﻿PREPARATION OF AMMONIUM SELENATE. 



53 



trate and Avash water were evaporated on a water-bath until the 

 "ammonium carbonate" was completely eliminated; then made 

 slightly acid with nitric acid, diluted to a large volume and treated 

 with a hot ten per cent solution of barium nitrate. A white pre- 

 cipitate of pure barium selenate was formed, which was placed on 

 filter paper, washed thoroughly with distilled water and dried in an 

 air bath for four hours at a temperature of from 115° to 130° C. 



Lead selenate was prepared in an analogous manner, by pre- 

 cipitating the selenic acid with a pure lead nitrate solution. 



Preparation of Ammonium Selenate, The materials used were : 



Barium selenate prepared as above. 



Lead selenate prepared as above. 



"Carbonate of ammonia" (Merck). 



The following analysis was made to determine the composition 

 of the "carbonate of ammonia": 



NHg 21.80 per cent. 21.40 per cent. 



CO2 27.73 per cent. 27.92 per cent. 



Procedure. Gram portions of barium or lead selenate were 

 weighed out into small beakers and treated with varying quantities 

 of distilled water and the powdered "carbonate of ammonia." 

 After different periods of time, the mixtures were filtered and the 

 residues washed Avith distilled water, until free from ammonia, and 

 then analyzed to determine the yields. 



Method of Calculating" Yields. The following is the reaction, 

 M being either lead or barium : 



MSeO^ + (NH J , CO3 = MCO3 + (NH J 3 SeO^. 



The residue of either the barium or lead carbonate obtained after 

 filtering from the ammonium selenate and the excess of the "am- 

 monium carbonate," were washed with distilled water until there 

 was no coloration produced when the wash-water was treated with 

 Nessler's reagent. This residue was dissolved in an excess of 

 standard hydrochloric acid and titrated with standard sodium 

 hydr'oxide solution. Methyl orange was used as an indicator. For 

 every molecule of barium or lead selenate decomposed by the "am- 

 monium carbonate," a molecule of barium or lead carbonate is 

 formed. The barium or lead carbonate thus produced reacts with 

 the hydrochloric acid and the amount of ammonium selenate may 

 be easily determined. 



