PKODUCTION OF DYES 325 



The next step is the conversion of anthranilic acid into phenyl- 

 glycine-ortho-carboxylic* acid by interaction with monochlora- 

 cetic acid. This reagent has to be manufactured separately, but 

 the electrolytic soda industry yields the necessary chlorine, 

 together with caustic soda which is also required in various 

 stages of the process. 



Lastly, the phenyl-glycine acid is melted with alkali or a 

 substitute for it, and the solution of the mass in water is oxidised 

 by exposure to a stream of air whereby indigo blue is precipitated. 



Phenyl-glycine-ortho-carboxylic acid 



C 8 I 

 produces first indoxyl 



C 6 H 4 <^CH 2 -C0 2 H 





and by oxidation indigo blue 



But the introduction of sodamide, NH 2 Na, an alkaline substance 

 which also acts powerfully as a dehydrating agent, has led to the 

 development of another process which runs on simpler lines 

 starting from benzene as the primary material. 



When aniline, C 6 H 5 NH 2 , is treated with chloracetic acid 

 phenyl-glycine, C 6 H 5 .NH.CH 2 .COOH, results. And this com- 

 pound, heated with sodamide or with sodium in the presence of 

 ammonia according to a process adopted by Meister, Lucius and 

 Briining, and now worked in England, gives a good yield of 

 indigo. The intermediate compound is indoxyl or its sodium 

 derivative as already explained. 



Indigo being insoluble in ordinary aqueous solvents two 

 special methods have to be used in applying it as a dye. One 

 which makes use of the solution in sulphuric acid has already 

 been referred to, but the other, of very ancient origin, is de- 

 pendent on the reduction of the blue to indigo white, a colourless 

 substance, which forms soluble salts with alkaline solutions. 



The vat is prepared by mixing the blue with a solution of lime 

 or caustic potash or soda and green vitriol (sulphate of iron), or 

 with vegetable matter such as bran. In some of the modern 

 processes the iron salt is replaced by metallic zinc or by a peculiar 



