Sec. 17.6] PREPARATION OF THIN FILMS BY ELECTROLYSIS 443 



ances or the resistance of the solution is too high, little or no current will pass 

 through the cell, and consequently the reaction rates will be negligible. 



The minimum electrode reaction potentials are determined by (1) the 

 back emf , which is the potential with which the particular reactions tend to 

 reverse themselves; (2) the "passivity" of the electrodes; and (3) the "polari- 

 zation" within the boundary. The last-named phenomenon results, pri- 

 marily, from localized concentration changes of the electrolysis solution; 

 passivity is due to electrode surface conditions for which discussions will be 

 found in standard texts. An electrode becomes polarized owing to the 

 progressive removal of one or more ions, by reduction or oxidation, within the 

 boundary; equilibrium is regained or maintained by rapid stirring of the 

 solution. The net result of the above three factors is to establish a minimum 

 potential below which little or no current will flow and no reactions will occur 

 and above which a rapid increase in current is obtained. 



The polarization of an electrode, which is required to produce a given 

 irreversible reaction under given conditions, is known as the overvoltage of 

 this reaction. Overvoltage values for the same reaction vary considerably, 

 depending upon the electrode material and surface condition. Thus the 

 hydrogen overvoltage for a platinized-platinum cathode is lower than for 

 smooth platinum, which, in turn, is lower than for mercury, etc. 



The importance of hydrogen production at the cathode may be illustrated 

 by the following example: If the electrode emf for the reaction or reduction 

 of A is 0.5 volt and the hydrogen overvoltage is 1.0 volt, reaction A will begin 

 when the potential slightly exceeds 0.5 volt and will continue with little or no 

 hydrogen evolution until the potential reaches 1.0 volt. At this point 

 hydrogen gas begins to form and, as the voltage increases, both hydrogen 

 formation and reduction of A occur together. The reduction of hydrogen 

 at the cathode is accompanied by the formation of hydroxyl ions. These 

 in turn may exercise considerable influence on the reduction of A or upon 

 the character of the reduced film. If this influence is deleterious, then 

 voltages below the hydrogen overvoltage should be employed. On the other 

 hand, increased alkalinity may be helpful; furthermore the presence of 

 hydrogen gas within the body of the solution may cause reductions that will 

 aid in the quantitative recovery of A. 



When the hydrogen overvoltage is much lower than the voltage necessary 

 for the reaction A and the applied voltage is between the two, then no reduc- 

 tion of A will occur and only hydrogen gas will be formed. To overcome 

 this, a different cathode material with a higher overvoltage may be used or 

 the voltage may be increased to the point where reaction A begins to occur, 

 whereupon the cathode becomes coated and the hydrogen overvoltage 

 increases to a value that is characteristic of a cathode made of metal .4 . Of 



