THE CHEMICAL COMPOSITION OF DYES 97 



nitrogen atoms that link two rings together. (The name is derived 

 from French azote ; short a.) 



The -N=N- group occurs once, twice, or thrice in the ion, 

 which may be a cation or an anion; the corresponding group- 

 names are mono-azo, disazo, and trisazo. The formula shows that 

 orange G is an anionic, mono-azo dye; there are two -SOg" 

 groups. The name refers to the colour, which is a very yellowish 

 orange. This is a typical background dye. 



Not all azo substances are dyes. Sudan IV, for example, 

 possesses the azo chromophore and is coloured (red). The 

 formula shows, however, that it lacks an auxochrome. It is in- 



CH3 



Sudan IV 



soluble in water, but very soluble in lipids. It is an important 

 lysochrome, used in histochemistry for the recognition of lipids. 

 It is presented to the tissues at saturation in 70% ethanol. Being 

 much more soluble in lipids than in this solvent, it accumulates in 

 lipid globules and colours them. As we shall see in the next 

 chapter, dyeing is a very different process from mere colouring 

 by solution. 



H 



