IMIÜOSULPHONATES. 67 



imidosiilphonate. The mother- liquor can be furtlier worked f(3r sul- 

 phate and imidosiilphonate, if worth the trouble. 



The reaction proves to be almost quantitative, for the salt obtained 

 in crystals is 80 per cent., while there is considerable loss of salt in the 

 mother-liquors adhering to the very voluminous masses of sodium 

 sulphate. The crude salt can be purified by recrystallisino- from 

 warm water rendered slightly alkaline as a precaution against 

 hydrolysis. 



Instead of sodium carbonate the equivalent quantity of normal 

 sulphite, or of metasulphite. pure and therefore freshly prepared, may 

 be used along with the additional sulphur dioxide indicated by the 

 equation already given. The mixture of nitrite and metasulphite be- 

 comes at once alkaline and rapidly heats up, so as to need cooling, in 

 accordance with what has been stated. Thh mode of procedure is 

 certainly less simple and less practicable than that described. It may 

 also be mentioned that a sufficient excess of metasulphite will of itself 

 convert all nitrite to nitrile salt, but as then addition of another acid 

 is required to eifect liydrolysis into imidosulphonate the liquor obtain- 

 ed is unsuited to give a good yield of imidosulphonate crystals. 



It is not unfrequeritly convenient to ])re])are disodium imidosul- 

 phonate from the trisodium salt. The latter salt can be preserved 

 without need of taking any precautions, which is not the case with 

 the disodium salt. Again, where the direct process for preparing 

 sodium imidosulphonate has been carried out with impure salts, or 

 with imperfect observance of the proportions to be used, and of other 

 details, it may not be so easy to crystallise out the disodium salt, 

 and here the very ready separation and purification of the trisodium 

 salt becomes advantageous. From the trisodium salt the disodium 

 imidosulphonate is prepared by treating the crystals with sulphuric 

 acid slightly diluted until neutrality to litmus is reached. The solu- 



