COMPOUNDS OF OXIDANT AND REDUCTANT 37 



From these equations, by substitution for [Rm] and [On] in equation 28 

 equation 33 is obtained: 



^ „, , RT So .RT m" RT Ko 



" nF {SrY nF n nF {KrY 



+ ^In (J^^+Wr (33) 

 nF Ko + [BY 



So = n[0„] + n[OM (34) 



where : 

 and: 



Sr = m[Rj + m[R„B,] (35) 



Equation 33 has been simphfied by assuming, as before, that no changes of 

 activity coefficients occur on oxidation or reduction; the apparent constants 

 represented by equations 31 and 32 have been treated as true constants. 



The shape of experimentally obtained titration curves gives informa- 

 tion about the values of a number of the constants in this equation. The 

 curves are obtained by fixing in each case all the varialiles except one. 



(i) As in Section 7.2.3. if all conditions are constant except the ratio of 

 ■So to (•Sfi)'', symmetry of the curve indicates that p = 1. 



(2) If p = 1, the slope of the titration curve under the same conditions 

 gives the value of /;. If ;> = 1, m = n. If p = m = Ji = I, and the tem- 

 perature is taken as 30°C., equation 33 reduces to: 



E^ ^ p + 0.0601 log — + 0.0601 log-^ 

 Sr Kr 



Kr^IBY (36) 



+ 0.0601 log 

 tion 3G becon 

 Eh = Eb + 0.06 log 



Ko + [B]" 

 If [B] is maintained constant, equation 3G becomes: 



So. (37) 



Sr 



where En indicates the potential at .50% reduction of the system at constant 

 concentration of free base. Two further conclusions derive from equation 36. 

 (5) If the potential increases with addition of coordinating substance, all 

 other conditions remaining constant. Ko is greater than Kr. If the potential 

 decreases under the same conditions, Kr is greater than Ko . 



(4-) If on addition of base, all other conditions being constant, the potential 

 approaches a limiting potential, it follows that q = r. At the limiting poten- 

 tial the last term of equation 30 is constant, so that the equation may be 

 written : 



Eft = E; + ki + 0.0601 log^""^ + k2 (38) 



I.e., 



E2 ^ Eo+ 0.0601 log— (39) 



Kr 



