76 A Manometric Actinometer for the Visible Spectrum 



during the first 10 min. When, after 230 min., the oxygen uptake had become very slow, 735 mm. 3 

 2 had been absorbed, that is, 2.5 moles 2 /mole of thiourea. Only 2 moles of oxygen are calculated 

 when the thiourea is oxidized according to the equation : 



NHo 



/ 

 CS + 2 Oo = CNNH 2 + H2SO4 . 



\ 



NH-2 (Cyanamide) 



The pyridine Solution was evaporated in vacuo, the residue taken up in water, and the insoluble 

 porphyrin removed by filtration. In the filtrate, about 74% of the calculated amount of H2SO4 was 

 precipitated by BaCl 2 . To another portion of the Solution AgNOs and ammonia, or copper acetate 

 and ammonia, were added to test for cyanamide. Silver produced a dark yellow precipitate and cop- 

 per a black precipitate. Another portion of the Solution was tested for urea by urease, which was 

 added from the sidearm of a manometer vessel to the acidified Solution (pH 5.5). Seventeen per 

 cent of the thiourea was recovered as urea. This urea may have been formed by hydrolysis of 



cyanamide: 



NH2 



/ 

 CNNH2 + H 2 = CO 



\ 

 NH 2 



Finally, it may be mentioned that the proporphyrin is slowly photooxidized in the Solution 

 Spectroscopic examination showed the appearence of a band in the red at about 675 m//. Very 

 probably this is the first step in the transformation of blood pigment to bile pigment discovered in 

 19305. 



Actinometer 



Two hundred mg. of thiourea, dissolved in 5 cc. pyridine ("medicinal" grade, 

 Mallinckrodt), are introduced into a rectangular vessel of about 14 cc. volume. 

 Two mg. of crystalline ethyl chlorophyllid are added, if red, yellow, or blue light 

 is to be measured. Ten mg. of crystalline Protoporphyrin are added if green or blue 

 light is to be measured. 



The vessel is then connected to a simple manometer of the type used for respi- 

 ration measurements. Pure oxygen is introduced into the gas-space. The vessel is 

 shaken in the thermostat at 20°C. in the dark, until the pressure remains constant 

 for at least 15 min. If the vessel then is illuminated by light which is absorbed by 

 the Chlorophyll or the porphyrin, thiourea is oxidized and the oxygen pressure in 

 the vessel decreases. 



It is an essential feature of this arrangement that the concentration of the Chloro- 

 phyll or the porphyrin is so high that light entering in a vertical direction through 

 the bottom of the vessel is completely absorbed, even when the vessel is in motion. 

 To test this condition it must be shown that the oxygen consumption remains 

 constant if the concentration of the dyes is increased. 



Quantum Yield 



The light beam leaving the slit of a monochromator was collimated by lenses and 

 reflected by a mirror vertically into the vessel. The cross-section of the beam 



