﻿PREPARATION OF AMMONIUM SELENATE. 



57 



. These tables show that almost theoretical yields may be ob- 

 tained. It is impossible to obtain a yield of 100 per cent because 

 a cei-tain amount of lead selenate dissolves in the ''ammonium car- 

 bonate" solution and in the wash- water. When the solution of 

 ammonium selenate and the washings are evaporated to drive out 

 the excess "ammonium carbonate" and to crystallize the ammo- 

 nium selenate, the lead selenate is precipitated. This is, of course, 

 filtered out in order to prevent contamination of the ammonium 

 selenate. In one experiment the amount of this lead selenate was 

 found to be 1.72 per cent; adding this to the yield of the anuno- 

 nium selenate obtained, gives a total of 99.47 per cent, which ac- 

 counts for practically all of the lead selenate. 



Results with Lead Sulphate : Since lead sulphate is analogous 

 to lead selenate, the following experiments were made to see what 

 effect "ammonium carbonate" would have upon lead sulphate. 

 One gram portions of lead selenate and ten ce. distilled water were 

 used in all of the experiments. The results may be tabulated in 

 the following manner: 



Amount of 



Time 



Yield 



Ammon. Carb. 



Treated. 



Per cent. 



0.31T1 gr. 



1 hr. 



47.75 



0.3171 gr. 



2 hrs. 



56.17 



0.3171; gr. 



4 hrs. 



56.17 



0.3171 gr. 



6 hrs. 



56.17 



0.6342 gr. 



1 hr. 



70.21 



0.6342 gr. 



2 hrs. 



71.21 



0.6342 gr. 



4 hrs. 



75.83 



0.6342 gr. 



6 hrs. 



75.83 



It may be seen from the above table that lead sulphate is not 

 nearly so rapidly decomposed by "ammonium carbonate" as is lead 

 selenate. 



SUMMARY. 



The object of this research was to determine the best condi- 

 tions for the preparation of ammonium selenate by the treatment 

 of either barium selenate or lead selenate with an excess of "am- 

 monium carbonate." This reaction forms insoluble barium car- 

 bonate or lead carbonate with ammonium selenate and the excess 

 of "ammonium carbonate" in solution. This mixture is filtered 

 and the filtrate evaporated. During the heating the "ammonium 

 carbonate" volatilizes and any barium or lead selenate which is 

 in solution is precipitated. This precipitate is filtered off and 

 the pure ammonium selenate allowed to crystallize. 



