1508 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



tions, and by changes in the absorption spectrum of methyl red (which is 

 an acid-base indicator), which may be caused by the presence of the base, 

 phenylhydrazine. (The quantum yield was determined by photometry 

 of methyl red !) Furthermore, in Ghosh's experiment, an induction period 

 of varying duration was noted, wliich affected unfavorably the reliability 

 of the rate measurements. 



Since light absorbed by methyl red could contribute to the reaction, 

 Livingston used a red band X > GOO m/z. (Ghosh and Sengupta worked 

 in the green, at X 435.8 m/x.) A green band (475-550 mju) was used for the 

 determination of methyl red. The initial concentrations of phenylhydra- 

 zine and methyl red (in methanol) were 5 X 10 ~^ and 1 X 10~^ mole/ 

 liter, respectively. The reaction was found to be of zero order (i. e., the 

 quantum yield remained constant) until about 80% of the dye was con- 

 sumed. The concentration of chlorophyll was varied between 0.2 and 75 X 

 10-^ mole/liter (chlorophyll a), and 1 to 7.5 X 10"^ mole/liter (chloro- 

 phyll h). The observed quantum yields scattered over the range from 0.09 

 to 0.15, showing no clear dependence on nature or concentration of the 

 chlorophyll used. The reason why Ghosh and Sengupta had found 6-7 

 times higher quantum yields could not be explained. Experiments with a 

 manometer, using an acetonic solution of allyl thiourea (0.5 mole/liter), 

 containing chlorophyll (1-2 X 10 ~^ mole/liter) and oxygen, indicated that 

 the quantum yield of this reaction (for which values up to 1.0 had been 

 found by Gaffron) was up to seven times higher than that of the methyl 

 red-phenylhydrazine reaction — thus indirectly confirming the low quan- 

 tum yield of the last-named reaction given above. The manometric experi- 

 ments also showed that no gas was produced in the reduction of methyl 

 red by phenylhydrazine (liberation of nitrogen could conceivably occur in 

 this reaction). 



Experiments showed that the occurrence of an induction period was 

 entirely dependent on the presence of oxygen. From the amount of oxy- 

 gen present in air-saturated methanol and the duration of the induction 

 period, it appeared that the quantum yield of chlorophyll-sensitized autoxi- 

 dation of phenylhydrazine (the reaction which probably takes place during 

 the induction period) may be of the order of unity, similar to the quantum 

 yield of the autoxidation of allylthiourea. 



In the green, at 435.8 m/x, the quantum yield of the methyl red-phenyl- 

 hydrazine reaction was found, in preliminary experiments, to be about 

 0.1, independently of changes in methyl red concentration; and despite 

 the fact that, in this case, a varying proportion of light was absorbed by 

 methyl red and not by chlorophyll. This indicates that the reaction can oc- 

 cur also by direct photochemical activation of the azo dye instead of by 

 sensitization. 



