CONTRIBUTIONS FROM THE CHEMICAL LABORATORY 

 OF HARVARD COLLEGE. 



A REVISION OF THE ATOMIC WEIGHT OF PHOSPHORUS. 



SECOND PAPER. — THE ANALYSIS OF PHOSPHORUS 



TRIBROMIDE. 



Bt Gregory Paul Baxter, Charles James Moore and Arthur 



Clarence Boylston. 



Presented Dec. 13, 1911. Received Dec. 7. 1911. 



In a recent investigation by Baxter and Jones ^ upon the atomic 

 weight of phosphorus by the analysis of trisilver phosphate, the con- 

 stant in question was found to have the value 31.04, if silver has the 

 atomic weight 107.88, or 31.03 if silver is given the value 107.87. 

 While this result is in good accord with those of most earlier experi- 

 menters in the same field, Ter Gazarian,^ on the other hand, from the 

 density of phosphine, has recently obtained a considerably lower 

 value, 30.91. So large a difference as this obviously needs explana- 

 tion, and the most promising method of solving the problem seemed 

 to be to analyze some other compound of phosphorus. 



In the research upon silver phosphate some uncertainty was in- 

 troduced by the fact that the salt contains only 7.7 per cent of phos- 

 phorus, so that the percentage error in determining the molecular 

 weight of silver phosphate is many times multiplied in the calculation 

 of the atomic weight of phosphorus. In this respect phosphorus tri- 

 bromide is somewhat better suited for the purpose, since it contains 

 11.5 per cent of phosphorus. Hence in spite of very considerable 

 difficulties in the preparation of the tribromide in a pure state, and 

 in its analysis, we were led to choose this substance for further in- 

 vestigation of the subject. 



In outline the method followed was to synthesize phosphorus tri- 

 bromide by the action of pure dry bromine on pure dry phosphorus 



1 These Proceedings, 45, 137 (1910); Jour. Amer. Chem. Soc, 32, 298; 

 Zoit. anorg. Chem., 66, 97. 



2 Jour, de Chim. Phys., 7, 337 (1909); 9, 101 (1911). 



