OXYGEN AFFINITY AND OXIDATION-REDUCTION POTENTIAL 275 



At half saturation the equation becomes: 



log K = n' log Xi/2 

 where Xi 2 is the pressure at half saturation; substituting for log K: 



log {\ - y) = - n' (log X - log X1/2) 

 and by substitution in the electrode equation one obtains: 



, 0.0601 



Ej, = Eq-\ n' (log X — log Xi 2) 



n 



Conant then uses this equation to obtain values for the Ex of oxyhemo- 

 globin and carboxyhemoglobin at defined partial pressures of the gases. At 

 25° C. and />H 7 he calculates that at 735 mm. mercury Ex for oxyhemoglobin 

 equals + 0.250 v., and at a similar pressure of carbon monoxide under the 

 same conditions Ex is equal to + 0.410 v. While the value of oxyhemoglobin 

 was inaccessible to experimental measurement, a value of + 0.417 v. was 

 found when the equilibrium between hemoglobin and ferricyanide was 

 measured in the presence of carbon monoxide at a pressure of 735 mm. 

 mercury. It should be noticed that the values for the Ex of the oxyhemo- 

 globin and the carboxyhemoglobin system are considerably higher than the 

 £g of +0.15 V. found for the hemoglobin-hem/globin system in the absence 

 of oxygen or carbon monoxide. In arriving at these results Conant assumed 

 that n equalled n' . While this is not always the case, the agreement between 

 predicted values and those found for carboxyhemoglobin shows that the 

 error, in this instance, was not large. 



5.2.4. Variation of Oxygen Affinity and Oxidation-Reduction Potential 

 with pH. Conant's treatment of the equilibrium between carboxy- or oxy- 

 hemoglobin, hemoglobin and hemoglobin, has not, so far, been extended to 

 deal with the effects of variations of pH. Wyman's work (3134,3136) is a 

 step in this direction. It is based, however, only in part on fresh experimental 

 work and in part on a theoretical treatment of the results of other workers, 

 including the rather over- worked data of Ferry and Green. We have pre- 

 viously pointed out the undesirability of relying solely on results obtained 

 from one hemoglobin which is unique so far as species and preparation are 

 concerned, and confirmatory experiments will probably lead to certain 

 modifications in the general treatment. 



This treatment is based on the finding of Ferry and Green that the disso- 

 ciation curves of oxyhemoglobin, obtained at various pH values, can be 

 reduced to a common pH by suitable adjustment of the p axis. We have 

 seen that Pauling has made this the basis of his statement that the influence 

 of pH is solely on the dissociation constant, and that the heme-heme inter- 

 action constant is independent of pH. Roughton and co-workers {2365) 

 have pointed out that if pH is kept constant and temperature varied, a 

 similar procedure may be used to reduce to a given temperature the oxygen 

 pressure-saturation curves. 



