50 G. S. A. Ranking— A Note on Indigo Manufacture. [No. 1, 



Now in fermented Indigo-liquor we have a body present which forms 

 by the action of air Indigo-blue. It must therefore be, so far as is 

 known at present, one of two bodies : Indigo-white or Indoxyl. Which 

 of these two it is the following considerations will help to decide. 



It is soluble in water and the solution has generally a faintly 

 alkaline reaction but may be acid. It therefore seems not to be Indigo- 

 white. 



On the other hand its solution shews a very characteristic green 

 fluorescence, and, moreover, if treated with HC1 and Fe 2 Cl 6 yields 

 Indigo-blue, which fact strongly supports the view that Indoxyl is 

 present : the above reaction being one of the characteristics of Indoxyl 

 (vide R. and S. loc. cit.) 



But supposing it to be Indoxyl, as it really seems that it may be, 

 how can we account for the fact that it reacts with the salts of the 

 metals just as does Indigo-white. To explain this we must briefly recon- 

 sider our position. We found that Indoxyl has two replaceable Hydrogen 

 atoms, one in the OH group, the other in the CH group, and that if both 

 these atoms are removed Indigo-blue is left. 



First consider what oecurs when an alkaline solution of Indigo- 

 white is treated with a solution of a metallic salt. .(See Table Column I). 

 A double compound is formed consisting of the metal and Indigo- 

 blue the metal having secondarily replaced the replaceable atom of 

 Hydrogen. These compounds may be shewn to consist of Indigo-blue 

 and the metallic base, by treatment with strong sulphuric acid * and 

 subsequent addition of water, when Indigo-blue will be thrown down 

 and the metal remain in combination with the acid. 



Now I imagine that, in the case of the Vat-liquor containing Indoxyl, 

 what happens is this, we recollect it has two atoms of replaceable 

 Hydrogen, and both of these are removed, thus 

 (a) 8 H 7 NO + 2 KOH. 



= C 8 H 5 NO.K.K. (Alkaline solution) + 2 H 2 0. 

 (6) 8 H 5 NO.K.K. + BaCl». 

 = C 8 H 5 NO. Ba + 2 K 01. 



The result being ultimately the same as in the case of white Indigo , 

 with this difference that the Indoxyl is the more powerful reducing 

 agent of the two, and needs more oxygen ; so that the ultimate 

 reactions of the metals with alkaline solutions of Indoxyl and Indigo- 



* 1. CgHVNO. Ba + H 2 S04 + = C s H 5 NO + Ba S0 4 + H 2 0, atmospheric oxidation 

 occurring as part of the reaction. 



N. B. — It should be noticed that the composition of the double compound of 

 Indigo-blue and metal in the case of Barium points to an original compound in 

 which there were two atoms of replaceable Hydrogen, such as Indoxyl. 



