440 1SOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 17 



17.4. Cathodes. The selection of a satisfactory metal for electroplating or 

 electrodeposition depends largely upon whether gravimetric as well as radio- 

 metric assay determinations are to be made. If the cathode film is to be 

 weighed, it is best to employ platinum disks; if not, copper, iron, etc., may be 

 used. Furthermore if the film is to be ignited or calcined, platinum is to be 

 preferred; it must be used if both heating and weighing are necessary or if 

 the film is later to be recovered quantitatively in a pure state. In most 

 cases gold may be substituted for platinum. 



It is generally assumed that cathodes of gold or platinum will be cleaned 

 and reused. Under some conditions it may be impossible to remove com- 

 pletely the radioactive film, in which case the disks must be discarded, or 

 the activity of each recleaned disk must be recorded before the next use. 

 When these alternatives are either impractical or uneconomical, gold-plated 

 copper disks may be substituted. A commercial plate 0.0001 in. is usually 

 sufficiently thick and is relatively inexpensive. After use, the plated gold 

 may be reclaimed, although it is safer to discard the disks rather than to 

 reuse the recovered gold since it may contain contaminating radioactivities. 



Although most of the elements that can be electroplated will plate out 

 on and adhere to copper, platinum, and gold, there is no hard and fast rule 

 that can be used to determine exactly what results may be obtained with 

 each element and each cathode. Both electroplating and electrodeposition 

 are greatly influenced by the character of the cathode surface, as well as by 

 the composition of the electrolysis solution. Cathodes of the same material 

 will vary from lot to lot and even from front to back. For these reasons, 

 duplicate determinations will frequently produce films greatly different in 

 appearance and possibly in composition or crystal structure. Presumably 

 identical electrolysis solutions and cathodes will frequently show wide varia- 

 tion in gas production, even though the currents and anode potentials are 

 the same. 



The formation of satisfactory films by electrodeposition is more critically 

 influenced by the character of the cathode surface and solution composition 

 than is such formation by electroplating. The greatest problem in electro- 

 deposition is to obtain a film that will adhere to the electrode, and it is for 

 this reason that etching of the surface may be advantageous. Inasmuch as 

 no definite instructions can be given in every case regarding the selection of a 

 satisfactory cathode, it is often necessary to try each metal in turn, starting 

 with platinum or copper. 



Since the cathode surface is so important to satisfactory film formation, 

 thorough cleaning of disks cannot be overemphasized. Surface impurities 

 result from rolling, handling, dust, oxidation, and attack by moist air con- 

 taining small amounts of such gases as sulfur dioxide, hydrogen sulfide, 

 hydrochloric acid, and nitric acid which are not usually excluded from the 



