ENERGY SUPPLY OF THE CELL 91 



This formula makes it possible to express the reduction potential 

 in terms of hydrogen pressure. 



As an example, the cysteine potential (Fig. 5) is 0.36 volts above 

 the hydrogen electrode (measured at pH 5). Assuming that the two 

 curves are parallel up to pH 0, we have 



P = 10~^2 atm. pressure 



This figure seems quite meaningless. But the reduction potential 

 rH is the negative logarithm of this number, in strict analogy to the 

 definition of pH. Thus, the rH of the cysteine-cystine system is 



12 or lup- The general formula for the computation of rH from 



measurements at any pH is derived from the above 



& = -.^^ log i^ = -o:^(log VP - log [Hi) 



RT 

 0.434F ^^^ [H+] 0.434F 



0.434i^ 

 0.434 Eh ' F 



(log Vp + ph) 



RT 



-f pH = - log Vp 



for sec, rH = 2(5^ + ph) 

 for pH 5, the cysteine potential is B* = +0.06 volt 



rH = 2(S-;SI + = - 



The higher the rH, the lower is P, and the farther is the system 

 removed from the reduction intensity of the hydrogen electrode. 



The computation of rH is permissible only if the 

 potential — pH curve is parallel to that of the hydrogen 



