BAXTEK. — ATOMIC WEIGHT OF PHOSPHOKUS. 589 



sibly as a lower bromide ^). At any rate the tribromide prepared in 

 this way contained far too little bromine, as was shown by the results 

 of its analysis. 



Much better results were obtained by simple distillation in a vacuum. 

 Although the boiling point of the tribromide is thus much lowered, 

 the dissociation of the pentabromide is sufficiently complete to secure 

 rapid elimination of the excess of bromine. As stated before, the dis- 

 appearance of the bromine can be followed by the color of the distillate 

 and residue. If material is collected for analysis as soon as the bro- 

 mine has apparently been eliminated it should yield at any rate a 

 minimum value for the atomic weight of phosphorus, unless decomposi- 

 tion of the tribromide into a lower bromide occurs during the distillation. 

 In the latter case, however, the composition of different fractions of 

 distillate would probably not be constant.^ 



The synthesis of the tribromide was effected in an apparatus con- 

 structed entirely of glass shown in the figure on page 590. About 

 fourteen grams of pure phosphorus were freed as completely as possible 

 from water by cooling and pressing between folds of hardened filter 

 paper. The phosphorus was weighed to a centigram and placed in the 

 distilling flask D, which had previously been filled with nitrogen through 

 the stopcock A. The flask was then fitted to the separating funnel B 

 by means of a well ground and polished joint C, but it was not con- 

 nected to the remainder of the apparatus at E, and the capillary tube F 

 opened into the air. 



In order to dry the phosphorus completely, the flask was surrounded 

 with boiling water and for four hours a slow current of pure dry nitro- 

 gen was forced in at A and flowed out at the end of the constricted 

 tube F. The melted phosphorus was agitated from time to time by 

 shaking the flask in order to liberate any steam which might have been 

 entangled mechanically and to bring new material to the surface of 

 the phosphorus. During this operation a portion of the phosphorus 

 distilled into the cool tube F, and a small amount was vaporized into 

 the air. Probably, too, a small amount of acids of phosphorus was 

 produced since slight oxidation of the phosphorus necessarily took 

 place while it was being introduced into the flask. As soon as the 

 phosphorus had been dried in this way, the end of the constricted tube 

 F was sealed, the stopcock A was closed, and the flask, after being 

 cooled, was exhausted as completely as possible through the stopcock 

 S by means of the Topler pump "W. All the ground glass joints were 

 made gas tight by means of a minimum quantity of syrupy phosphoric 



" Besson and Foamier, Compt. Rend., 150, 120 (1910). 



