OXIDATION-REDUCTION POTENTIALS 25 



It must be remembered that many organic oxidation- 

 reduction systems, particularly the indicator dye systems, 

 involve weak bases or acids, and accordingly their 

 behaviour and the potentials to which they give rise will 

 depend on the pK of the solutions in which they are 

 active. As a result, it is necessary to maintain the pK 

 value of the solutions constant by the use of buffers and 

 to record the ^^H value at the time of the measurement 

 in order that the values of Eh may be of significance. 



Another important fact which must be borne in mind 

 when considering oxidation-reduction potentials is that 

 Eh is purely a measure of intensity of effect and not of 

 capacity. It gives information as to whether a given 

 substance will oxidise or reduce another substance but 

 not as to how much of the second compound can be 

 oxidised or reduced. In this respect it is analogous to pH, 

 which describes the intensity of acidity or alkalinity but 

 gives no indication of how much alkali or acid may be 

 required to alter the pH by a given amount. Or, again. Eh 

 is analogous to temperature, which indicates the intensity 

 of heat or cold but gives no clue as to how much heat 

 must be added to or subtracted from a given body in 

 order to alter its temperature to some other value. We 

 know that a body with a high temperature v/ill lose heat 

 to one of lower temperature or vice versa, but from a 

 knowledge of the temperatures alone we cannot predict 

 what the final temperature of the pair will be. 



The application of oxidation-reduction potentials will 

 be considered in their appropriate places in connection 

 with bacterial respiration and metabolism. 



For further information : — 



W. M. Clark, " The Determination of Hydrogen Ions," Third Edition- 

 London, 1928. 



W. M. Clark, " Recent Studies on Reversible Oxidation - Reduction in 

 Organic Systems." OJmn. Rev., 2 (1925-26), 127. 



L. F. Hewitt, " Oxidation-Reduction Potentials in Bacteriology and 

 Biochemistry," Third Edition, L.C.C, London, 1935, 



