TMTDES OF BENZENEDISULPHONIC ACIDS. 7 



of the solution and adding water to it, and seeing whether it gave 

 any precipitate or not. When the reduction was complete, air 

 was passed through the mixture to remove the excess of sulphur 

 dioxide ; and then some water was added. On neutralising the 

 solution with potassium carbonate, voluminous crystals of the 

 potassium salt of the imide filled the liquid, which, however, 

 dissolved again on warming. The solution was concentrated on 

 a water-bath, and set aside to crystallise. 3.25 grammes of 

 the pure potassium salt, crystallised in needles, were thus ob- 

 tained. In order to get the free imide, an excess of a solu- 

 tion of mercurous nitrate was added to the warm solution of 

 the potassium salt, when the mercurous salt appeared as a 

 white crystalline precipitate. This precipitate was washed, sus- 

 pended in water, and decomposed by a current of hydrogen 

 sulphide. The precipitated sulphide was filtered off, and the 

 solution was evaporated on a water-bath until it became some- 

 what syrupy. On setting it aside for a while, the solution gave 

 prismatic crystals of the imide, containing two molecules of water 

 of crystallisation. The water of crystallisation can be removed 

 at 100°. 



When a considerable quantity of the imide is to be prepared, 

 the conversion of the potassium salt into the mercurous salt, 

 with the subsequent removal of mercury by means of hydrogen 

 sulphide becomes a very tedious operation. It is better to evapo- 

 rate the reduced alcoholic solution at once with the addition of 

 a little water, until the crystals begin to appear. In this way 

 a crop of tolerably pure imide can be obtained. The mother 

 liquors from the recrystallisation can be worked up for various 

 salts of the imide, or the free imide may be obtained through 

 the mercurous salt as described above. 



