386 SULPHINDIGOTIC ACID. 



stable, take up oxygen rapidly from the air, and deposit 

 indigo-blue. 



Sulphindigotic acid (Sulphocoerulic acid), C 8 H 4 NO. 

 S0 2 .OII, is formed when 1 part indigo is digested for 

 three days, at 30-40, with 15 parts concentrated sul- 

 phuric acid. Pure wool is then placed in the diluted 

 solution. Upon this the acid formed is deposited, there 

 remaining in the liquid only the excess of free sul- 

 phuric acid. The wool, which is dyed blue, is now 

 well washed with water; and the acid extracted by 

 means of ammonium carbonate; the solution evapo- 

 rated at as low a temperature as possible; and the 

 residue washed with alcohol for the purpose of re- 

 moving another acid, hyposulphindigotic acid, which 

 is formed, together with sulphindigotic acid, particu- 

 larly when indigo is dissolved in fuming sulphuric 

 acid; thereupon the substance is dissolved in water; 

 precipitated with lead acetate; and the lead salt, 

 suspended in water, decomposed by sulphuretted 

 hydrogen. On evaporating the filtered solution at a 

 but slightly elevated temperature, the acid remains 

 behind in the form of a blue, amorphous mass, easily 

 soluble in water and alcohol. 



Its salts are amorphous. The potassium salt, C 8 H 4 NO. 

 S0 3 K, and the sodium salt occur in commerce under the 

 name of indigo-carmine, and are prepared on the large 

 scale by adding potassium acetate, or Glaubers salt, to a 

 diluted solution of indigo-blue in sulphuric acid ; wash- 

 ing out the blue precipitate with solutions of the salts 

 employed; and pressing. They form copper-colored 

 masses, which appear blue in a finely-divided condition, 

 and dissolve with blue color in pure water. 



If, in the preparation of sulphindigotic acid, less 

 (only 8 parts) sulphuric acid is employed, on subse- 

 quently diluting with water, a blue precipitate is 

 thrown down, consisting of 



Sulphophoenicic acid (Sulphopurpuric acid), 

 C 16 H 9 N 2 2 .S0 2 .OH, which dissolves in pure water, free 

 of acids, and forms purplish-red salts with bases ; these 



