STAINING, PRACTICAL AND THEORETICAL 



AZO GROUP 



Chromophore: — N=N — 



The azo dyes form by far the largest group of dyestuffs. There 

 are at the present time over one thousand manufactured. About 

 one hundred of these are used in biology. The vast majority of 

 them are acid dyes. There are a number of different classes of 

 azo dyes. Space does not permit more than a very brief outline 

 of two or three broad divisions of this vast group. 



The azo dyes can be subdivided into mono-, dis-, tris-azo, etc., 

 according to whether they contain one, two, or three or more 

 azo groups to the molecule. 



About eighty mono-azo dyes are used as microscopical stains. 

 All but a few of them are acid dyes. Most of the acid mono-azo 

 dyes in biology are used as cytoplasmic stains. The molecular 

 weights of these range from about 350 to about 696. The basic 

 ones fall into a lower molecular weight range. Some of the acid 

 dyes, with molecular weights around 690 will stain collagen as 

 well as cytoplasm. Those between about 350 and 500 will stain 

 keratin selectively, under suitable conditions. 



There are over seventy dis-azo dyes used as microscopical 

 stains. All of these are acid except two. 



Only eight tris-azo dyes have been used in biology up to the 

 present time. These are all acid dyes. One of them, chlorantine 

 fast green BLL, possesses a very high molecular weight (1324). 

 This dye stains collagen as well as cytoplasm. 



Some examples of azo dyes are given : 



42 



