EFFECT OF pH 



studied. If at pH 7-1 the E^ is — 0-314 volt, then at pH 7-2 the potential will be 

 — 0-317 V. This difference of 3 millivolts is within the limits of experimental error 

 when working with biological material. 



It should be noted that, if the hydrogen ion concentration is kept constant, 

 the last term in equation (18) becomes a constant, so that we get back to the general 

 electrode equation (10). 



The value of E^^ (the electrode potential at 50 per cent, oxidation) varies with 

 pH. 



DERIVATION OF rH 



The effect of pH on the electrode potential (Ej,) depends upon a number of 

 factors as mentioned above. In order to avoid confusion, Clark and Cohen (1923) 

 introduced the term rH which includes E^ and pH. The term rH is convenient 

 in theoretical discussions, but it has been used unjustifiably to describe the condition 

 of biological systems such as the interior of cells. Since these are systems of unknown 

 constitution and incompletely studied properties, the relation between Ej, and pH 

 cannot be defined and hence no term containing both factors can be obtained in our 

 present state of knowledge, whilst the use of rH makes a tacit assumption that there 

 is a known simple relationship between the Ejj and pH of the system. For this and 

 other reasons, Clark rightly insists that the use of the term rH, originally introduced 

 by himself, should be discontinued. The results of a considerable nmnber of 

 investigators on oxidation-reduction conditions in biology have, however, been 

 quoted in terms of rH. In order to make the interpretations of such results com- 

 prehensible to readers the following description of rH is included in this monograph, 

 but the author does not wish to encourage the further use of the very misleading 

 term rH. . 



Just as pH is the logarithm of the reciprocal of the hydrogen ion concentration 

 of a system, i.e. : — 



(20) pH = log ^^ 



so rH is the logarithm of the reciprocal of the partial pressure of hydrogen gas (P) 

 in equilibrium with the system, that is : — 



1^ 



P 



The value of rH is derived from consideration of the simple reversible oxidation- 

 reduction system : — 



(22) R© + H® ^ Ox. + H 



to which the ordinary mass action equation may be applied, i.e. : — 



[R©] [H®] ^^ 



(21) rH = log:5- 



^-^) [Ox.] [H] - ^^ 



The general electrode equation (18) for the system is : — 



(24) E, = K + ^ In [^^ 



