PRELIMINARY EXPERIMENTS. 73 



It was found that the sulphate decomposes slightly when fused, a difficulty 

 which was later overcome. Again the quantitative solution of the silver 

 sulphate was a very tedious process. Eight grams fused in a platinum 

 crucible and placed in 1.5 liters of water required six weeks for its com- 

 plete solution, even with frequent agitation. The action on the glass 

 during this long time, the danger of the access of gaseous impurity, and 

 the loss of valuable time all made this difficulty a serious one. 



Argentic sulphate can be readily dissolved by placing twice its weight 

 of concentrated sulphuric acid in the crucible and heating at about 300 ; 

 on cooling, the acid sulphate of silver crystallizes out. 1 Upon adding water 

 to this acid sulphate the normal sulphate is formed as a fine powder, which 

 can readily be washed into the precipitating flask and dissolved. The 

 large excess of sulphuric acid thus introduced was far from desirable, 

 however, for it tends to cause considerable occlusion of argentic sulphate 

 in the precipitated chloride. In order to test this question, a solution, pre- 

 pared as described above, and containing 7.4 grams of argentic sulphate, 

 was precipitated with hydrochloric acid. The precipitate was washed by 

 decanting eleven times with very dilute hydrochloric acid and then dis- 

 solved in ammonia and diluted. The argentic chloride was then reprecipi- 

 tated with hydrochloric acid ; the solution should contain all the sulphate 

 previously occluded. After settling, this mixture was filtered, and the per- 

 fectly clear solution was evaporated in a platinum dish over an alcohol 

 lamp until ammonic chloride crystallized out on cooling, the volume being 

 then about 0.1 liter. To this were added about 10 grams of baric chloride. 

 An undoubted precipitate of baric sulphate was produced, which proved 

 that there is considerable occlusion of sulphate by argentic chloride pre- 

 cipitated from solutions containing a large excess of sulphuric acid. 



For the sake of comparison, approximately the same amount of argentic 

 chloride prepared from nitrate was dissolved in ammonia, and 3.5 mg. 

 of sulphate added. The solution was treated exactly like the preceding 

 operations, and yielded about the same amount of precipitate. This 

 experiment gives an approximate idea of the extent of the occlusion. 



In searching for a means of wholly eliminating the sulphate it was 

 found that a small proportion of argentic sulphate in argentic chloride 

 could be completely converted into chloride by fusion in a current of hy- 

 drochloric acid gas. This observation led to the development of a new 

 process in which the entire reaction was carried out in this manner, fused 

 argentic sulphate being wholly converted into chloride without the need 

 of dissolving in water. Thus the operation was greatly simplified, and the 

 chances of error diminished. 



iSchultz, Pogg. Ann., 133, 143 (1868). 



