76 PROCESSES OUTSIDE THE LIVING CELL CHAP. 4 



levels. This must be due to an enzymatic mechanism preventing a 

 primary back reaction of type (4.13), and accelerating the completion 

 of the oxidation process. A secondary back reaction (reoxidation of 

 ferrous oxalate by oxygen) actually was observed by Hill, but this 

 reaction is comparatively slow and does not prevent a partial escape of 

 oxygen into the atmosphere, or the fixation of oxygen by hemoglobin. 



A "hidden," i. e., instantaneously reversible, photochemical oxidation* 

 of water by illuminated cations has also been postulated in the explanation 

 of two forms of the "Becquerel effect," the "photovoltaic" effect, in 

 which the illumination of an oxide-, halide-, or dyestuff-coated electrode 

 causes a change of potential, and the " photogalvanic effect," in which 

 a similar change is induced by the illumination of an electrolyte in contact 

 with an inert electrode. Both effects must be caused by short-lived 

 chemical changes in the surface layer of the electrode, or of the electrolyte. 

 The most probable change is the displacement of an oxidation-reduction 

 equilibrium (c/. Rabinowitch 1940). It has been suggested by Baur 

 (1918, 1919) for the photogalvanic effect, and by Audubert (1934) for the 

 photovoltaic effect, that one partner in the oxidation-reduction equi- 

 librium in aqueous electrolytes is water (the other being the specific 

 photosensitive component — dyestuff, salt, or oxide). However, Svensson 

 (1919) found no evolution of oxygen or hydrogen in illuminated photo- 

 galvanic systems; neither was he able to observe the formation of hydro- 

 gen peroxide or ozone. Baur suggested therefore, that the decomposition 

 of water remains in a "hidden stage," that is, stops short of an actual 

 evolution of oxygen or hydrogen, because of the efficiency of the back 

 reactions. 



5. Photoxidation of Water by Dyestuffs 



In connection with the problem of photosynthesis, the photochemical 

 oxidation of water by dyestuffs, either in consequence of a reduction of 

 the dyestuff itself, or by true sensitization as in Hill's experiments, is of 

 great interest. 



Many cationic dyestuffs are oxidants, capable of being reduced to 

 so-called leuco dyes. Their oxidation-reduction potentials are much too 

 low to enable them to oxidize water in the dark— the strongest known 

 organic oxidants have potentials of -0.4 volt at pH 7, which is still 0.4 

 volt above the potential of the oxygen electrode at the same pH. How- 

 ever, the absorption of a light quantum, even of a "red" light quantum 

 of about 600 mM, corresponding to 45 kcal per einstein, should make the 

 free energy of reaction (4.16) negative. 



(4.16) D* • H2O > DH2 + ^ O2 



In other words, light-excited dyestuff molecules contain enough energy 



