1903] Davis — Atomic Weight of Thorium. 47 



In some cases the amount of sulphuric acid is determined 

 by precipitation as BaS0 4 . This is also a very objectionable 

 process, because of the well known property of barium sul- 

 phate to occlude foreign substances and hence should be 

 excluded from atomic weight work. While the values obtained 

 from the sulphate method may lie close to the real value of 

 thorium, they probably deviate from it by several points. 



From the above considerations it was first decided in this 

 work to make some determinations by the sulphate method in 

 order to test its accuracy. For the material used we are 

 indebted to Dr. H. S. Miner, of the Welsbach Lighting- Co., 

 Gloucester, N. J., who generously supplied us with a large 

 quantity of commercial thorium nitrate. This thorium we 

 purified by several of the accepted methods of purification. 

 Half kilo of the nitrate was dissolved in three litres of water 

 and precipitated as hydroxide with ammonia. This was thor- 

 oughly washed, then dissolved in HC1. The solution was 

 reprecipitated by ammonia. The other part was neutralized 

 by ammonia until a slight precipitate persisted and the tho- 

 rium precipitated by the addition of sodium nitrite. The 

 precipitate was thoroughly washed, then dissolved in acid and 

 the precipitation repeated. 



One portion we attempted to purify by changing to the 

 chromate, but on subsequent treatment, the chronium wts so 

 difficult to remove that this portion was not used in any of 

 our work on the atomic weight of thorium. 



Another half kilo, after precipitation by ammonia, was 

 purified by dissolving in ammonium carbonate solution, and 

 then precipitated by ammonia. 



Then a portion of this precipitate was purified by the citrate 

 method. A saturated solution of citric acid was saturated 

 with thorium hydroxide and stirred for twenty-four hours. 

 After the citric acid would take up no more thorium hydrox- 

 ide, the undissolved hydroxide was filtered off, and the fil- 

 trate was heated in a water-bath with boiling water. A 

 heavy white precipitate came down. This was filtered through 



