196 OXIDATION-REDUCTION POTENTIALS 



Thus definite numerical values may be given to the reduction intensities of bacterial 

 cultures. 



OXIDATION-REDUCTION AND ANAEROBIOSIS 



Now consider Figure 6 in more detail. The zero of potential at pH = o is the 

 arbitrary zero of the normal hydrogen electrode. As pH increases the potential of the 

 hydrogen electrode becomes more negative and follows the straight line at the left. 

 Parallel to this, and about 1.23 volts more positive, is the theoretical line of the normal 

 oxygen electrode, an impracticable electrode. Now reverse the point of view. Con- 

 sider a reducing agent capable of decomposing water with the liberation of hydro- 

 gen at one atmosphere. It should give a hydrogen electrode potential. Consider 

 an oxidizing agent capable of decomposing water with the liberation of oxygen at 

 one atmosphere. It should give an oxygen electrode potential. But the hydrogen 

 electrode at any given hydrion concentration becomes more positive by about .03 

 volt for each power of 10 by which the hydrogen pressure (in atmospheres) is 

 lowered. Likewise, the ideal oxygen electrode should become .015 volt more 

 negative for each power of 10 by which the oxygen pressure (in atmospheres) is 

 lowered. Thus if the oxygen pressure is lowered to io~^^ and the hydrogen pressure 

 to lo"^'' atmosphere, an oxygen electrode and a hydrogen electrode should give a po- 

 tential on the line of Figure 6 marked H2 io~^^, O2 lo"''^ Conversely, a half-reduced 

 indophenol system (lying near this line) should indicate a hydrogen or oxygen partial 

 pressure of the values indicated, if a state of equilibrium is attained. The foregoing 

 condition is extremely important. As a matter of fact, many of the indophenols are 

 kept reduced by living cells even though oxygen is bubbled through the suspension. 

 This fact alone is sufficient to suggest not only the relative inertness of atmospheric 

 oxygen but also the caution to be observed in applying equilibrium data to kinetic 

 affairs. It reveals at once the difficulty in obtaining an end-point indicator for oxygen 

 which can be rationally formulated by methods comparable with those used in acid- 

 base titrations. 



It was not long after the advent of anaerobic culture of bacteria that Gunning 

 criticized the methods, claiming that they were not adequate to remove the last traces 

 of the oxygen he thought necessary to life. Finally, he appeared before the French 

 Academy to claim that by a better approach to strict removal of free oxygen he had 

 succeeded in stopping bacterial growth. Needless to say, the claim has received little 

 support; but what interests us now is Pasteur's reply to Gunning. He stated that he 

 was convinced of the fact of anaerobiosis because he had observed bacteria to thrive 

 in solutions which maintained indigo in the reduced state. 



If it can be shown that in any given case conditions are favorable to the attain- 

 ment of true equilibrium, calculation shows the following: In a reduced solution of 

 indigo disulphonate at pH = 7.o, the potential should be less than —.125 — .060 = 

 — .185 (Tables I and II). From the formula 



— .185 = 1 .23— .06 pH+ .015 log Po.. , 



Po2=io~''''-5 atmosphere. Since a gram mol of oxygen contains about 10'' discrete 

 molecules and occupies about 24 liters at one atmosphere, the foregoing condition 



