Ö6 



DIVERS AXD HAGA. 



slow and then at once stopped when the pajjer i« just permanently 

 reddened. The hquor at this stage contains, besides a very httle 

 sulphate, nothing but the two salts, nitrilosulphonate and metasulphite, 

 which, omitting notice of intermediate stages, have been derived as 

 follows : — 



2NaNOo+3Na2C03 + 8S02=2N(S03Na)3 + Na2SA+3C02 



The apparently redundant metasulphite is necessary in order to get 

 the nitrite converted wholly into nitrile salt. Its presence is almost 

 equally essential for the preservation from destruction of the imidosul- 

 phonate about to be formed. 



When full neutrality is reached, or perhaps just passed — it is 

 impossible to say — the liquor after standing a few minutes suddenly 

 becomes strongly acid and warm, and evolves much sulphur dioxide. 

 Hydrolysis has occurred, but thanks to the metasulphite present, only 

 through one stage, and the liquor now holds only imidosulphonate, 

 sulphate, and sulphur dioxide — 



2N(S03Na)3+ Na^S A + OH2 = 2HN(S03Na)2+ 2Na2S04+ 2SO2. 



Sulphurous acid appears to have no immediate hydrolysing action upon 

 the imidosulphonate, but as it oxidises to sulphuric acid it must be 

 removed. A ra])id current of air is, therefore, sent through the liquor 

 for half an hour or so, after which the addition of a very little sodium 

 carbonate suffices to render it slightly alkaline and quite stable. It is 

 evaporated, so far as may be necessary, on the water-bath and then 

 cooled down to 0° or a little below, whereby in the course of some 

 hours most of the sulphate is made to crystallise out. A second 

 evaporation and cooling is generally needed to separate still more 

 sulphate, after which renewed evaporation and ordinary cooling brings 

 about the separation in well-formed crystals of much of the disodium 



