption band, and there are 

 so many suitable red acid dyes that haematein is never used in this 

 form. 



As we shall see (p. 193), certain acid dyes become basic when 

 rather strongly acidified, but here again haematein is useless, for a 

 different reason. At the pH at which it becomes a basic dye, it 

 is orange, and for reasons that will be mentioned (p. 229), we 



