DIFFERENT OXIDANTS 1573 



gave values as low as 55%; on this basis, the oxygen yields were 100 ± 

 4% of the theoretical value for the reaction (D = Dye) : 



(35.33) D + H2O > H2D + K O2 



The rate decreased with increasing dye concentration (3.3 X 10""^ — * 3.3 X 

 10-2 mole/liter), probably because increased competition of the dye for 

 light absorption brought the absorption by chlorophyll below the level 

 needed for light saturation. 



With blue dyes, such as 2,6-dichlorophenol-indophenol, the absorption 

 by the dye was so strong that the rate became too slow for manometric 

 measurements. By using a very dilute solution, both in respect to the dye 

 and in respect to chlorophyll (5 X 10-^ mole/liter), the occurrence of the 

 dye reduction could be proved by ol^servation of the decoloration of the 

 dye in light. l<]xperiments of this type were made with eight dyes at con- 

 centrations of the order of 5 X 10"^ mole/liter or less, at pH 6.6 and 8.0, 

 with the results shown in Table 35. VIII. 



Table 35.VII1 

 Dye Reduction by Chloroplasts (after Holt and French, 1948) 



-Eo' (volt)" 

 at pH 6.6 at pH 9.0 



Positive results (at pH 6.6 and 9.0) 



Phenol-indophenol 0.254 0.083 



2,6-Dichlorophenol-indophenol . 247 . 089'' 



o-Cresol-indophenol 0.217 0.089 



Positive results (at pH 6.6 only) 



Thionine<^ 0.074 (-0.001) 



Negative results 



l-Naphthol-2-sulfoindophenol . 147 . 003 



Methylene blue 0.024 -0.050 



Indigo tetrasulfonate -0.027 -0.114 



Indigo disulfonate -0. 104 -0.199 



" Throughout this book, oxidation-reduction potentials are given on Lewis and Ran- 

 dall's scale on which strong oxidants have high negative potentials, cf. table 9. IV. 



'' Slow reaction. 



"= No oxygen evolution was found; reaction may thus be with a cellular hydrogen 

 donor, such as ascorbic acid, rather than with water. The decolorization of thionine was 

 incomplete, and reversed itself in the dark. 



This table shows, in agreement with Aronoff's findings {cf. above), that 

 the photochemical effectiveness of the several oxidants (saturation rate of 

 oxygen liberation in strong light) increases generally with their oxidation 

 potential. However, the parallelism is not strict, and specific effects seem 

 to occur, as shown by the inactivity of the l-naphthol-2-sulfoindophenol. 

 It must be considered that the observed rate is the net balance of a photo- 

 chemical forward reaction and a dark back reaction. The back reaction 

 can involve either the final reduction product (such as the leuco dye), or a 



