A REVISION OF THE ATOMIC WEIGHT OF PHOSPHORUS. 169 



and Trannoy show that the gas prepared in this way by them contained about 

 3 per cent of hydrogen, probably derived from calcium contained by the phos- 

 phide. In this case some calcium nitride would be formed, since the phosphide 

 was made in air; and this would produce ammonia as an impurity in the phos- 

 phine. Although the gas was purified by fractional distillation, according to 

 Gazarian's statements hydrogen is difficult to eliminate, and a proportion of 

 only 0.4 per cent would be sufl&cient to lower the atomic weight of phosphorus 

 0.1 unit. Ammonia would be even more difficult to remove, since its boiling 

 point is only 50° higher than that of phosphine. The effect of a given percent- 

 age of impurity is, however, much less with ammonia than with hydrogen, 

 although in the same direction. 



From the preceding brief summary it is evident that the uncertainty in the 

 atomic weight of phosphorus is as great as o.i unit, and that, as Brauner re- 

 marks at the conclusion of his review of the subject, "a revision of the atomic 

 weight of phosphorus with modem means is urgently necessary." 



The analysis of silver phosphate was selected as one of the most promising 

 methods of attacking the problem, since the per cent of silver can be determined 

 exactly by a method which has been carefully studied, especially in this labora- 

 tory. The accuracy of the result will therefore depend primarily upon the 

 success attained in preparing silver phosphate in a perfectly definite and pure 

 state. The greater part of the following research was devoted to the solution of 

 this problem, which van der Platts found so difficult. 



The analysis of the halogen compounds of phosphorus offers certain difficul- 

 ties owing to the ease with which these substances are decomposed by water, 

 and to the necessity for oxidizing the phosphorous acid resulting from the de- 

 composition of the halogen compounds with water before the addition of silver 

 nitrate. An investigation upon the tribromide of phosphorus is now in progress 

 in this laboratory. Phosphonium compounds were found utterly unsuited for 

 exact analysis on account of their instability. 



PURIFICATION OF MATERIALS. 



Water. — All the water used in this research was made from the laboratory 

 supply of distilled water by distillation, first from an alkaline permanganate 

 solution, and then, after the addition of a trace of sulphuric acid, through a 

 block tin condenser. 



Ammonia. — The best commercial ammonia was distilled into the purest 

 water. 



Nitric Acid. — The best commercial concentrated acid was twice fractionally 

 distilled through a platinum condenser, with the rejection of the first third of 

 the distillate. Every sample was shown to be free from chloride by careful 

 nephelometric tests. 



Hydrochloric Acid. — The best commercial C. P. acid, diluted with an equal 

 volume of water, was distilled through a platinum condenser. 



