346 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Owing to its insolubility it is converted into indigo white or into 

 indigotin sulphonic acid so as to be used as a dye. 



Indigo blue has been synthesised by various methods. The 

 synthetic product is cheaper than the natural and is gradually displac- 

 ing the natural product as a dye. 



Indigo White. 



The insolubility of indigo blue 

 renders it useless as such for dyeing 

 purposes. On reduction by zinc 

 dust and alkali, hydrosulphite, or 

 by electrolysis it is converted into indigo white. In air this solution 

 reoxidises and forms indigo blue. 



In dyeing the indigo blue is reduced, the cloth soaked in the solu- 

 tion and exposed to the air. Insoluble indigo blue is deposited on the 

 fibres. 



. Indigo white can be precipitated from solution in absence of air 

 as white crystals which dissolve in alcohol, ether and alkalies with a 

 yellow colour. 



Indirubin or Indigo Red and Isoindigotin. 



Natural indigo blue is generally associated with small quantities of indigo 

 red. This is formed by a combination of i molecule of indoxyl with i 

 molecule of the isomeric oxindole : 



NH 



00 



CO OC 



NH 



V 



NH 



NH 



Combination occurs between the a and fi carbon atoms. If combination 

 occurs between the two /? carbon atoms, isoindigotin is formed. 



I ndole-/?- Acetic Acid. 



This compound was first found amongst 

 C'CH 5 -GOOH th e putrefactive decomposition products of 

 'H proteins. It is often present in urine and is 



NH found particularly in cases of intestinal disorder. 



It crystallises in platelets melting at 164 and is soluble with 

 difficulty in water, but easily in alcohol and ether. On heating it 

 decomposes into carbon dioxide and scatole. 





