228 ILLINOIS STATE ACADEMY OF SCIENCE 



cedure is as follows: The mixed sulfates are reduced to 

 sulfides by heating with charcoal or are converted to the 

 carbonates by boiling with sodium carbonate solution. 

 The sulfides or carbonates so obtained are dissolved in 

 hydrochloric acid and the resulting solutions subjected 

 to fractional crystallization. This method of concentra- 

 tion depends upon the fact that when a saturated solu- 

 tion of radium-barium chloride is cooled from 100° to 

 0° the crystals formed are much richer in radium than 

 the original solution. Accordingly, if a solution of the 

 mixed chlorides is evaporated until there remains not 

 quite enough solvent to keep all the salts in solution,, 

 there will be a tendency for the radium chloride to crys- 

 tallize out, while the mother liquor will become corres 

 pondingly richer in barimn chloride. After this process 

 has been repeated many times, it is found that the radium 

 is nearly all concentrated in the crystal fractions, while 

 the solutions at the "soluble end" of the series contain 

 no radium. It has been found that the concentration of 

 radium takes place more rapidly if this process of frac- 

 tional crystallization is carried out by the use of bro- 

 mides in place of chlorides. This is explained by the fact 

 that the bromides are more soluble than the chlorides. If 

 a saturated solution of the chlorides is cooled from 100° 

 to 0°, about 50 per cent of the solute crystallizes out; but 

 under the same conditions the bromide solution will give 

 up only about 34 per cent of the salts. Hence, there is a 

 more rapid concentration of the radium if this salt is 

 used. 



The concentration of the radium in any fraction may 

 be calculated from the equation: 



C = AK" 

 in which" is the number of crystallizations, A is either 

 the actual or assumed concentration of some dish to 

 start with, and K, called the enrichment factor, is a con- 

 stant when the crystallizations are carried out under ex- 

 actly similar conditions. It represents th-e relative con- 

 centration of the radium in the crystals to that in the 

 original material. It has been shown^ that the enrich- 

 ment factor is practically independent of the degree of 



1. John L. Niermann. Jour. Phys. Chem. 2^, 192 (1920). 



