386 SULPHINDIGOTIC ACID. 



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

 indigo-blue. 



Sulphindigotic acid (Sulphocoerulic acid), C 8 HWO. 

 S0 2 .OH, 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, hyposidphindigotic acid, which 

 is formed, together with sulphindigotic acid, particu 

 larly when indigo is dissolved in fuming sulphuric 

 acid; thereupon the substance is dissolveid 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 IKNX). 

 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 



