INFLUENCE OF pH 



199 



The only exceptions to the above Eh/pH relationship are found in 

 the case of the cyanide compounds, and the imidazole (pilocarpine, 

 histidine) hemochromes. In the latter, it is possible that the measure- 

 ments were made in the region of the pK of the hemtchrome and that 

 too few points on either side of this have been measured (c/. Pig. 11). 

 In the former it is quite definite that the oxidation-reduction potential 



- 0.400 - 



- 0.300 



0.200 



0.100 



+ 0.100 



+ 0.200 



Pyridine hemochromogen 



10 



11 



12 



pH 



Fig. 11. Relation of potential to pH (after Barron, 180). Note that Ei 

 decreases along the ordinate. 



is invariant over a wide range of pH in the alkaline region (Fig. 10 

 and Fig. 11). 



The structural interpretation of these facts is b}'^ no means clear. 

 The suggestions of Clark and Davies for the hem/chromes have 

 already been discussed (Section 4.2.). In the case of the cyanide 

 compounds it is assumed by Davies that the affinity of the heme and 

 hematin for cyanide is so great that competition between cyanide 



