PHOTOCHEMISTRY 275 



inhibitors, changing Ught absorption, and various types of secondary 

 reactions. In the same way intermittent exposure is sometimes of help 

 in determining whether or not the primary photoprocess is followed by 

 dark thermal reactions. A sector wheel may be revolved in the path of 

 the light in such a way as to reduce the intensity, for example, by half. 

 Under these conditions half of the wheel is open and half of it closed and 

 the time of exposure is reduced to one-half by this revolving wheel irre- 

 spective of the speed at which the wheel rotates. The time between 

 illuminations, however, is changed by changing the rate of rotation and 

 if each period of illumination is followed by a chemical reaction which 

 takes considerable time, the total amount of reaction may vary with 

 the speed of rotation. In many cases, however, the chain reaction 

 following the primary photochemical process is completed in much less 

 time than the time between illumination. Under these conditions, of 

 course, the intermittent illumination is unable to contribute any informa- 

 tion concerning the reaction. 



Impurities. — Many photochemical reactions are quite sensitive to 

 small amounts of impurities such as oxygen, for example. This is 

 particularly true with chain reactions in which a foreign substance may 

 act to carry a chain or may act in the opposite way to inhibit the reaction 

 by breaking a chain. As in all exact chemical work, the materials should 

 be subjected to extensive purification. Only rarely is it possible to use 

 commercial chemical substances without further purification. Irf many 

 cases impurities have no particular influence on the reaction, but one 

 must either ascertain this fact in advance or prove it experimentally. 



EXPERIMENTAL TECHNIQUE 



In quantitative photochemical investigations it is necessary to use 

 light which is nearly monochromatic in character and to measure the 

 amount of radiation absorbed. In the early researches on photo- 

 chemistry these requirements were not often realized and there was 

 serious disagreement among different workers. Frequently when the 

 whole spectrum is used, radiation of one wave-length may produce one 

 reaction while light of another wave-length may produce an entirely 

 different one, and m fact may even reverse the reaction produced by the 

 first radiation. Valuable information concerning the nature of the 

 reaction is obtained from a knowledge of the quantum yield — the number 

 of molecules reacting per quantum of radiation absorbed. As explained 

 before, this quantity often enables one to determine the nature and 

 extent of secondary chemical reactions which may accompany the 

 primary photochemical process. For this reason considerable effort is 

 justified in measuring accurately the amount of radiation absorbed. The 

 energy in ergs which is transmitted from the rear of the cell is subtracted 

 from the energy striking the front of the cell with suitable corrections 



