AMIDOSULPHONIC ACID. 227 



this salt may be dissolved in four or five times its weight of water, the 

 solution nearly saturated ice-cold with sulphur dioxide, and set aside 

 at the ordinary temperature for a day in a flask closed not quite air- 

 tight. After it has thus stood, the sulphur dioxide remaining is to he 

 expelled by a current of air, and the solution placed in a good desiccator, 

 where the acid soon begins to crystallise out in thick plates. The 

 crystals, well drained, are to be washed two or three times with a little 

 ice water. The yield should approach four-fifths of the weight of the 

 hydroxylamine sulphate. Since sodium nitrite can be converted in- 

 expensively into its own weight of hydroxylamine sulphate, hardly a 

 better source of the acid might seem possible to be had : nevertheless, 

 it will never be resorted to in work on the large scale, for the acid itself 

 can be obtained from sodium nitrite direct, more easily than hydroxyl- 

 amine sulphate can be obtained from it. This process of getting it 

 will now be described. 



Preparation from sodium nitrite, through imidosulphonic acid. — Briefly, 

 this process consists in fully sulphonating sodium nitrite by means of 

 sulphur dioxide and sodium carbonate, hydrolysing the nitrilosulpho- 

 nate to acid sulphate and amidosulphonic acid, neutralising with 

 sodium carbonate, crystallising out sodium sulphate, and precipitat- 

 ing the amidosulphonic acid by addition of large excess of concentrated 

 sulphuric acid. But to get the largest yield of acid most easily, some 

 such particulars as the following must be observed. Sodium nitrite, 

 2 mol., and sodium carbonate, 3 mob, are put into enough water to 

 make the whole weigh 18 times as much as the sodium nitrite ; then 

 sulphur dioxide is passed in until a solution acid to litmus is obtained. 

 Most usually, the solution then very quickly changes of itself, but a 

 drop of strong sulphuric acid may be added to start the change, which 

 is that of the nitrilosulphonate into imidosulphonate and acid sulphate. 

 Marked heating up occurs, and a large evolution of sulphur dioxide, 



