PHYSICS, CHEMISTRY AND ENGINEERING 149 



DETERMINATION OF ATOMIC WEIGHTS OF THE 

 RARE EARTH ELEMENTS 



B. S. Hopkins, University of Lllinois 



Theoretically the determination of the atomic weight of an 

 element is exceedingly simple. It consists in preparing a pure 

 compound of the element and then transforming it completely 

 into some other compound of the element. The change in 

 weight which is produced by the change in composition gives 

 sufficient data for the calculation of the atomic weight. In 

 practice, however, the determination of the atomic weight of 

 any number of the rare earth group becomes very difficult. 

 This difficulty is apparent when we consider that the rare earth 

 elements are found only in complicated mixtures and that the 

 properties of these elements are so similar that the preparation 

 of a pure compound is attended by well-nigh insurmountable 

 difficulties. The methods used for the determination of the 

 atomic weights of members of this group may be illustrated by 

 the steps necessary for the preparation of pure yttrium ma- 

 terial and the calculation of the atomic weight by the use of 

 the ratio Y 2 O s to 2YC1 3 . 



In purifying yttrium material, the mineral, such as gadolin- 

 ite, was ground finely and extracted with HC1 or H 2 S0 4 . Sil- 

 ica was removed by dehydration. Then the rare earths pres- 

 ent in solution were precipitated by hot oxalic acid. The ma- 

 terial was then converted to the anhydrous sulfates which were 

 dissolved in water and solid Na 2 S0 4 added to precipitate the 

 members of the cerium group. The members of the yttrium 

 group, which are not precipitated by Na 2 S0 4 , may be separated 

 from each other in two ways ( 1 ) by the difference in the solu- 

 bility of some salts such as the bromates; (2) by slight differ- 

 ences in basicity which permits their gradual separation 

 through the method of fractional precipitation. Utilizing the 

 difference in solubilities the method of fractional crystallization 

 was used by placing a quantity of the mixed bromates in a 

 flask and adding sufficient water to take the whole into solu- 

 tion. Then part of the water was evaporated on the steam 

 bath and the material left to crystallize. Obviously the least 

 soluble crystallized first, leaving the more soluble portions in 

 solution. This liquid was thoroughly drained away from 



