32 II. METHODS OF INVESTIGATION 



E, = E.-'^\n^, (14) 



nF [Ox^^] 



Oxidation-reduction potentials are frequently depicted graphically by 

 plotting the value of Eh against the percentage of reduction. It will be 

 seen that equation 14 is the equation of a family of curves of identical shape, 

 disposed parallel to one another along the Eh axis according to the value of 

 Eo, the latter being a characteristic constant for the particular system. 

 Figure 1 shows a series of hypothetical curves corresponding to three dif- 

 ferent values of Eo- The curves are seen to be symmetrical about the values 

 of Eo, which lies on a point of inflexion. The slope at this point gives the 

 value of n. 



7,2.2. Effect of pH. The equations which have been derived are useful only 

 if the concentration of the oxidant in its ionic form is known. The fraction 

 of total oxidant present in this condition varies with pH, since at certain 

 values of the latter, combination of the ionic oxidant with hydroxyl ions 

 will occur, fractions of the oxidant assuming successively the forms 

 (OH) ac'"-i'+, (OH)oOj:("-2)+ . . . (OH)„0.r. Consequently with a given con- 

 centration of total oxidant the value of Eh varies with pH. The same con- 

 siderations hold for other systems in which the ionic relationships of oxidant 

 and reductant are different {cf. equation 1, Sect. 7.2.1. above). 



^Vhen, as is usual, only the concentration of total reductant and oxidant 

 is known, it is necessary to replace the concentration of ionic forms in the 

 electrode equation by equivalent expressions involving the total concentra- 

 tion of the components, and the various dissociation constants concerned. 

 The general relationship is complicated and it would serve no useful purpose 

 to investigate it further. The nature of the potential change is better appre- 

 ciated by examination of individual cases as they arise. 



To illustrate the effect and the general procedure adopted it will suffice 

 to examine further the system with which we have so far been dealing, since 

 this is the simplest type of system likely to be encountered in hematin 

 chemistry. It is not to be assumed that any actual system will conform in 

 practice to this simple relationship; in fact, due to the intervention of other 

 factors, sometimes incompletely understood, most cases investigated have 

 been found to be rather more complex. 



If the presence of carboxylic acid groups in the porphyrin side chains is 

 neglected, their dissociation being considered as unaffected by the valency 

 change, heme and a number of heme derivatives possess an iron atom with 

 a residual charge of zero. On oxidation, the iron acquires a positive charge 

 of one unit, enabling it to combine reversibly with a single hydroxyl ion. 

 The simplest complete equation which could be envisaged for the system is: 



-t -|-OH~ 



Red ^ Ox+ =± OxOH (15) 



+e -OH- 



for which the oxidation-reduction ecjuation has been derived (equation 14). 



F [(lr+] 



