88 PHYSIOLOGY OF BACTERIA 



(6) REDUCTION POTENTIAL AND ACIDITY 



The interpretation of reduction potentials is made 

 quite difficult and complicated by the fact that it varies 

 with the hydrogen ion concentration. 



This variation is not always regular. It depends 

 upon the state of ionization in the system, and different 

 systems are not affected in the same way by the same 

 change in pH. The system FeCU — FeClg is practically 

 uninfluenced by acidity. Ordinarily, however, the poten- 

 tial drops with increasing pH, i.e., with increasing 

 alkalinity. This is especially true with the hydrogen 

 and the oxygen electrode. 



The system cysteine-cystine, which will interest us 

 later, has been investigated by Dixon and Quastel (1923), 

 and they found a simple rectilinear relation between 

 potential and pH up to pH 9.5, as may be seen from 

 Fig. 5. The same is true with glutathione though the 

 range investigated by the authors is smaller. The effect 

 of pH upon the potential of the hydrogen electrode is also 

 indicated in this figure. 



The relation between potential and pH is not always 

 linear, as may be seen from Fig. 6, which represents the 

 system methylene blue — methylene white taken from 

 Clark, Cohen and associates (1928, VIII). A definite 

 potential is required to change the blue dye to the 

 colorless leuco-compound, but this potential varies 

 greatly with the acidity of the solution. At pH 7, a 

 potential equal to our arbitrary zero point is sufficient 

 to turn the color, at pH 10 it requires a potential of 

 — 0.100 volts to do the same, while at pH 4, a potential 

 of +0.150 volts would be sufficient. 



This figure also shows short stretches of the potentials of the 

 hydrogen electrode and the oxygen electrode and demonstrates the 



