BLEACHING OF CHLOROPHYLL IN METHANOL 1489 



steady-state bleaching, tenfold extension of the half-life of the bleached 

 material). The back reaction was changed, by the addition of oxalic acid, 

 from second to first order (in respect to the concentration of bleached ma- 

 terial). An enhancement was produced also by the addition of methyl 

 red (an azo dye whose reduction by phenylhydrazine is sensitized by chloro- 

 phyll) ; 10~^ mole/liter methyl red increased the stationary bleaching by a 

 factor of four. A particularly strong enhancing effect was caused by iodine 

 (10~^ mole/liter) ; for example, in one experiment the steady-state bleach- 

 ing increased from 0.2 to 26%, and the half-life of the bleached state from 

 0.5 to 20 sec. In this case, too, the back reaction became a first-order reac- 

 tion. Irreversible bleaching was completely suppressed by iodine. In pure 

 carbon tetrachloride, reversible bleaching was noticeable, but it was largely 

 obscured by irreversible bleaching, which was much faster than in meth- 

 anol, and continued for some time in the dark. (Such an after-effect was 

 not observed in methanol, or in methanol-carbon tetrachloride mixtures.) 



The quantum yield of irreversible bleaching of air-saturated methanolic 

 solution of chlorophyll a was estimated by McBrady and Livingston as 

 7f,., = 4 X 10~^ (c/. similar earlier estimates on page 497, Vol. I). They 

 also estimated the quantum yield of reversible bleaching (from the photo- 

 stationary state and the rate of back reaction, or from the initial rate of 

 bleaching), with the results shown in Table 35.1. The rate of the reverse 

 process may change from case to case, by a factor of 200 or more, but the 

 rate (quantum yield) of the forward reaction was found to change much less 

 — only from 1.3 X 10~* to 10 X 10~* in the examples given in Table 35.1. 



Table 35.1 



Kinetics of Reversible Bleaching of Chlorophyll (after McBrady and 



Livingston)* 



Forward 



reaction 



Back reaction quantum 



yield, 



Solvent Order Rate constant 7 X 10* 



Methanol + oxalic acid 1 . 28 sec. ~^ 7 



Methanol + ecu 2 2.8 X 10^ mole liter -i sec. -» 1.3 



Methanol (pure) 2 "At lea.st 20 times higher" ~ 4 



Methanol + I2 1 ? '--10 



* 



Cf. below for data of Livingston and Knight. 



The values, jrev. = 10 X 10"'' and jrev. = 3 X lO"*, given earlier by 

 Porret and Rabinowitch, and by Livingston, respectively, fall into the same 

 range. (It is worth remembering that all these 7's are minimum values, 

 since their calculation is based on the assumption that the reversibly 

 bleached chlorophyll absorbs no red light at all.) 



In dealing with the mechanism of reversible bleaching, McBrady and 

 Livingston did not go beyond discussion of the several possibilities enu- 

 merated in Volume 1 (pages 489 and 514). One interesting new suggestion 



