CHAPTER 17 



THE PREPARATION OF THIN FILMS OF 

 RADIOACTIVE ELEMENTS BY ELECTROLYSIS 



Rayburn W. Dunn 



17.1. General Considerations. Although many methods have been pro- 

 posed for the preparation of thin films that may be used for radioactivity 

 measurements, probably the most convenient and satisfactory is the elec- 

 trolytic. Under properly controlled conditions it is possible either to elec- 

 troplate or to electrodeposit many of the elements quantitatively from solu- 

 tions of their salts, and the resulting film can be made quite uniform in 

 thickness. If the apparatus is correctly designed, the area that this film 

 covers will be both accurately fixed and reproducible. 



With certain minor modifications, the electrolytic methods generally 

 employed for quantitative chemical analysis are readily applied to radio- 

 assay. For the most part, these methods are for electroplating rather than 

 electrodepositing. The term "electroplating" is usually taken to mean the 

 formation, by electrolysis, of a closely adhering metallic film. "Electro- 

 deposition," on the other hand, may have this same meaning, or it may refer 

 to the electrolytic formation of a nonmetallic layer, closely adherent or not, 

 on the electrode. It is in the latter sense that it is employed here. 



Among the elements that have been successfully electroplated from aqueous 

 solution are copper, silver, gold, zinc, cadmium, mercury, tin, lead, anti- 

 mony, bismuth, chromium, iron, cobalt, nickel, and platinum. Aluminum 

 and several other metals that are not reducible in aqueous solution may be 

 plated from organic solvents. In the group of electrodeposited elements 

 are, among others, uranium, oxygen, and fluorine as U0 2 and UF 4 , and 

 thorium, iron, carbon, and nitrogen, as ThFe(CN) 6 . Quantitative proce- 

 dures for electrolyzing most of these elements are well known, although only 

 a few have as yet found use in radiochemistry. 



In general the methods for commercial electroplating cannot be suc- 

 cessfully applied in analytical work, although the chemical principles and 

 reactions of both procedures may be identical. This is essentially true 

 because the conditions of temperature, ionic species and concentration, 

 current and voltage required for quality plating do not necessarily satisfy 

 the requirements for quantitative recovery. It is advantageous, however, 

 to obtain a smooth, shiny film whenever possible. Rough films are likely to 

 be porous and may occlude extraneous salts from the electrolysis solution. 



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