A REVISION OF THE ATOMIC WEIGHT OF 



PHOSPHORUS. 



THE ANALYSIS OF SILVER PHOSPHATE. 



INTRODUCTION. 



Although phosphorus is one of the best known and most important ele- 

 ments, present knowledge concerning its atomic weight is somewhat inadequate. 

 The early determinations of this constant by Dulong,^ Pelouze,^ Berzelius,' 

 and Jacquelain^ are widely discrepant and have no particular significance. 

 Those by Schrotter, Dumas, van der Platts, and Berthelot, on the other hand, 

 all give values not far from 31.0, and this value has been selected by the Inter- 

 national Committee on Atomic Weights. Although these investigations have 

 already been critically discussed by Clarke,^ Brauner,® and others, a few of the 

 more important sources of error are briefly pointed out here. 



Schrotter,^ the discoverer of red phosphorus, converted weighed quantities 

 of this substance into phosphorus pentoxide by combustion in a stream of 

 oxygen. As the mean of ten determinations which varied from 30.94 to 31.06, 

 he obtained 31.03 for the atomic weight of phosphorus. The oxygen used was 

 slightly moist, as Brauner has pointed out, since, although it was dried by 

 phosphorus pentoxide, it was finally passed through a tube containing calcium 

 chloride! The phosphorus pentoxide formed during the combustion must have 

 retained this small amount of water, which would make the atomic weight of 

 phosphorus appear too low. Schrotter admits that the combustion was incom- 

 plete, and since this error would tend to raise the atomic weight of phosphorus, 

 he concludes that the true value is 31.00. 



Dumas ^ titrated the trichloride of phosphorus against silver after decompos- 

 ing the trichloride with water. Since the sample used did not boil at constant 

 temperature, but distilled between 76° and 78°, it must have been impure. If 

 it contained oxychloride, as Clarke has suggested, the atomic weight of phos- 

 phorus would be found too high. Dumas overlooked the solubility of silver 

 chloride and therefore used the wrong end-point in these titrations. Further- 



» Ann. Chim. Phys., 2, 149 (1816). » C. R., 20, 1053 (1845). 



* Lehrhuch, 5th Ed., 3, 1188 (1845). * C. R., 33. 693 (1851). 

 ' A Recalculation of the Atomic Weights, Smith. Misc. Coll., 19 10. 



* Abegg, Handb. der anorg. Chem., 1907, vol. 3, part 3, p. 366. 

 ' Ann. Chim. Phys. (3), 38, 131 (1853). 



* Ann. Chem. Pharm., 113, 28 (i860). 167 



