l82 DYEING 



A few acid dyes of this group are important for the production of 

 na\y blue on wool. Only two or three are used in microtechnique. 

 Azocarmine G, red, is exceptional among acid dyes in being used 

 for colouring chromatin (p. 205). Induline WS, blue-black, is 

 useful for collagen. This is an acid dye related to safranine, but 

 with the hydrogens of the amino-groups replaced by phenyl 

 rings. Usable specimens, blended with other dyes to produce a 

 blacker colour, are sometimes sold as nigrosine, and this name 

 occurs frequently in the older literature of microtechnique. There 

 is unfortunately considerable variation among dyes sold as nigro- 

 sine, and some of them are unsatisfactory as collagen-dyes. 



THE AZO DYES 



We now leave quinonoid rings behind us and pass to an entirely 

 different chromophore, -N=N-. The azo dyes nowadays occupy 

 a predominant place in the textile industry. A huge variety of reds, 

 oranges, and yellows is available, as well as other colours. The 

 number of acid azo dyes suitable for the direct colouring of wool 



Skeleton formula for azo dyes 



exceeds that of all such acid dyes of other groups put together. 

 Many azo dyes can be used with mordants, and about three- 

 quarters of all the dyes that are used with mordants in the textile 

 industry are of the azo-group. The number of azo-dyes that are 

 useful in microtechnique is, however, rather limited. 



It is a remarkable fact that no living organism produces an azo 

 dye. They are made synthetically by the action of nitrous acid or 

 nitrites on amino-derivatives of benzene. If aniline hydrochloride 

 is treated with sodium nitrite, a nitrogen atom leaves the sodium 



13 ^1 -r \ yl 



">— NH3 CI- + < >NH3 Cr + NaNOs 



Aniline hydrochloride Aniline hydrochloride Sodium nitrite 



— N=N— <^ ^NHg CI- + NaCl + 2H2O 

 Amino azohetizene hydrochloride 



nitrite and associates through a double bond with a nitrogen atom 

 of an amino-group, thus introducing itself as a link between two 

 rings. The resulting ion gives a yellow basic azo dye. 



