1885.] The Chemical Constitution of Isatin. 195 



the solution is allowed to flow slowly into the mixture of isatin and 

 acetic acid, which thereby becomes heated. As soon as the slightest 

 ebullition is noticeable, the flask must be cooled by ice-water. After 

 the mixture has become quite cold, the process is repeated as above, 

 with frequent shaking, care being taken that the temperature does 

 not rise above 50°, as otherwise the product undergoes partial decom- 

 position. 



After each addition of chromic acid, the colour of the liquid, at 

 first red, becomes darker ; when about the half of the chromic acid 

 has been added, a brown flaky substance begins to separate out, the 

 quantity of which goes on increasing, and before the whole has been 

 run in, the bottom of the flask is seen to be coated with a dirty grey 

 layer. After the addition of all the chromic acid and acetic acid, 

 the flask, with its dark-brown coloured contents, is allowed, after 

 frequent shaking, to stand in cold water for 12 hours. Should this 

 he neglected, the liquid becomes heated again of itself (the reaction 

 not being yet at an end), and this must be avoided, as it is accom- 

 panied by partial decomposition. 



At this stage the product contains as yet no isatoic acid. To com- 

 plete the oxidation, the flask with its muddy contents is now allowed 

 to stand for some hours in water, kept at 40° to 50° ; a higher tem- 

 perature is hurtful. The contents thus undergo a distinct change ; 

 the brown flakes vanish, a crystalline substance taking their place, 

 and the liquid finally acquires the deep green colour of acetate of 

 chromium. The product is lastly heated for several hours at about 

 60° ; during this stage isatoic acid separates out in the form of a 

 yellow crystalline powder. 



To make this separation as far as possible complete, the whole is 

 poured into 500 c.c. of extremely dilute sulphuric acid ; the precipi- 

 tate thus got, after being washed with cold water with the aid of a 

 filter-pump, and spread out on paper to dry, is almost quite pure isatoic 

 acid. The yield from 100 grams isatin is 80 grams, i.e., 72 per cent, 

 of the calculated amount. It is finally obtained absolutely pure by 

 one crystallisation from boiling acetone. Other solvents are unsuit- 

 able for the purification of isatoic acid ; water, which, even when warm, 

 dissolves it with difficulty, causes decomposition, with evolution of 

 carbonic acid. Ethyl and methyl alcohols etherify at the boiling 

 temperature a part of the isatoic acid, as may be easily recognised from 

 the agreeable odour which the crystals acquire. Even the minute 

 quantities of methyl alcohol which acetone contains show their pre- 

 sence in this way. 



Isatoic acid crystallises from hot acetone (of which about 23 parts at 

 the boiling temperature are required for the solution of 1 part of 

 acid) in hard yellow prisms, which appear to be almost rectangular ; it 

 melts at 230°, with decomposition, carbonic acid being given off, 



