388 ISATOSULPHURIC ACID. 



stitution-products of isatin are decomposed by fusing 

 caustic potassa like isatin, substitution-products of ani- 

 lin being formed. 



Isatosulphuric acid, C 8 H 4 M) 2 .S0 2 .OH, is formed 

 by the action of potassium bichromate and sulphuric 

 acid on sulphindigotates (indigo-carmine). Difficultly 

 crystallizable, very easily soluble acid ; monobasic. Its 

 barium salt, (C 8 H 4 NS0 5 ) 2 Ba + 4H 2 0, forms brass-red, 

 strongly lustrous, crystalline scales, but slightly soluble 

 in cold water. 



Trioxindol (Isatic acid), C 8 H 7 N0 3 . The violet solu- 

 tion of isatin in potassa-ley becomes yellow when 

 boiled, and then contains potassium isatate. The free 

 acid is exceedingly unstable ; when the attempt is 

 made to set it free by means of another acid, it breaks 

 up into isatin and water. 



The substitution-products of isatin conduct them- 

 selves towards caustic potassa in like manner. They 

 yield chlorinated or brominated isatic acids, which are 

 likewise exceedingly unstable in a free condition. 



Dioxindol (Hydrindic acid), C 8 IKN"0 2 , is formed by 

 the action of sodium-amalgam on isatin, to which is 

 added water, by reduction of the isatic acid, which is 

 at first formed. Transparent, rhombic prisms ; easily 

 soluble in water and alcohol; fuses at 180, and decom- 

 poses at 195, anilin being formed. The aqueous solu- 

 tion in contact with air turns red, oxygen being taken 

 up and isatin formed. Combines with acids and bases, 

 forming salts. Treated with chlorine or bromine it 

 yields crystallizing substitution-products. Treated with 

 nitrous acid in an alcoholic solution, it is at first con- 

 verted into a crystalline substance, nitrosodioxindol^ 

 C 8 H 6 (^0)^"0 2 , insoluble in water ; fusing at 300-310 ; 

 further action converts it into ethyl benzoate and other 

 products. It yields oil of bitter almonds when gently 

 warmed with nitric acid or silver oxide. 



