THE INDIRECT ATTACHMENT OF DYES TO TISSUES 209 



action of chromium might be apphcable to the other metals; but 

 unfortunately the ordinary use of chromium in industry is radic- 

 ally different, for it is applied as sodium or potassium dichromate, 

 that is to say in an anionic complex, whereas in microtechnique we 

 use the metals as cations. It is an extraordinary misfortune that 

 in fixation we cannot profit from industrial chemistry" because we 

 use anionic chromium while the tanner uses cationic, and in dyeing 

 we cannot profit much because we use cationic chromium while the 

 professional dyer generally uses anionic. 



In short, in microtechnique w^e generally use iron, aluminium, 

 and cationic chromium to mordant for haematein, carmine, and 

 certain oxazine dyes, while the industrial dyer generally uses 

 anionic chromium to mordant for azo dyes. 



The chemistry of mordanting in industry has been thoroughly 

 investigated and must be mentioned very shortly here.^^^' ^^*^' ^^-' "^ 

 The acid azo dye commonly has -OH groups ortho to the azo- 

 group. On reaction with the mordant at high temperature, the 



_OH HO_ _0^t 0_ 



\_/^ IN x^_^ ^_^ IN IN ^^^ 



Skeleton fortnulae of an azo dye before and after linkage with chromium 



chromium atom makes covalent links with the oxygens of the 

 hydroxyl groups and obtains a dative covalency from one of the 

 nitrogens, at the same time showing a negative electric charge. 

 It follows that the mordant dye complex has the character of an 

 acid dye, which can react with wool by making salt-linkages with 

 amino-groups of the protein. The dye generally possesses sul- 

 phonic groups (not shown in the skeleton formula), which again 

 are acidic and can react in the same way.^^^ Thus the mordant/dye 

 complex reacts with positively-charged groups in the wool, or 

 indeed may even be held in place by the mere fact that it forms too 

 large a particle to escape from the pores of the wool.^^^ There is 

 strong evidence that one chromium atom reacts with two mole- 

 cules of the dye,*^^ but this is not represented in the simplified 

 formula shown. 



Long and complicated researches have led to the conclusion that 

 the mordant/dye complex of the textile dyer is to be regarded as 

 acting as though it were an acid dye, but in microtechnique we can 

 generally tell whether any colouring agent is acting as an acid or a 

 basic dye by a momentary glance down the microscope. The fact is 

 o 



