CONVERTERS OF SOIAR ENERGY — ^HOTTEL 161 



being studied at the Institute. The system chosen consists of an or- 

 ganic dye, thionine, and ferrous iron in the form, for example, of a 

 ferrous sulfate solution. The two components form in the solution 

 a reversible oxidation-reduction system. 



(dyestuff) +Fe++?± leukodyestuff+Fe+++ 



t 

 (colorless) 



Ferric iron is a much stronger oxidizing agent than thionine ; there- 

 fore, in the dark, all the thionine is in the form of the dye, and all 

 the iron in the ferrous form. If, however, the mixture is illuminated 

 by the light absorbed by thionine — i. e,, visible light in the region 

 5000-7000 A. (green, yellow, red light) — the thionine molecules are 

 activated by light and become capable of oxidizing ferrous iron. 

 Since the reduced thionine is colorless, the reaction is recognized by a 

 decoloration of the solution. This bleaching proceeds to a steady 

 state, whose exact character depends on the intensity of illumination. 

 In this state, the velocity of the photochemical bleaching reaction 

 is exactly compensated by that of the back-reaction restoring the 

 equilibrium. As soon as the light is switched off, the system reverts 

 to its original state. 



Experiments have been conducted on the kinetics of this interesting 

 photochemical process, using a photometric method for the determina- 

 tion of the concentration of the dye under different conditions. Of 

 more interest in the present connection is the electrochemical effect of 

 light in the thionine-iron system. As the composition of the solution 

 changes through illumination, its electrode potential is also changed ; 

 if two platinum electrodes are placed in the solution and the electro- 

 lyte surrounding one of them is illuminated while the other is kept 

 dark, a potential difference is established between the two electrodes 

 and a current flows from the dark to the illuminated electrode. 

 The problem of the photogalvanic effect demonstrated by this experi- 

 ment has two elements : the first and simpler question is that of the 

 electromotive force produced by a given illumination; the second is 

 that of the current that can be drawn from such a photogalvanic 

 cell. 



So far, experiments have been concerned with the first part of the 

 problem. The photogalvanic potential of the thionine-iron system 

 has been measured in relation to the concentrations of all the compo- 

 nents and the light intensity. A pronounced maximum of potential 

 is found at a certain concentration of the dyestuff, and a strong in- 

 crease in effect with decreasing acidity of the solution. From such 

 experiments, it has been possible to develop a quantitative picture of 

 the photogalvanic effect in satisfactory agreement with the experi- 

 mental results. The next step is a study of the factors affecting cur- 



