488 



PHOTOCHEMISTRY OF PIGMENTS IN VITRO 



CHAP. 18 



had no effect on bleaching. Oxygen, on the other hand, suppressed the 

 bleaching entirely, even in a concentration as low as 10~® mole per liter. 

 It must be stressed that oxygen inhibits reversible bleaching, and does 

 not merely make it irreversible : the quantum yield of irreversible photoxi- 

 dation of chlorophyll in methanol (< 10""^; cf. below, page 497) is at 

 least one order of magnitude smaller than that of reversible bleaching 

 (> 10"'; cf. above, page 487). 



Carbon dioxide had no effect on reversible bleaching, while formic acid 

 was found by Porret and Rabinowitch to increase it from 1% to as much 

 as 10 to 30%. These very strong bleaching effects still were practically 

 completely reversible and could be suppressed by traces of oxygen. 

 Livingston, too, found that the rate of restoration of bleached chlorophyll 



>le/liter 



Fig. 53. — Reversible bleaching of chlorophyll in methanol by ferric chloride (after 

 Rabinowitch). Extent of bleaching in the photostationary state plotted as function 

 of [FeCls] concentration, for various amounts of FeCU. Broken curves indicate the 

 probable effect of the dark reaction. 



is made three or four times slower by the addition of 10~^ mole per liter 

 of formic acid. Experiments with other acids proved that these effects 

 are specific for formic acid (and not due to hydrogen ions). 



In addition to oxygen, the reversible bleaching of chlorophyll solutions 

 also is inhibited by ferrous chloride. This reminds one of the reversible 

 discoloration of chlorophyll by ferric chloride in the dark, which, too, 

 can be inhibited by ferrous salts (page 464). Observations by Rabino- 

 witch (unpublished) showed that the equilibrium between chlorophyll 

 and ferric and ferrous chloride (in methanol) is displaced in light: chloro- 

 phyll solutions containing ferric and ferrous chloride in proportions 

 which do not cause marked discoloration in the dark are reversibly 



