COMPOUNDS OF OXIDANT AND REDUCTANT 39 



i.e., 



(.-!)[«.,]=(,-. -^). 



I.e., 



\ — a 



[RB,] = ^ X S 



1-^ 



X 



^ a:(l - g) - g ^ ^ 



{x - y) 

 therefore: 



B = Ss- ^^^ + '^ [xil - «) - a]S 



where : 



(x - y) 

 = Sb - ZS 



Z = i^yj±jl X [a:(l -a) -a] (46) 



(x - y) 



Hence the required equation may be written: 



£a = £^ + 0.0601 log— + 0.0601 log^ 



Sr Kr 



+ 0.0601 log ^^ + ^^^ ~ ^ ^^- (47) 



If it is now assumed that the same number of molecules of base, namely 

 two, combines with both oxidized and reduced metalloporphyrin, and So is 

 made equal to Sr {i.e., 50% reduction), q = r = 2 and a = 0.5. Under 

 these conditions: 



Z = (^Z+lH^Jlil (48) 



(x - y) 



E, - K = 0.06 log^ + 0.06 log K«_ + ('Sb - Z S)^ 



In the systems under consideration, the main interest centers not on the 

 oxidation-reduction potential relationships as such, but on wliat information 

 they can provide on the constitution of the substances taking part in the 

 system, and their relationships with one another. Equation ^J) should lead 

 to values for the dissociation constants Kq and K/e, but the complexity of 

 the last term, involving Z, makes the use of the equation difficult. Clark 

 has shown, however, that graphical approximations may yield useful results. 



and: 



