278 BIOLOGICAL EFFECTS OF RADIATION 



assuming the validity of Beer's law (page 260). Samples of material 

 can be removed for chemical analysis by titration or other means. 

 Usually very sensitive methods of measurement are necessary because 

 the amount of change is small. Especially when making use of mono- 

 chromators, micro- or semimicrotechnique is necessary. A convenient 

 microtechnique for analyzing gases has been developed by Blacet and 

 Leighton (1). 



GENERAL PROCEDURE 



In the following paragraphs the general procedure for conducting 

 photochemical investigations will be outlined. Specific illustrations 

 will be given later. At the outset it is safe to assume that the photo- 

 chemical reaction may be quite complex and it becomes necessary to 

 exclude as far as possible all secondary reactions and impurities before 

 drawing conclusions regarding the main reaction. For example, the 

 photochlorination of organic compounds is seriously affected by the 

 presence of small amounts of oxygen. Under ordinary conditions in 

 the air, the photochemical reaction studied will be one involving oxygen 

 and the production of phosgene as an intermediate step. The direct 

 addition of chlorine to an organic compound, either saturated or unsatu- 

 rated, can be interpreted only in case oxygen is absent. Many other 

 cases of similar nature are known where traces of acid or of inorganic 

 salts are involved in the observed reaction. 



It is not sufficient in quantitative work to place a vessel containing 

 the reacting system in front of a source of light and follow the photo- 

 chemical change in the vessel. As the reaction proceeds, the original 

 material is destroyed and the absorption of light may become consider- 

 ably lessened. The rate of the reaction then slows down not because 

 of any fundamental change but simply because less of the light is 

 absorbed. Qualitatively, one can determine the nature of the product 

 which is formed by the photochemical reaction, but no quantitative 

 conclusions can be drawn from the rate of the reaction. In many cases 

 it is important to know the rate and to know just how much energy is 

 absorbed by the photoactive material. The most fundamental quantity 

 to be measured is the quantum yield <J>, the ratio of the number of mole- 

 cules reacting to the number of quanta actually absorbed. When this 

 value is known, conclusions regarding the fundamental process can be 

 drawn and the extent of the photochemical reaction can be predicted 

 from known values of the light intensity and the absorption and the 

 time of exposure. In case the absorbing material is so concentrated 

 that all the light is absorbed throughout the whole reaction, the absorp- 

 tion does not change during the course of the reaction and the calculations 

 are somewhat simpUfied. There are certain objections to this procedure, 

 particularly when a large amount of material is used. 



