REVERSIBLEJ PHOTOCHEMICAL REACTIONS 489 



bleached in light. This bleaching is of the same order of magnitude as 

 that observed in pure, oxygen-free, chlorophyll solutions (i. e., ~ 1%), 

 but it is not suppressed by oxygen. The bleaching experiment with 

 ferric chloride can be repeated indefinitely with one and the same solution 

 (provided that only red light is used, since blue light causes an irreversible 

 decomposition of the "oxychlorophyll," cf. page 465). Figure 53 shows 

 the stationary bleaching as a function of the concentration of ferric 

 chloride. A saturation at [FeCls] values as low as 5 X 10"^ mole per 

 liter is exhibited. At the larger [FeCls] values, the bleaching becomes 

 weaker, probably because the thermal equilibrium between chlorophyll, 

 ferri and ferro ions is shifted towards oxidation. 



According to pages 484 et seq. reversible bleaching of chlorophjdl could 

 be caused by its tautomerization, dismutation, oxidation, or reduction. It 

 was mentioned previously that the effect in pure, oxygen-free solutions 

 is proportional to the square root of light intensity. If bleaching were 

 caused by tautomerization, the back reaction would be monomolecular, 

 and the effect would be proportional to the first power of light intensity, 

 as shown by the following equations (t for tautomeric) : 



light 



(18.7) Chi* . tChI 



dark 



(lS.8a) - ^^^ = kl (bleaching) 



(18.8b) -I- ^£^ = fc'CtChl] (back reaction) 



kl 

 (18.8c) [tChl] = Tj (stationary state) 



It was stated above that a proportionality of the stationary bleaching 

 with the square root of light intensity indicates a himolecular hack reaction, 

 e. g.: 



light 



(18.9) Chi* + X . A + B 



dark 



(18.10a) -^^^ = kl (bleaching) 



(18.10b) + ^^^ = fc'[A] X [B] = k'lAJ (back reaction) 



y (stationary state) 



where A[Chl] is the amount of chlorophyll missing during the illumina- 

 tion. While the effect of light intensity excludes tautomerization as the 

 cause of reversible bleaching, the low concentration of chlorophyll (~ 10"* 

 mole per liter) seems to exclude dismutation. (The results of Weiss and 

 Weil-Malherbe, 1944, seem to indicate that dismutation is not entirely 

 impossible even at these low concentrations.) This seems to leave an 

 oxidation-reduction reaction with the solvent (or an impurity?) as the 



