1494 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



phyll while oChl designated an unstable oxidation product, perhaps a free 

 radical. 



Irreversible bleaching not only fails to be accelerated, but is even some- 

 what slowed do^vn by heating (by about 15% when the temperature in- 

 creases from 16 to 36° C). This indicates that the back reaction which re- 

 verses the first step of oxidation has a somewhat higher temperature coef- 

 ficient than the forward reaction with which it competes. 



In continuation of experiments by Rabinowitch and Weiss on the re- 

 versible decoloration of chlorophyll solutions by ferric and eerie salts and 

 the accelerating effect of ferrous ions on the back reaction in reversible 

 photobleaching (page 464, Vol. I), Knight and Livingston found that re- 

 versible bleaching of chlorophyll in methanol is enhanced by cerous chlo- 

 ride, lanthanum chloride, and barium chloride (in concentrations from 2 X 

 10 ~^ to 2 X 10"^ mole/liter). The back reactions become first-order reac- 

 tions, with a half-period of 7-8 sec. in the presence of the cerium salt, and 

 20 sec. in the presence of the lanthanum salt. With repeated illumination, 

 the absorption at 650 m/x increased irreversibly, indicating the gradual for- 

 mation of a product absorbing red light even more strongly than chloro- 

 phyll itself. (In interpreting these results, one should recall the observa- 

 tions of Rabinowitch and Weiss on the effect of salts on the reversible reac- 

 tion of chlorophyll with ferric chloride.) 



When iodine was added to methanol, the back reaction was first order 

 with a half-period of 25 sec. at 30° C. 



With increased temperature (23 -^ 47° C), the relative stationary 

 bleaching in the presence of iodine increased by approximately 50%, de- 

 spite the fact that the back reaction was accelerated by about a factor of 3 

 (still remaining a first-order reaction). 



Under all conditions, the extent of steady bleaching remained propor- 

 tional to the square root of light intensity. It was found to be proportional 

 to [I2] at concentrations up to 10 ~^ mole/liter; the half-life of the bleached 

 state was, in this range, independent of [I2]. A chlorophyll-sensitized reac- 

 tion of iodine with methanol appeared to be the cause why the bleaching 

 effects were found to decrease with repeated exposure to light. 



In discussing these results, Livingston first reestimated, from figure 

 35.2, the quantum yield of irreversible bleaching in the oxygen-saturated 

 state, and found jirr. — 4.5 X 10 ~^ in agreement with the earlier estimates 

 (cf. Vol. I, p. 497). Maximum quantum yield of reversible bleaching was 

 calculated by extrapolation as jrev. ^ 2.8 X 10~^ i. e., at least 6 times 

 the value found in McBrady's work (table 35.1). In carbon tetrachloride, 

 the quantum yield of irreversible bleaching was much higher — about 2.5 

 X 10 ~*, but this is still a low value compared to the quantum yield of re- 

 versible bleaching. In the presence of iodine, the quantum yield of revers- 



