221 



Xew Methods for the Preparation of Salts of Selenic Acid. 



By Frank C. Mathers and .7. Otto Frank. 



The purpose of this paper is to describe the preparation of metalli • 

 selenates from ammonium selenate. Ammonium selenate is the best start- 

 ing material on account of the very satisfactory method' by which it may 

 be prepared. An outline of the nietliod for preparing ammonium selenate is 

 as follows : Selenic acid was first prepared by oxidizing a solution of selen- 

 ium in nitric acid with potassium permanganate. This solution after the 

 removal of the manganese dioxide precipitate by filtration was precipitated 

 with lead nitrate. The lead selenate thus formed was washed thoroughly 

 until free from soluble salts. The treatment of this solid lead selenate 

 with an excess of a strong solution of ammonium carbonate produced lead 

 carbonate and ammonium selenate in solution. Pure ammonium selenate 

 crystallized from the filtrate upon evaporation while the excess of am- 

 monium carbonate was volatilized. 



There are three methods by which this ammonium selenate may be 

 changed into metallic selenates. They are as follows : 



1. The crystallization of a solution of ammonium selenate with a 

 metallic nitrate will give crystals of the most insoluble salt which can be 

 formed by any combination of the four ions in solution. The most soluble 

 combination of the ions will remain in the mother liquor. The solubility 

 of the possible combinations of the four ions is in this order beginning with 

 the most soluble, ammonium nitrate, metallic nitrate, ammonium selenate, 

 and ammonium metallic selenate or metallic selenate. Whether the metal- 

 ie selenate or the double ammonium salt is formed depends upon their 

 relative solubility except that by increasing the amount of ammonium 

 selenate the double salt may be produced in cases where the metallic salt 

 is ordinarily formed. For example, equivalent amounts of ammonium 

 selenate and copper nitrate give copper selenate while two equivalents of 

 the ammonium selenate to one of copper nitrate gives the ammonium cop- 

 per selenate. 



•Mathers and Bonsib, ,7our. Araer. Chem. Soc. 33,703 (1911) ; Indiana Univer- 

 sity Studies. S41 (1910). 



