LEONOR MICHAELIS 



and free radicals, once they have been established, have no chance to 

 undergo bimolecular reactions by which they may be rapidly elimi- 

 nated. Radicals can be preserved, in this manner, to an extent w^hich 

 by far exceeds their equilibrium concentration postulated by thermo- 

 dynamics. Lewis showed that many organic compounds exposed in 

 this way to ultraviolet radiation become colored; in suitable cases he 

 identified, by spectrophotometrical comparison, the colored substance 

 with the free radical obtained previously by chemical oxidation. The 

 peculiar merit of this method is the fact that free radicals can be de- 

 tected in the cases in which they would be unnoticeable in the state of 

 equilibrium because of their low concentration. On the other hand, 

 since the appearance of color is not necessarily evidence for a free 

 radical, a scrutiny of the phenomenon is necessary for each individual 

 case. 



In order to arrive, from these considerations, at the discussion 

 of the final problem we have in mind, the varying degree of inclination 

 of an organic substance toward oxidation (or reduction) must be con- 

 sidered. Here we distinguish two essentially different properties. 

 To state that a substance is easily, or difficultly, oxidized by an oxi- 

 dizing agent may mean one of two things: that the speed of such an 

 oxidation is great or small (a topic belonging to a discussion of chemical 

 kinetics); or that the final state attainable by the interaction of the 

 oxidizing agent and the oxidizable substrate is a complete, 100% 

 oxidation, an incomplete oxidation, or almost no oxidation at all 

 (a problem of thermodynamics). 



We shall start with the problem in thermodynamics, in par- 

 ticular, the case in which both the oxidizing agent and the substance 

 to be oxidized form reversible oxidation-reduction systems. If leuco- 

 methylene blue is the substance to be oxidized and potassium ferri- 

 cyanide the oxidizing agent, when the two are mixed in equivalent 

 proportions practically all of the leuco dye is oxidized to the dye and 

 all of the ferricyanide is reduced to ferrocyanide. But when leuco- 

 methylene blue is mixed with an indophenol dye, both the oxidation 

 of leucomethylene blue and the reduction of indophenol will be in- 

 complete, and the final state is a mixture of four substances, the two 

 leuco dyes and the two dyes. Furthermore, if leucomethylene blue is 

 mixed with safranine, no change will occur, at least within practical 

 measurable limits. Safranine does not oxidize leucomethylene blue 



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