198 



V. HEMATIN COMPOUNDS 



value, but that with reduction in pH an inflection point is found 

 below which Eh becomes invariant with pH ( — (A£/,)/(ApH) = 0). 

 The nature of the curves (Fig. 9) indicate that this is due to a dissoci- 

 ation of the oxidant, i.e., the hemzchrome, which may be represented: 



[FeOHl + H+5=?[Fel+ + H2O 

 the two negative charges on the side chain carboxyl groups being 



- 0.1 



- 0.2 



- 0.3 



- 0.4 — 



« ' ' I I I ' ' I ■ I I I I ' I < I i I 1'^^ " 



10 11 n 



pH 



13 



Fig. 10. Relation of potential to pH (after Clark, ^5^): (1) iron protoporphyrin 

 (estimated by Barron); (2) cyanide iron mesoporphyrin; (3) cyanide iron hemato- 

 porphyrin; (4) cyanide iron protoporphyrin (after Barron); (5) a-picoHne iron hemato- 

 porphyrin; (6) pyridine iron mesoporphyrin in alcohol-water; (7) nicotine iron meso- 

 porphyrin in alcohol-water; (8) nicotine iron protoporphyrin; (9) pyridine iron 

 hematoporphyrin; (10) nicotine iron protoporphyrin in alcohol-water. 



omitted. That the dissociation concerned is connected with a group 

 attached to the iron atom, and is not associated with the side chain 

 carboxyl groups is supported by the fact that etioporphyrin hemo- 

 chromes in which no carboxyl groups occur, show the same Eh/pH 

 relationship. 



