CHANGE IN AGGREGATION 



35 



7.2.3. Change in Aggregation. If reduction results in a change in the degree 

 of aggregation of the components, a further comphcation is introduced. To 

 illustrate the effect, a system will be assumed in which the monomeric form 

 of the oxidant requires one electron for reduction. The general form of such 

 a system is: 



{OxX^ + m^pRed,, (26) 



where pm = n. The electrode equation, derived from equation 14 then 

 becomes: 



[RedJ' 



Efi = Eq — — In - , , 



nF [(Ox„)"+] 



(27) 



Owing to the presence of the higher order term, equation 27 does not give 

 a titration curve symmetrical about Eo. Figure 2 shows the curve plotted 



c*j 



20 



40 



60 



80 



100 



PER CENT OXIDATION 



Fig. 2. Hypothetical oxidation-reduction curve showing effect of change of aggregation. 



for the case where n = p = 2 and to = 1. In this case, £o lies at the point 

 of 50% oxidation; other values of m and p result in its displacement from 

 this central position. The difference should be noted between this curve 

 and those of Figure 1 (Sect. 7.2.1.). It follows that symmetry of an experi- 

 mental titration curve is evidence that p = 1 and no change of aggregation 

 occurs, while, if a curve lacking symmetry is obtained, change of aggrega- 

 tion must be presumed. 



