44 II. METHODS OF INVESTIGATION 



as abscissa, the unique point falls on the supplementary curve very close 

 to 0.0181 volt above the aforementioned intersection. We shall show that 

 it is not always possible to report a reliable value for Eq. When deprived of 

 this base line, one may use the obvious, alternative method shown in Figure 

 3; namely, to intersect the appropriate curve with a line parallel to the 

 orienting diagonal and 0.0181 volt above. For the curves shown and on the 

 scale used in Figure 3 it would be difficult to discern any difference along 

 the upper part of the curves whether the abscissa were made log (Sb — S), 

 log Sb, or the log {Sb — 1.5 <S) which is demanded for the location of the 

 unique point where: 



0.5 log Ko = log [B] 



If necessary a drawing of larger scale may be used and the principle described 

 for the finding of Kr applied to the estimation of Kq. 



If at and near the center of an association curve Ko is much greater than 

 [B]'^ and Kh is much less than [BY, the value of r will be determined by the 

 maximal slope of the association curve, which is given by equation 50." 



7.2.5. Interaction between Oxidation-Reduction Systems. Consider two 

 systems A and B, represented at constant pH by the electrode equations: 



£,^ = £;^-«|'ln^ (51) 



^.„ = ^.^,-^'lng-» (5.) 



and let En . be more positive than £o/j. If solutions containing these separate 

 systems are mixed, they will in general interact in a manner to make^*^ 

 equal to Eh^. At equilibrium, by subtracting equations 51 and 5'i: 



riF [8o^] [Srb\ 



The direction of the Interaction thus depends not only on the values of 

 the characteristic potentials £'„ of the two systems, but also on the initial 

 ratios of oxidant to reductant. It is not to be expected that a system of 

 higher characteristic potential will always oxidize one of lower. Under 

 certain conditions of initial ratio of oxidant to reductant, the reverse will be 

 the case, the same equilibrium point being reached from whichever side it 

 is approached. This has sometimes been forgotten in biological work. 



Under the conditions prevailing within the cell, it should also be remem- 

 bered that there is frecjuently no true e(|uilibrium between various oxidation- 

 reduction sy.stems (r/. Chapter VIII). 



7.2.6. Eleclroactivity and Electroinaclivity. In the foregoing discussion it 

 has been tacitly assumed that when a bright platinum or gold electrode is 

 inserted into a solution containing an oxidation-reduction system, a potential 



