442 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 17 



solution. Of necessity the cation must be introduced into the electrolysis 

 solution in such a valence state that it will not form this precipitate in the 

 solution itself. 



Neglecting valences and electrolysis reactions for the moment, one other 

 consideration is important, namely, the independent role that B plays in 

 forming and maintaining the precipitated film. The reaction that occurs 

 may be represented as 



A (ion) plus B (ion) +±AB (precipitate) 



The equilibrium constant for this reaction, Kab, equals the product of the 

 concentrations of A and B; the value of each of the latter is squared or cubed, 

 etc., depending upon whether one, two, or more ions are concerned in forming 

 the precipitated molecule. Therefore the greater the concentration of B in 

 solution, the smaller will be the concentration of A when equilibrium is 

 reached and, thus, the more quantitative the recovery. 



The electrodeposited film, however, is not only slightly soluble in water, 

 but is also usually dissolved by acid or base, or by complex ion formation. If 

 the rate of solution by any one reaction or combination of these reactions is 

 faster than the rate of valence change and precipitation, no electrode film will 

 form. A rather delicate balance of these reaction rates is usually necessary 

 if quantitative results are to be achieved and such side reactions must be 

 eliminated or minimized. 



17.6. The Electrolysis Current and Voltage. A convenient power supply 

 for electrochemical reactions is the ordinary 6-volt storage battery. Voltages 

 above 12 are not usually necessary, and a parallel or series-parallel arrange- 

 ment of two or four batteries will supply as much power as will, in general, be 

 needed for routine analytical work. Electrolyses from nonaqueous solution, 

 however, may require much higher voltages, in which case a vacuum-tube 

 d-c power supply will be much more convenient than either batteries or a 

 generator. 



In addition to the voltage, three other independent variables will influence 

 the electrolysis current, namely, the distance between anode and cathode, the 

 resistance of the electrolysis solution, and the electrode-solution boundary 

 resistances. The electrolysis-solution resistance, in turn, is dependent upon 

 the ion concentration and upon the temperature of the solution; increasing 

 either one decreases the resistance. 



The desired electrochemical reactions occur only at the boundaries joining 

 the electrodes and the electrolysis solution; therefore the potential drop 

 across each of these boundaries is of considerable importance. These 

 electrode voltage drops determine which reactions will occur, and the current 

 that flows determines the reaction rates. If either of the boundary resist- 



