VARIATION OF EXZYME ACTIVITY WITH pH 



663 



It is now necessary to examine how the pH functions determine the 

 pZ/-pH and F„/-pH curves. The variation of the reciprocals of the com- 

 mon functions with pH is shown in Fig. 14-2. The reciprocals are plotted 

 because the concentrations of the individual ionic species are given by ex- 

 pressions of the type (B) = (B^)//^, so that the curves represent the frac- 

 tions of the various species that are present. However, it is more impor- 

 tant to investigate the behavior of the logarithms of the pH functions since 



Fig. 14-2. Plots of the reciprocals of the pH functions. The solid curves are 



for a dibasic acid with TpK^' = 6 and p-K'a" = 7. The dashed curves are for a 



monobasic acid with piCg — 7. 



it is these that appear in Eqs. 14-42 and 14-44. Following Dixon (Dixon and 

 Webb, 1958, p. 127), we shall use the expression p/ = — log / and thus 

 Eq. 14-42 may be re\vritten as: 



P^,' = pZ, + p/e + Vfs - Vfe 



(14-45) 



Inasmuch as piii, is assumed to be constant, the changes in p-K"/ with pH 

 will be determined by the dependency of the p/ terms on pH. Figures 14-3 

 and 14-4 show this dependency for the systems plotted in Fig. 14-2. In the 

 case of a dibasic ionization, the form of the curves will depend on the 



