74 BISMUTH, LEAD, AND TIN. 



sulphuric acid. The whole solution, containing the 

 suspended precipitate, is evaporated to expel the nitric 

 acid, until the sulphuric acid begins to volatilize ; it is 

 then diluted with water, and the sulphate of lead col- 

 lected upon a weighed filter, which has been dried at 

 120, and washed with spirit of wine. A filter which 

 has not been weighed may be employed, if care be 

 taken to remove as much as possible of the precipitate 

 from the filter, and to incinerate the latter carefully 

 apart, so that no reduction of lead may take place. 

 (Moreover, see No. 49.) 



After the above operation it is difficult to obtain the 

 binoxide of tin perfectly free from lead. It is much 

 surer to fuse the alloy with a mixture of carbonate of 

 potassa and sulphur, to extract the tin as a sulphide, 

 and proceed as in No. 49. 



49. BISMUTH, LEAD, AND TIN. 



The alloy is oxidized with moderately strong nitric 

 acid, the mass mixed with an excess of ammonia and 

 sulphide of ammonium, and digested for some time in 

 a closed flask. In this way the tin is entirely dissolved 

 as a sulphur-salt. The solution is filtered off from the 

 other sulphides, which are then washed with very weak 

 sulphide of ammonium, and dried. 



From the solution the bisulphide of tin is precipi- 

 tated by dilute hydrochloric acid, filtered off, washed, 

 and dried. It is then gradually heated, together with 

 the filter, in a porcelain crucible, with free access of 

 air; at first gently, and ultimately to redness, so that it 

 may be entirely converted into binoxide of tin. A 

 fragment of carbonate of ammonia is held in the cruci- 

 ble at the end of the operation, to remove any sulphuric 

 acid which may have been formed. 



