BAXTER AND STEWART. — PRASEODYMIUM CHLORIDE. 173 



the oxidizing power of the oxide was determined. Other weighed 

 portions of oxalate were oxidized with permanganate. The ratio Pr 2 3 : 

 3C2O3 gave the average result 140.98. Finally, weighed amounts 

 of oxalate, the praseodymium content of which had been found as 

 above, were ignited to black oxide and this in turn to trioxide in hydro- 

 gen. The trioxide was changed to sulphate by solution in nitric acid 

 and evaporation with sulphuric acid. Excess of acid retained by the 

 salt was found by titration. In eight experiments the ratio Pr 2 3 : 

 Pr^SO^ yielded an average value 140.96. The mean of the four 

 methods is 140.97, which is essentially identical with the result of 

 Brauner's earlier work. 



Auer von Welsbach 7 next published the results of three determina- 

 tions by the Bunsen method, without details, 140.64, 140.50, 140.56, 

 average 140.57. 



Finally Feit and Przibylla 8 purified praseodymium material from 

 neodymium by crystallization of the double magnesium nitrate, and 

 from lanthanum by crystallization of the nitrate from nitric acid 

 solution. The higher oxide, prepared by ignition of the oxalate, was 

 dissolved in standard sulphuric acid and the oxygen evolved was 

 measured, as well as the excess of sulphuric acid. The oxygen evolved 

 was subtracted from the weight of the black oxide before computing 

 the atomic weight from the relation of trioxide to sulphuric acid used. 

 The average result of three experiments is 140.54. 



Thus it can be seen that while the investigations of Jones, Scheele, 

 von Welsbach, and Feit and Przibylla indicate a value for the atomic 

 weight of praseodymium between 140.5 and 140.6, that of Brauner, 

 which was carried out with equal or greater care, and with material of 

 undoubted purity, points to a value at least as high as 140.9. The 

 International Committee upon Atomic Weights has chosen the lower 

 figure, and recommends the value 140.6. 



The various difficulties likely to be met in carrying out the methods 

 used in the earlier determinations have been many times discussed, 

 and a resume of the situation is given in the paper by Baxter and 

 Chapin on the atomic weight of neodymium. 9 It is worth pointing 

 out that in addition to the dangers there mentioned, methods involving 

 the use of praseodymium oxide are subject to error from the tendency 

 of this substance to form a higher oxide of somewhat uncertain com- 



7 Sitsungsb. Akad. Wiss. Wien, 112, 1037 (1903). 



8 Zeit. anorg. Chem., 50, 258 (1906). 



9 Loc. cit. 



