632 ADOLF F. V O I G T 



The assumption is always made that complete mixing occurs, but this 

 is only true if the tracer and carrier are in the same chemical form or in 

 forms that exchange rapidly. For example, iodine tracer in the form 

 of free iodine will exchange rapidly with iodine in solution as iodide 

 ion, but not iodate. It is frequently necessary to take the tracer 

 and carrier together through several chemical processes involving 

 valence changes before it can be assumed that they are actually mixed. 

 Once such mixing is obtained it may be desired to purify the tracer 

 or at least to make certain that it is pure {45) . In addition to the or- 

 dinary chemical operations that can he used for such purposes, in- 

 cluding those listed above, certain special methods have been adapted 

 for this purpose. One of these, which makes use of the adsorptive 

 ability of some precipitates, is the procedure of "scavenging." Pre- 

 sumably carrier has been added only for the element desired and an>' 

 radioactive impurity is still free of carrier or nearly so. A much 

 larger fraction of the active impurity than of the desired element is 

 then likely to be removed on an adsorbing precipitate. Care must 

 be taken in choosing the precipitate. For example, in purifying 

 strontiunj tracer from less basic elements such as yttrium or zirco- 

 nium one could precipitate a hydroxide such as ferric hydroxide at a 

 pH up to about 8 and remove large amounts of the yttrium or zir- 

 conium impurities by coprecipitation without affecting the stronitum. 

 If the reverse purification (the removal of strontium from yttrium) 

 were desired, however, one could not use this reaction since the 

 yttrium would be more likely to coprecipitate than the strontiiun 

 impurity. The beauty of radiochemistry is that the progress of such 

 purification steps can be followed by comparing the characteristics 

 of the activity separated on a scavenger precipitate with the main ac- 

 tivity. Such comparison is important since the precipitate may just 

 carry down some of the main activity and a single measurement of 

 its radioactivity would not suffice to distinguish between impurity 

 and product. Two general methods of comparison are by half-life 

 and by absorptive cliaracteristics. Since the important tracers 

 usually have long half-lives, comparison on this basis would require 

 long periods of time. Therefore, the absorptive characteristics, the 

 ratio of /3 to 7 activity, the shape of the absorption curve of the ^ 

 rays in aluminum, or that of the X or 7 radiation in aluminum or lead 

 are frequently used for this identification. If such curves are identi- 

 cal for the scavenged and main activity it can be assumed either that 

 the tracer is pure or that that particular reaction is not removing the 



