78 



A Manometric Actinometer for the Visible Spectrum 



light, the loss being effected only by reflection. When 0.2 mg. of ethyl-chloro- 

 phyllid per cc. of acetone were added, the transmission in the blue, the yellow or 

 the red was nearly zero; when 1 mg. of Protoporphyrin per cc. of the pyridine was 

 added, the transmission in the green was nearly zero. 



It follows, from the data recorded in the table, that the accuracy of the method 

 is sufficient for most photochemical experiments, inorganic or biological. The light 

 intensities mentioned in the table should not be exceeded. One micromole of 

 quanta entering the vessel in a period of 15 min., is about the right intensity. The 

 pressure changes then are about 20 mm. and can be read with an accuracy of 5%. 



Average = 0.98 



We started our experiments with dioxane as solvent. If it was pure and free of 

 peroxide, the quantum yield seemed to be one. Otherwise, the yield was greater 

 and could be decreased by Cyanide or increased by copper salts. When peroxide- 

 free dioxane, was kept overnight in a bottle which was not airtight it gave a strong 

 peroxide reaction with KJ. Therefore, dioxane was discarded as a solvent. Thus far 

 we have not found a more satisfactory solvent than pyridine, which has a sufficiently 

 low vapor pressure and dissolves thiourea, ethyl chlorphyllid and Protoporphyrin 

 sufficiently. 



Applications 



The actinometer seems particularly suitable for determinations of the quantum 

 yield in photosynthesis. We first place the actinometer in the path of a light beam 

 whose intensity is unknown and measure the oxygen uptake. In the same position 

 we then place a vessel of the same dimensions filled with living green cells which 

 also absorb the light completely. If we compare equal periods of irradiation, we 

 obtain the quantum yield of photosynthesis by the simple equation: 



O2 developed O2 

 O2 absorbed hv. 



The general usefulness of the actinometer becomes apparent if we bear in mind 

 that the instrument integrates the light intensities over time and number of quanta. 



