REVERSIBLE PHOTOCHEMICAL REACTIONS 



487 



was of the order of 10 seconds. Thus, ay must have been of the order 

 of 0.001. This means y = 10~^, if the bleached product absorbs no 

 red light at all, and y > 10~', if the red absorption band is only weakened. 

 In Livingston's experiment, a 500-watt carbon arc was used, and approxi- 

 mately 4 X 10^^ quanta were absorbed per sec. in 10 ml. of a 2 X 10~^ 

 molar solution of chlorophyll, corre- 

 sponding to n-kv = 0.03. The sta- 

 tionary bleaching was correspond- 

 ingly weaker — of the order of 0.1%. 

 The back reaction was slower than 

 in the experiments of Porret and 

 Rabinowitch — its half period was 

 of the order of 100 seconds. This 

 leads to ay values of the order of 

 3 X lO""*. Similar quantum yields 

 were obtained by direct evaluation 

 of the rate of bleaching (from the 

 initial slope of the curves in Fig. 52). 

 According to Porret and Rabin- 

 owitch, stationary bleaching is pro- 

 portional to the square root of light 

 intensity. This can be understood 

 if one assumes that the rate of bleach- 

 ing is proportional to light intensity, 

 while the rate of the back reaction 

 is proportional to the square of the 

 concentration of the bleached prod- 

 uct. Livingston confirmed this by a 

 direct determination of the rate of 

 the back reaction (by analysis of the 

 declining sections of the curves in 

 Fig. 52), which he found to obey a 

 second-order law. However, the bi- 

 molecular constant of the equation: 



(18.6a) d[Chl]/d< = fc[bChl]2 



■•-200-» •^200-»' 



Time, seconds 



Fig. 52. — Reversible bleaching of 

 chlorophyll in oxygen-free methanol 

 (after Livingston 1941). Curve A shows 

 the approach to a photostationary state 

 after an illumination period of 200 sec- 

 onds. Curves B and C correspond to 

 very short illumination periods. The 

 regeneration of color occurs by a second- 

 order reaction (rate of restoration of color 

 proportional to the square of bleaching), 

 as shown by the shape of the descending 

 sections of curves A, B, and C. 



(b for bleached) varied strongly with 



the purity of the solvent (the values 



corresponding to the three curves in figure 52 were 0.65 X 10^ 5.9 X 10^, 



and 8.3 X 10^ respectively). 



Addition of hydroquinone or allylthiourea, compounds whose autoxida- 

 tion is sensitized by chlorophyll, or even the substitution of a 50% 

 methanol and 50% isoamylamine mixture for pure methanol as solvent, 



