212 DYEING 



Aluminium behaves similarly, but both the simple sulphate and 

 potassium alum are colourless, and there is no production of colour 

 to call attention to the change in chemical composition of the 

 cation on solution. The cationic complexes in solution are usually 

 represented as [A10H]^+ and [A1(0H)2]^ 



The production of hydrogen ions in the course of these changes 

 results in varying degrees of acidity. The following figures relate 

 to solutions of the crystalline salts (ordinary laboratory chemicals, 

 as used in microtechnique): — 



pH 



chrome alum, 5% . . . . .1*8 



iron alum, 5% . . . . . • i'9 



,, 2*5 /o • • • • . 2'I 



potassium alum, 5% ..... 3-2 



The complex cations that we have been discussing, in solutions 

 rendered acid by the act of dissolving, are the essential part of the 

 mordant — the part that must react with the dye and with the tissue, 

 so as to leave a link between them. As we have seen, the attach- 

 ment with the dye can occur either before or after the mordant 

 metal has fixed itself to the tissue. It will be convenient to consider 

 first the nature of a soluble lake as used in the single-bath process; 

 that is to say, we shall forget the tissue for the moment and think 

 only of the reaction of the dye with the mordant. 



Dyes that form lakes possess a phenolic -OH group, which 

 plays an important part in lake-formation. Sodium hydroxide will 

 react with phenol to form sodium phenolate and water, and ferric 



? 



Na 

 Sodium phenolate 



iron will react in a comparable way. A much firmer bond can be 

 made between certain phenols and iron, however, because they 

 can provide a second link with the metal. For instance, salicylic 

 acid reacts with iron in the way just described, the metal replacing 

 the hydrogen of the phenolic -OH group. ^^^ This happens when 

 salicylic acid is treated with ferric chloride. This, however, is not 

 all that happens. ^^^ The organic acid has an oxygen atom con- 



