9-1 Blumenthal — Preparation of Selenic Acid. 



nitrate was added to the concentrated solution, and the'free acid 

 nearly neutralized with ammonium hydroxide, white barium 

 selenate was precipitated. Though impure on account of its 

 tendency to occlude foreign material, this precipitate proved to 

 be practically free from selenious acid. 



Preparation of the Reagent. — To 10 grm. of selenium 

 dioxide dissolved in 75 cm3 to 100 cm3 of water were added 

 6 grm. of potassium bromate and 4 cm3 of strong nitric acid. 

 This mixture was heated on the water bath until perfectly 

 colorless. After the second evolution of bromine, which 

 occurred when the volume was quite low, a little more bromate 

 was added to decompose the excess of nitric acid and the 

 heating was continued. When the addition of a crystal or two 

 of bromate caused no further evolution of bromine, the 

 colorless solution was made slightly ammoniacal, heated to the 

 boiling point, and treated with a hot dilute solution of barium 

 nitrate. The precipitated barium selenate, after settling, was 

 filtered with suction, washed thoroughly with hot water, and 

 finally transferred to a large crucible and dried at low redness. 

 The weight of this product was about 25 grm., while the theo- 

 retical amount obtainable from the selenious acid taken was 

 about 25*1 grams. After weighing, the selenate was treated 

 in a porcelain crucible with 5 cm3 of strong sulphuric acid (sp. 

 gr. 1*8) and some 20 cm3 of water. The pasty mass so obtained 

 was heated about two and one-half hours on the water bath, 

 and finally diluted, filtered, and washed. The filtrate and 

 washings were made up to one liter, and an aliquot was taken 

 for analysis by the method of Fierce.* This analysis showed 

 about 94 per cent of the selenium taken to be present. From 

 the determination of the selenious acid present by the method 

 of Gooch and Clemonsf it was found that 85 per cent of the 

 selenium was present as selenic acid, the presence of selenious 

 acid in the final product being due in this case to the fact that 

 the barium precipitation was made in alkaline solution, under 

 which conditions barium selenite is also precipitated. The 

 total yield of selenic acid was 78 per cent of the theoretical. 

 Later experiments have shown that the contamination by 

 selenious acid may be to a large extent avoided by precipitating 

 while the solution still contains free nitric acid. 



Sodium Selenate. 



The preparation of sodium selenate, to be used with sul- 

 phuric acid in place of free selenic acid, was also investigated. 



Preliminary Test. — Elementary selenium (1 grm.) was 

 fused in platinum with sodium peroxide (2 grm.) and sodium 

 carbonate (6 grm.). The mass, after dissolving in water and 

 filtering, barely bleached a few drops of dilute permanganate, 



* This Journal (4), i, 416. \ Ibid. (3), 1, 51. * 



