ARSENIC AND TIN. 79 



dissolves the sublimate, the solution added to the sul- 

 phide of ammonium, and the whole liquid carefully 

 acidified with hydrochloric acid which precipitates the 

 sulphide of arsenic. Some powdered chlorate of potassa 

 is added to the liquid, without filtering, and heat ap- 

 plied till there remains only pure sulphur, which is 

 filtered off. From the filtrate, the arsenic acid is then 

 precipitated by ammonia and sulphate of magnesia as 

 in No. 61. 



All the tin is left in the bulb in the form of dark 

 brown bisulphide of tin, mixed, however, with a vari- 

 able quantity of sulphur, so that the amount of tin 

 cannot be immediately inferred from the weight of the 

 residue. In order to determine the tin, the contents of 

 the bulb are thrown into a porcelain crucible, moistened 

 with nitric acid, and ignited, with access of air, until 

 the tin is entirely converted into white binoxide, which 

 is then weighed. The quantities of tin and arsenic 

 are afterwards calculated for the whole quantity of the 

 original oxidized mass. 



A simpler method is based upon the solubility of 

 sulphide of arsenic in bisulphate of potassa, while the 

 sulphide of tin is insoluble. The mass oxidized by 

 nitric acid is digested with a solution of caustic potash 

 and sulphur until it is completely dissolved (or with 

 the exception of a basic sulphide, from which it may 

 be filtered). The solution is then mixed with an excess 

 of sulphurous acid, digested, and boiled until about 

 two-thirds of the water has evaporated and all the sul- 

 phurous acid. The sulphide of tin is filtered off and 

 washed with a concentrated solution of common salt, 

 and not with water. This may then be separated from 

 the precipitate by a solution of acetate of ammonia 

 slightly acid, but the liquid must not be mixed with the 

 salt washings. The sulphide of tin is dried and con- 

 verted into the oxide by roasting in the air. The 



