223 



Zinc oxide was selected because of its pronoimced basic properties. 

 Ammonium selenate, dissolved in water, was treated with an excess of zinc 

 oxide. The evolution of ammonia was slow at room temperature but was 

 more rapid at the boiling point. Zinc selenate and not the double salt was 

 obtained by crystallization of the solution, llie yield was only about 25 

 per cent, of the theory. Analysis of the ZnSeOi.THjO showed 18.8 per 

 cent, zinc while the theory is 19.4. 



3. This method makes use of copper selenate prepared as described 

 above. Any metal more electropositive than copper will precipitate copper 

 from a copper selenate solution and form a selenate of the metal that was 

 used. This is the best method of making the metallic selenates since a 

 pure solution of the metallic selenate can be obtained. The previous prep- 

 aration of the copper selenate from the ammonium selenate is exceptionally 

 easy. Selenates of cadmium and zinc were prepared by treating solutions 

 of 15 grams copper selenate dissolved in 50 cc. of water with an excess of 

 metallic cadmium and metallic zinc respectively. The yield of cadmium 

 selenate was 14.1 grams and of zinc selenate 10.7 grams while the theory 

 was 14.7 and 16.3 respectively. 



SUMMABY. 



Ammonium selenate, on account of the very satisfactory method for 

 its preparation, is the most suitable starting material for the making of 

 metallic selenates. The three methods that were tried for preparing 

 metallic selenates from ammonium selenate are : 



1. Crystallization of a solution containing ammonium selenate and a 

 metallic nitrate will produce crystals of the double ammonium selenate (or 

 the metallic selenate in some cases) since this salt is the most insoluble 

 which can be produced by any possible combination of the four ions. The 

 yields are good. 



2. Ammonium selenate can be decomposed by liasic metallic oxides 

 with the formation of llie metallic selenate and of free ammonia. This is 

 not a satisfactory method and the yields are low. 



3. The treatment of copper selenate solution with any metal which is 

 more electropositive than copper will precipitate metallic copper and form 

 the corresponding metallic selenate. This is a very satisfactory method 

 and the yields are practically quantitative. 



