ENERGY SUPPLY OF THE CELL 83 



Instead of using hydrogen and oxygen, less extreme 

 reagents might be used, and the potential difference 

 would be smaller. 



An oxidation-reduction potential is established when an electrode 

 is placed in a liquid which does not react with it chemically. This 

 potential indicates the intensity of reduction. It does not tell 

 anything about the amount of substance that can be oxidized or 

 reduced. This potential is comparable, as a simile, to the hydrogen 

 ion concentration which tells us the intensity of the acid, but not 

 the amount of acid present. 



Q I I I I I I I I * t I I I I I ' - 



Fig. 3. — Differences in potential between a mercury electrode in bacteria 

 cultures, and a calomel half cell: A, B, and C: a mixed culture with different 

 amounts of oxygen; D: B. subtilis in dextrose-peptone solution. 



A single potential cannot be measured; we have to 

 combine two potentials and thus obtain an electric 

 current which gives us the difference between the two 

 potentials. Potter in the above experiments measured 

 the differences between the oxidation-reduction potential 

 of the sterile medium and that of the culture. Gillespie 

 coupled the electrode in the culture with a calomel half 

 cell, and thus measured all his potentials against a stand- 

 ard electrode of definitely known potential. The dif- 

 ferences are recorded in volts.. Figure 3 represents a 

 putrifying solution of peptone in a deep layer, containing 

 a mixed culture of bacteria carefully protected against 

 agitation. Curve A gives the potential difference of an 

 old culture when the access of air was carefully avoided. 



