THE MEASUREMENT OF 0/R POTENTIALS 



1X48-23 



BEDUCTION INDICATORS 



E'o to pH (30°) 



Table 6; the values listed are E'o in volts) 



*At 25° 



fSee footnote 3 in text. 



Color standards. Since the compounds listed in Tables 6 and 7 are 

 practically one-color oxidation-reduction indicators, color standards 

 of sufficient approximation can be prepared simply by graded dilu- 

 tions of the colored component, the oxidant. It should be borne in 

 mind that some of the compounds are also acid-base indicators, 

 therefore it may be necessary to set up the color standards in a buffer 

 at the same pH as the solution or culture under test. 



Color imetric measurement. The general principles of color com- 

 parison, as outlined for the indicator method of pH-determination, 

 are applicable here. In addition, special precautions are required to 

 make certain that the measurement is a valid one. An indicator 

 may fade in a test solution for reasons other than simple reduction. 

 The compound may precipitate or adsorb on suspended particles, or 

 it may be decomposed; in such cases judicious treatment with 

 a suitable oxidizing agent (e.g., ferricyanide, or air) will not 

 immediately restore the initial color of the oxidant. Moreover, 

 many reversible oxidation-reduction systems are so sensitive to 

 oxygen as to require extreme precaution for its exclusion. This ap- 

 plies to the electrometric method as well as to the colorimetric. 



TABLE 8 



It is a fact that many biological systems act as if they contain, 

 at any moment, only minute amounts of electromotively active 

 oxidation-reduction substances, therefore the addition to such a 

 system of even a small amount of indicator-oxidant may suffice to 

 oxidize the system at once without appreciable reduction of the indi- 



