THE MEASUREMENT OF PH AND TITRATABLE ACIDITY 



97 



The relation of percentage reduction to potential as defined by the last 

 term in Eq. (10) is given in Table 12. For example, if methylene blue is 

 observed to be 80 per cent reduced at pH 7, 



Eh = 0.011-0.018 = -0.007 volt. 



Color standards. Since the compounds listed in Tables 10 and 1 1 are 

 practically one-color oxidation-reduction indicators, color standards of 

 sufficient approximation can be prepared simply by graded dilutions 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. 



Colorimetric 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 decom- 

 posed; 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 exclu- 

 sion. This applies to the electrometric method as well as to the 

 colorimetric. 



Table 12 



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

 moment, only minute amounts of electromotively active oxidation-reduc- 

 tion 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 indicator. This drawback cannot 

 be overcome except by allowing sufficient time for the biological system 

 to overcome the poising^ effect of the added indicator. However, the 



1 Poising action of an oxidation-reduction system is analogous to buffer action of an 

 ^cid-base system. (Compare paragraph on buffer action, p. 89.) 



