RICHARDS AND MERIGOLD. ATOMIC WEIGHT OF URANIUM. 385 



Purification of Materials. 



As the source of uranium, commercial " chemically pure " uranium 

 acetate was used.* This was first converted to the chloride on account of 

 the greater solubility of this compound, — by precipitation as ammonium 

 uranate and redissolving in dilute hydrochloric acid. To the hot and 

 slightly acid solution, pure sulphuretted hydrogen was added to satura- 

 tion. The free acid was then neutralized with amnionic hydroxide, a 

 slight excess of the alkali was added, and more sulphuretted hydrogen 

 was run in. In this way some uranyl sulphide was precipitated, in order 

 to sweep down with it any colloidal sulphides of the higher groups which 

 might otherwise escape removal. The excess of sulphuretted hydrogen 

 was boiled off, and after standing over night the supernatant liquid was 

 decanted through a washed filter. 



The next step depended upon the fact that uranium remains iu a 

 solution of the double carbonate of ammonium and uranium, in the 

 presence of an excess of ammonium sulphide, while all the other members 

 of the aluminum and iron groups are thrown down by this reagent. 

 Consequently amnionic hydrate and ammonium carbonate in slight excess 

 were added to the filtrate, forming the double carbonate. If the solu- 

 tions are concentrated, the double carbonate is precipitated when more 

 than a slight excess of amnionic carbonate is used. This happened in 

 some cases, when it was necessary to redissolve the precipitate in dilute 

 hydrochloric acid and again add ammonic carbonate in more dilute 

 solution. About fifty grams of carbonate per litre was found to give the 

 best results. Ammonic hydroxide was then added to the hot solution, 

 and sulphuretted hydrogen in excess. After stauding over night the 

 solution was filtered. In several of the more concentrated solutions, a 

 considerable quantity of the salt crystallized out. These crystals were 

 worked up separately, as they were probably purer than the solution. 

 On boiling the solution to decompose the excess of ammonium sulphide, 

 some of the ammonic carbonate was decomposed, causing the precipita- 

 tion of some uranium sulphide. This precipitate was discarded, as it 

 might have contained iron, or other analogous metals which had previ- 

 ously escaped precipitation. Dilute hydrochloric acid in slight excess 

 was added, and the carbon dioxide was expelled by boiling. The free 

 acid was then almost neutralized with pure ammonic hydroxide, and 



* This method of uranium purification, with some modifications and additions, 

 is similar to that employed by Zimmermann. Annalen der Cliemie u. Pharmacie, 

 232, 299. 



vol. xxxvn. — 25 



