KINETICS 1591 



such drifts must be ascertained when the pH method is used to measure the 

 rate of the Hill reaction. 



Another potentiometric method for following the Hill reaction is the 

 measurement of the oxidation-reduction potential. This is feasible when- 

 ever the system contains one — and only one — electrode-active oxidation- 

 reduction couple (such as ferrous ion-ferric ion, ferrocyanide ion-ferricyanide 

 ion, or dye-leuco dye). This method was described by Spikes, Lumry, 

 Eyringand Wayrynen (1950''-). They found it practicable with ferricya- 

 nide as oxidant. However, the oxidation-reduction potential often showed 

 considerable drifts even in the absence of oxidant. For example, when 

 crude suspensions of cell content from sunflower leaves were illuminated, 

 the redox potential of the medium changed rapidly. The drift continued 

 for 12 hours. This may indicate that the Hill reaction was proceeding at 

 the cost of a natural oxidant present in the cell juice (as described in the 

 early experiments of Hill). Macerated cell material from spinach showed 

 no such prolonged drift. 



Chloroplasts separated from the plasma and cell juice showed only a 

 small change of potential with time during illumination without added 

 oxidant. If the chloroplasts were boiled, the potential was not affected, 

 even if ferricyanide was added. 



If, however, the chloroplasts were "live," a change of potential was 

 observed in light, when a complete Hill solution, or ferricyanide alone, or 

 quinone were added to them. The strongest change was produced when 

 ferricyanide was used in a buffered solution (to prevent complications due 

 to changes in pR). With this system, phosphate-buffered at pR 6.85, 

 potential vs. time curves could be determined under a variety of condi- 

 tions. 



Spikes et al. (1954) have developed a device to automatically record 

 redox potentials in cell suspensions. 



The same method was also used by Wessels and Havinga (1952, 1953; 

 c/. Wessels 1954) in experiments with different quinones and quinonoid 

 dyes as Hill oxidants, and by Gerretsen (1950'-^, 1951), who combined the 

 measurement of redox potential with that of pH changes in a study of light 

 reactions in crude leaf juice. 



It will be noted that the displacement of an oxidation-reduction equilib- 

 rium in light produces a nonequilibrium state in which one redox system 

 (e. g., quinone-hydroquinone) is in the reduced, and another one (e. g., 

 O2/H2O) in the oxidized state. Barring immediate effective spatial separa- 

 tion of the two couples, an electrode immersed into the illuminated mixture, 

 will follow — entirely or preponderantly — the change in the composition of 

 the component with the greater electrode activity. The situation is simi- 

 lar to that in the so-called "photogalvanic cells" (described by Rabino- 

 witch) in which a platinum electrode, immersed into an illuminated mix- 



