ENERGY SUPPLY OF THE CELL 



89 



linear relation of these potentials to pH. The hydrogen electrode 

 indicates at what potential hydrogen can be formed. At pH 0, 

 the potential is sufficient; at pH 4.5, which is about the acidity in 

 cultures of Bad. coli, the required potential for hydrogen liberation 



Fig. 6. — Reduction potential of methylene blue at different acidities. 



is about —0.4 volts and in alkaline solutions, it becomes so strongly 

 negative that hydrogen formation by alkali-forming bacteria appears 

 very difficult. 



The reduction potential as ^uch, even if expressed in 

 volts on the Eh scale, tells us very little about the 

 reducing power if we do not also know the pH of the 

 culture. This was the aim of Gillespie in determining 

 the hydrogen electrode potentials (Table 11). The 



