Sec. 17.5] PREPARATION OF THIN FILMS BY ELECTROLYSIS 441 



air in the laboratory. The greasy coating due to handling should be removed 

 before the oxide and salt coating, unless removal of both can be accomplished 

 at the same time. The noble metals, gold and platinum, are readily cleaned 

 by immersion in warm concentrated sulfuric acid-dichromate cleaning solu- 

 tion, and, if desired, this treatment may be followed by boiling in either con- 

 centrated nitric or hydrochloric acid. 



Copper and iron may be degreased with organic solvents, or by immersing 

 for a very short period in the dichromate cleaning solution, followed by 

 washing with hot water, or they may be cleaned with fine sandpaper. After 

 any of these procedures, immersing in cold dilute hydrochloric or nitric acid 

 will complete the cleaning. Nitric acid will attack and etch the copper 

 surface faster than hydrochloric, while the reverse is true with iron, and 

 advantage may be taken of these rate differences to effect cleaning with or 

 without appreciable etching. As pointed out above, it may sometimes be 

 advantageous to etch the cathode during cleaning, using aqua regia or sand- 

 blasting if necessary. Rinsing the disks in acetone before drying is particu- 

 larly recommended for cathodes that are easily oxidized. 



17.5. Electrodeposition. The electrodeposition process involves not one 

 but two different chemical reactions; namely, oxidation or reduction and, 

 concurrently, precipitation of an insoluble salt on the electrode. Electro- 

 deposition, unlike electroplating, can occur on either the anode or the 

 cathode. Anodic depositions, although reported in the literature, have not 

 as yet been successfully applied to the problems of radiometry. Anodic 

 films of lead sulfate and oxide, and of manganese oxides, for example, have 

 been produced, but procedures for quantitative recovery by this means 

 have not been worked out. 



The underlying principle upon which electrodeposition is based can be 

 explained as follows: A cation A, which can exist in solution in either of two or 

 more valence states, is oxidized at the anode and reduced at the cathode. 

 An anion B, also in solution, but neither electrolytically oxidizable nor reduci- 

 ble, forms an insoluble compound either with the reduced cation but not the 

 oxidized, or with the oxidized but not the reduced. If the reduced cation 

 deposits with B, then cathodic deposition is obtained, and by the converse 

 process, anodic deposition may be obtained. These may be represented as 

 follows: 



Anode 

 Cathode 



which are the cation electrode reactions, where x has a value between 1 and 3 

 and the quantity (x + y) has a value between 2 and 5. If the anion B has a 

 valence of — z, it will form two salts with A , namely, A Z B X and A z B {x+y ). For 

 electrode film formation, one of these must be insoluble in the electrolysis 



