92 PHYSIOLOGY OF BACTERIA 



electrode. It is nothing but an expression of the distance 

 of the system from the hydrogen electrode. Therefore, 

 no rH can be computed for methylene blue which does 

 not go parallel to the hydrogen electrode, nor for ferrous- 

 ferric systems. Clark, Cohen and associates (1928, 

 supplement) warn against the general use of the rH 

 because it may not always be useful, and may be even 

 wrong. 



(c) REDUCTION POTENTIAL INDICATORS 



The electrometric measurement of reduction poten- 

 tials requires a number of carefully adjusted instruments, 

 and there are occasions, such as living protoplasm, where 

 direct measurements could not be made at all. 



It is a very old experience that bacteria have reducing 

 properties; indigo, litmus and methylene blue are the 

 best-known test substances for microbial reduction. 

 But little attention had been paid to differences in the 

 reduction of these compounds. The conception of 

 reduction intensity as contrasted against reduction 

 capacity was lacking then. 



Clark, Cohen and various associates studied the 

 reduction potentials of a number of dyes. They 

 measured the potential of the dye in the completely 

 reduced stage, and in the various stages of oxidation. 

 It was found that the oxidation took place within a 

 comparatively small range of potential. Decolorization 

 occurred when about 70-95% of the dye is reduced. 

 With different dyes, the relative position of the potential 

 differed widely. The reduction potentials of a few of 

 the tested indicators are shown in Fig. 7. It is seen that 

 50% of the methylene blue is reduced at an electrode 

 potential of —0.002 volts. A bacterial culture which 

 produces exactly this potential, will show a blue color 



