Sec. 17.3] PREPARATION OF THIN FILMS BY ELECTROLYSIS 439 



Colloids are frequently used to improve the electroplate, but no specific 

 rules can be given to govern their use. 



17.2. Apparatus. Many types of electrolysis cells and stirrers have been 

 designed and used by the various groups that are investigating the field of 

 radioassay. Unfortunately, many of these developments, like those of the 

 chemical procedures, have not found their way into the published literature. 



The simplest electrolysis apparatus will have a rotating anode of platinum 

 or platinum-10 per cent iridium wire. The wire is usually coiled in a flat 

 spiral and serves both as a conductor and a stirrer. A platinum dish con- 

 tains the electrolysis solution and serves as the cathode. Alternatively, 

 cathodes consisting of thin disks are probably the most convenient for both 

 gravimetric and radiometric measurements. Cathodes of 1 or l}^ in. in 

 diameter are most frequently employed. These vary in thickness from 

 0.0005 to 0.005 in. depending upon the cost of material, the ease of handling, 

 and the effect of background scattering on the counting rate. 



A glass cylinder and gasket, each having the same inside diameter and an 

 outside diameter of either 1 or 1}^ in., generally constitute the body of the 

 cell. The base is constructed so that either it serves for the cathode con- 

 nection itself, or a separate connection is incorporated in it. Finally some 

 means must be provided to hold the glass tightly against the base with the 

 gasket and cathode in between. 



17.3. Anodes. Platinum wire or gauze anodes have been almost univer- 

 sally used for laboratory electrochemical reactions, principally because they 

 are very passive. Platinum has some limitations, however, of which cost 

 is not the least. Electrolysis of acid chloride solutions produces free chlorine 

 at the anode, and in the presence of this gas, platinum loses its passivity to 

 some extent and small but appreciable amounts are dissolved. This reaction 

 increases with hydrochloric acid concentration, temperature, and current 

 density but is minimized if the platinum is alloyed with 10 per cent iridium. 



Similar attack occurs if electrolysis takes place in an alkaline solution 

 containing free ammonia. This attack will not be very great if the current 

 density and ammonium hydroxide concentration can be kept low; however, 

 it may account for cathode deposits of 0.1 to 0.2 mg if lengthy electrolyses 

 are carried out. 



Substitutes for platinum have not proved too satisfactory for quanti- 

 tative analytical work since most are attacked in either acid or alkaline 

 solution. If the cathode film need not be weighed, alloys of the 18-8 variety 

 of stainless steel sometimes may be used; carbon or graphite may be employed 

 if anodic oxygen is not formed and if the rods are not appreciably porous. 

 Tantalum cannot be used because it forms an oxide coating that prevents 

 the flow of current from solution to anode, although the reverse process 

 occurs with ease. 



