KINETICS 



1597 



ured the rate of reduction of ferricyanide by the redox potential method 

 found that the initial rate declined with increasing [FeCye"^], from 1.30 

 to 0.12 mole ferricyanide reduced per mole chlorophyll per minute. Ap- 

 proximately, this initial rate was proportional to [FeCy6~^]~S indicating 

 a strong self-inhibition not found by Holt and French. On the other hand, 

 the change in rate as the reaction proceeded followed closely the zero order 

 law. Spikes et al. suggested, as possible explanation of this apparent con- 



160 - 



^-^•~A-A-A-A-£ 



120 



TIME, minutes 



Fig. 35.22A. Effect of quinone concentration on oxygen production by chloro- 

 plast suspensions (after Clendenning and Gorham, 1950): (A) crude leaf mace- 

 rate; (B) separated and resuspended chloroplast material. 



tradiction, that the ferrocyanide produced the same inhibition as ferricy- 

 anide. With quinone, where Clendenning and Gorham found clear evi- 

 dence of self-inhibition, Spikes et al. observed a proportional increase of 

 initial rate with the concentration of the oxidant! 



Wessels (1954) also found that the reduction (he used the dye DCPI 

 as oxidant) proceeds linearly with time, indicating no dependence of rate 

 on the concentration, [DCPI] (cf. figure 35.22B). 



Maximum Yield and Back Reactions. What proportion of the theoretical 

 amount of oxygen can be obtained in the Hill reaction with a given oxidant 

 depends (a) on the extent of side reactions which destroy the oxidant, e. g.. 



