VANADIUM. 211 



VANADIUM. 



Roscoe's determination of the atomic weight of vanadium was the first 

 to have any scientific value. The results obtained by Berzelius * and by 

 Czudnowicz f were unquestionably too high, the error being probably 

 due to the presence of phosphoric acid in the vanadic acid employed. 

 This particular impurity, as Roscoe has shown, prevents the complete 

 reduction of V 2 O 5 to V 2 O 3 by means of hydrogen. All vanadium ores 

 contain small quantities of phosphorus, which can only be detected with 

 ammonium molybdate a reaction unknown in Berzelius' time. Fur- 

 thermore, the complete purification of vanadic acid from all traces of 

 phosphoric acid is a matter of great difficulty, and probably never was 

 accomplished until Roscoe undertook his researches. 



In his determination of the atomic weight, Roscoe J studied two com- 

 pounds of vanadium, namely, the pentoxide, V 2 O 5 , and the oxychloride, 

 VOC1 3 . The pentoxide, absolutely pure, was reduced to V 2 O 3 by heating 

 in hydrogen, with the following results : 



7-7397 g rm - V 2 O 5 gave 6.3827 grm. V 2 O 3 . 17-533 P er cent, of loss. 



6.5819 5-4296 " i7-57 



5-1895 4-2819 " 17.489 



5.0450 4.1614 " 17.515 



5. 4296 grm. V 2 O 3 , reoxidized, gave 6. 5814 grm. V 2 O 5 . 17.501 per cent, difference. 



Mean, 17.509, =b .005 



Hence V = 50.993, .0219. 



Upon the oxychloride, VOC1 3 , two series of experiments were made 

 one volumetric, the other gravimetric. In the volumetric series the com- 

 pound was titrated with solutions containing known weights of silver, 

 which had been purified according to the methods recommended by 

 Stas. Roscoe publishes his weighings, and gives percentages deduced 

 from them ; his figures, reduced to a common standard, make the quan- 

 tities of VOCL given in the third column proportional to 100 parts of 

 silver. He was assisted by two analysts : 



* Poggend. Annal., 22, 14. 1831. 



t Poggend. Annal., 120, 17. 1863. 



t Journ. Chem. Soc., 6, pp. 330 and 344. 1868. 



