90 PHYSIOLOGY OF BACTERIA 



hydrogen ion potential is probably the lowest potential 

 a bacterial culture can reach, and the last column of 

 Gillespie's table indicates how far removed the reduction 

 intensity of the culture is from that of hydrogen. If the 

 potential is measured in this way, it is independent of 

 the pH, or rather, it is already corrected for pH. 



The same principle was employed by Clark, Cohen and associates 

 (1928, II) to elaborate the reduction potential, rH. This might be 

 best illustrated graphically. In Fig. 5, the change of potential of 

 the hydrogen electrode with pH is given. The hydrogen electrode 

 corresponds to a hydrogen pressure of 1 atmosphere. If the pressure 

 is less, the reducing tendency will be less, the oxidizing intensity will 

 be stronger, and so we obtain, for lower hydrogen pressures, parallel 

 lines above that for the hydrogen electrode. This pressure can be 

 calculated from the formula for the potential of the hydrogen 

 electrode : 



F = -^] Vp ^^ 1 Vp 



' F [H+] 0.434P ^ [H^] 



where P is the hydrogen pressure, R the gas constant, F the Faraday 

 constant, T the absolute temperature. For a constant temperature , 



Tirp 



the expression ^ 404/p is constant, and for 30°C., it becomes 0.06. 



En = -0.06 log j^ 



= -0.06(log VP - log [H+]) 

 — log [H+] = pH by definition 



Eh = -0.06(log VP + pH) 

 For pH = 0, we obtain 



Eh = -0.06 log VP 



0;^ = - log Vp 



