296 



BIOLOGICAL EFFECTS OF RADIATION 



Nitrate ion. — When potassium nitrate is dissolved in water and 

 subjected to radiation in the short ultra-violet region, nitrite and free 

 oxygen are produced according to the reaction: 



2NO7 -^ 2N0^ + O2 

 The quantum yield depends on the wave-lengths of the light and on the 

 pH of the solution. At a pH of 9.9 Villars (50) found the following 

 values : 



At wave-length 2536 A the quantum yield varied from 0.05 at a pH of 

 6 to 0.3 at a pH of 10 to 14. At the shortest wave-length there is prob- 

 ably enough energy available to cause direct rupture of the nitrate ion 

 into the nitrite ion and atomic oxygen. At the longest wave-length 

 the energy is insufficient for this process. The strong dependence on the 

 pH of the solution indicates that the reaction is more complicated than 

 given by the simple decomposition of the nitrite ion. The low quantum 

 yield, less than unity, can be attributed to loss of energy by collision to the 

 surrounding solvent or to loss of energy in vibration of the atoms in the 

 ion, but it may be due to the existence of more complex reactions. 



Oxidation of Ions hy Dissolved Oxygen. — The oxidation of an aqueous 

 solution of sodium sulfite by dissolved oxygen is typical of a large group 

 of reactions. It is strongly inhibited by small traces of organic material, 

 such as hydroquinone, benzaldehyde, and similar substances. This 

 marked inhibition is characteristic of chain reactions. That the oxidation 

 of sodium sulfite by dissolved oxygen is a chain reaction was demonstrated 

 by Biickstrom (2), who found that about 50,000 molecules react per 



o 



quantum of radiation at 2536 A. 



The auto-oxidation of stannous chloride is a similar reaction; both 

 the large quantum yield and the marked inhibition by various substances 

 show the reaction to be a chain reaction. The reaction was studied in 

 aqueous and in nonaqueous solutions by Haring and Walton (20, 21). 

 The complex ion produced by the union of stannous chloride and hydro- 

 chloric acid absorbs ultra-violet light and in the presence of dissolved 

 oxygen is changed over into stannic chloride. The reactions in water are 

 somewhat complicated by hydrolysis, but similar results are obtained in 

 nonaqueous solvents in which no hydrolysis can take place. Rough 

 measurements of the quantum yield showed that many molecules react 

 for each photon absorbed. 



Picric acid was found to be one of the most powerful inhibitors. In 

 Hcetic acid solvent, for example, one millimole of stannous chloride was 

 oxidized in 2 min., whereas, with 0.02 gram of picric acid present, less 



