PHOTOCHEMISTRY 293 



(42) suggested that the reaction is strongly catalyzed by dust particles, 

 and there is good evidence that supports this view. 



Heidt (23) found that the quantum yield decreases as the concentra- 

 tion of hydrogen peroxide decreases and approaches a limiting value 

 somewhere in the neighborhood of unity. In these experiments the 

 solutions were nearly free from dust particles and the radiation was of 

 high intensity, conditions that apparently lead to the suppression of the 

 chain reaction. 



Hydriodic Acid. — One of the simplest photochemical reactions is the 

 decomposition of hydriodic acid. Gaseous hydrogen iodide is transparent 

 through the visible region of the spectrum and to 3000 A. At wave- 

 lengths shorter than this it absorbs radiation continuously without fine 

 structure. The energy of radiation at 3000 A corresponds to 95,000 

 calories per mole, and this is sufficient to break the hydrogen-iodide bond 

 and liberate atoms. The atoms are thrown out with kinetic energy, 

 depending on the frequency of the light. Inasmuch as this kinetic energy 

 is not subject to the limitations of the quantum theory, it is possible for 

 all wave-lengths to be utilized, thus giving a continuous absorption. 

 Hydrogen molecules and iodine molecules are formed by the combination 

 of atoms. There is little chance that hydrogen atoms will combine with 

 iodine atoms to give hydriodic acid, and there is no chance of a chain 

 reaction. Hence the overall quantum yield is 2 molecules per quantum, 

 as given by the following equations: 



HI + Av = H + I 

 H -1- HI = Ho + I 

 I + I = I2 



The reaction is one of the simplest in photochemistry, and the 

 quantum yield is not affected by light intensity or by concentration. 

 It is practically the same in gases at low pressures (35) and at high 

 pressures, and even in liquid hydriodic acid (9) or when dissolved in 

 liquid hexane (54). This reaction has been studied by several investi- 

 gators. The hydrogen iodide is placed in a quartz vessel and is sub- 

 mitted to monochromatic radiation. Radiation of 2536 A from a 

 mercury-vapor lamp, filtered through a mixture of chlorine and bromine, 

 is satisfactory. The reaction cannot be followed by pressure change 

 because there is no change in the number of gaseous molecules. It has 

 been followed by titrating the amount of iodine liberated after various 

 intervals of time. It could be followed colorimetrically by the production 

 of iodine. 



Warburg (51) found that in this reaction the number of molecules of 

 hydriodic decomposed per quantum was the same at different wave- 

 lengths, 2070, 2536, and 2820 A. The number of molecules decomposed 

 per erg of radiation varied considerably. This is taken as one of the 

 experimental supports for the Einstein relation and the quantum theory. 



