4 A Manometric Actinometer for the Visible Spectrum 7 ' 

 Otto Warburg and Victor Schocken 



The actinometer to be described in this paper is based upon the work of H. v. 



Tappeiner 1 , who discovered the sensitization of photooxidation processes by dyes; 



upon that of S. E. Sheppard" 2 , who discovered the role of sulfur Compounds, 



especially of allyl thiourea, in the photochemical process of photography ; and upon 



the manometric and bolometric method developed at Dahlem for the measurement 



of the quantum yield of photosynthesis 3a * b . In 1926, Warburg and Gaffron 4 



determined the quantum yield of photooxidation of allyl thiourea when sensitized 



by Chlorophyll or porphyrin by these methods. The result was that 1 molecule of 



oxygen reacts, if 1 light-quantum is absorbed by the sensitizers, that means that 



the quantum yield was 



<p = - — = l. 

 h- v 



Due to many circumstances, the technical procedure of 1926 was too complicated 

 to be used for practical actinometric purposes. We have simplified the procedure 

 in such a way that quantum intensities can now be measured in the greater part 

 of the visible spectrum by the chemical process of photooxidation wherever simple 

 manometrie equipment is available. We replaced the differential manometer by 

 the simple Haldane-Barcroft manometer; Acetone, which because of its high vapor 

 pressure, is unsuitable for manometric experiments, was replaced by pyridine, 

 which, thus far, is the most appropriate solvent we have found. Allyl thiourea was 

 replaced by unsubstituted thiourea, a simplification by which we eliminated a slow 

 dark reaction. 



We have tried many substances as reductants in our system but none equaled 

 thiourea. Even substances such as hydroquinone, which are easily oxidized by many 

 oxidants, could not be photooxidized at all. Thus, Sheppard's discovery of the 

 specific reactivity of certain sulfur Compounds proved as important in photo- 

 oxidation as in photography. 



We have tried many dyestuffs but none equaled Chlorophyll or prophyrin as 

 sensitizers. We used crystalline ethyl chlorophyllid and crystalline Protoporphyrin. 

 As light absorbers they are complementary, the Chlorophyll transmitting the green 

 and the Protoporphyrin transmitting the red. If mixed together they absorb the 

 greater part of the visible spectrum. 



Photooxidation of Thiourea 



One mg. of thiourea (13.2 micromoles) was dissolved in 5 cc. of pyridine. One-tenth mg. of Proto- 

 porphyrin was added. The mixture was introduced into a rectangular vessel of 14 cc. volume which 

 was connected to a manometer. The gas Space contained air. When shaken in the thermostat at 

 20° C. and illuminated with a 200 w. incandescent lamp at 20 cm., 135 mm. 3 Oo were absorbed 



* Aus Archives of Biochemistry 21 (1949): 363. 



