THE MEASUREMENT OF 0/R POTENTIALS 



ix,8-21 



tively-active contaminants attain dominance and an observed po- 

 tential becomes unstable and difficult to interpret. 



In contrast to the above mentioned reversible processes which are 

 readily amenable to Eh measurement, there are a great many oxida- 

 tion-reductions that proceed by a variety of mechanisms that do not 

 permit formulation and precise measurement in terms of equili- 

 brium states. Electrode potentials in such cases are difficult to 

 interpret and of uncertain significance. 



In cell suspensions and bacterial cultures, especially when de- 

 prived of free access of oxygen, there develops with time a progressive- 

 ly more negative potential which traverses the zones characteristic of 

 reversible oxidation-reduction indicators (see next section). Polari- 

 zation of the electrode or a small dose of an oxidant may reverse the 

 trend of reduction potential temporarily, but the trend is resumed 

 after a while to levels of potential that may sometimes be associated 

 with the type of cell and the various metabolites in the suspension or 

 culture. Duplicate electrodes in such systems may not be in good 

 agreement at the start, but they will reach about the same limiting 

 value in time. For examples, see Clark, Cohen, et al. (1928), Allyn 

 and Baldwin (1932), and Hewitt (1936). 



THE COLORIMETRIC METHOD 



The empirical use of substances such as litmus or methylene blue 

 as indicators of reduction in bacterial cultures is well known. For 

 the determination of various degrees of reduction intensity an ap- 

 propriate series of indicators is necessary. Among those available 



TABLE 6 



A SELECTION OF OXIDATION-REDUCTION INDICATORS 



E'o at pH 7, (30°) 

 (Values of E'o between pH 5 and 9 will be found in Table 7) 



Compound 



Phenol-7ri-sulfonate-indo-2,6-dibromophenol . . . 



TO-Chlorophenol-indo-2,6-dichIorophenol 



o-Chlorophenol-indophenol 



2,6-Dichlorophenol-indophenol 



2,6-Dichlorophenol-indo-o-cresol 



l-Naphthol-2-sulfonate-indo-2,6-dichlorophenol 



Lauth's violet (Thionin) , 



Cresyl blue 



Methylene blue 



Indigo tetra sulfonate 



Methyl Capri blue 



Indigo trisulfonate 



Indigo disulfonate 



Gallophenine 



Brilliant alizarine blue 



Phenosafranine 



Tetramethyl-phenosafranine 



Saf ranin T 



Induline scarlet 



Neutral redj 



Rosindone sulfonate No. 6 



(Hydrogen at 1 atmosphere) 



*At 25°. fSe^ footnote 3 in text. 



E' 



g- 

 h. 

 i. 



i- 



k. 



1. 



m. 



n. 



o. 



P- 



q- 



r. 



s. 

 t. 

 u. 



0.273 V. 



0.254 



0.233 



0.217 



0.181 



0.119 



0.062 



0.047 

 +0.011 

 -0.046 

 -0.061 

 -0.081 

 -0.125 

 -0.142* 

 -0.173* 

 -0.252 

 -0.273 

 -0.285 

 -0.299 

 -0.324 

 -0.385 

 (-0.421) 



