1X48-22 MANUAL OF METHODS FOR PURE CULTURE STUDY 



TABLE 



SELECTED OXIDATION- 



Relation at 

 (Letters refer to compounds listed in 



are reversible oxidation-reduction systems, the oxidants of which are 

 usually colored and the reductants practically colorless. A number 

 of such indicator systems have been characterized and may be 

 employed, with due precautions, in determining an oxidation-reduc- 

 tion potential colorimetrically. 



A selection of such indicators^ is listed in Tables 6 and 7. Similar 

 tabulations are given by Hewitt (1936). Fuller details can be found 

 in Clark, Cohen, et al. (1928) and Cohen (1933, 1935). Table 6 

 gives the names of the indicators, listed in the order of their E'o values 

 at pH 7.0; and Table 7 gives the corresponding E'o values at suc- 

 cessive levels between pH 5.0 and 9.0. The magnitude of the salt 

 and protein errors of these compounds has not been established. 



Each indicator system listed in Tables 6 and 7 involves a two- 

 electron transfer, and the relation of E'o to other factors at fixed pH is 

 given by equation 9. 



RT [Reductant] 



Eh = E'o -■ In ■ 



2F [Oxidant] 



(9) 



Converted to ordinary logarithms after insertion of numerical values, 

 this equation becomes, at 30°C, 



[Reductant] 



Eh = E'o- 0.030 log (10) 



[Oxidant] 



The relation of percentage reduction to potential as defined by the 

 last term in equation 10 is given in Table 8. For example, if methy- 

 lene blue is observed to be 80% reduced at pH 7, Eh = 0.01 1-0.018 = 

 -0.007 volt. 



^A special comment is necessary in regard to neutral red (compound t in Tables 6 

 and 7). It undergoes reversible reduction in tlie usual manner, and the colorless 

 solution of reductant formed upon rapid reduction reoxidizes very rapidly when ex- 

 posed to air. However, the reductant on standing in solution at pH 4 to 6 for a little 

 time undergoes transformation to a fluorescent substance which is stable for days in the 

 presence of air, but reoxidizes rapidly upon acidification. As an oxidation-reduction 

 indicator, therefore, neutral red must be employed with due caution and can be used 

 only for rough comparisons. 



