OXYAMIDOSULPHONATE ; DIVERS AND HAGA. 7 



nitrite suddenly appears, and so long after as the joint precipi- 

 tation of brown oxide is easily checked by stirring. AVhen the 

 point is reached where the oxide only redissolves slowly and 

 no longer gives place to a yellow one of hyponitrite, no more 

 silver solution must be added. To add much more would be to 

 cause permanent precipitation of silver oxide, which, if at all 

 large in amount, is apt to be accompanied by silver sulphite. 

 The quantity of silver sulphate needed may be as much as 40 

 grams, which means 7 or 8 litres of solution ; of silver nitrate 

 about 44 grams will be wanted, dissolved in 4 litres, or more, 

 of water. 



Half an hour after precipitation, the solution is to be 

 poured off even though still a little turbid, and the precipitate 

 washed by décantation ; for very slowly deposition of a mirror 

 of metallic silver from the sulphite in solution begins and goes 

 on for days. In order to separate the hyponitrite from the 

 metallic silver and its oxide, and perhaps chloride, it has to be 

 dissolved in dilute acid and reprecipitated. If every trace of 

 nitrite is to be kept out of the hyponitrite, nitric acid can 

 hardly be used, because it always contains, according to my 

 testing, a very minute quantity of nitrous acid. Sulphuric acid 

 is therefore the acid to take. Since the hyponitrite must be 

 kept in solution the shortest time possible for filtration, the use 

 suggests itself of acid not very dilute in order to reduce the 

 volume of liquid to be filtered. But high dilution is better, 

 because the stability of hyponitrite falls off rapidly with in- 

 creasing concentration, and, besides this, the use of sulphuric 

 acid not dilute enough causes separation of silver sulphate. A 

 one per cent, solution of the acid is suitable,_after it has been 

 well cooled in ice, some five litres of it being probably required. 



