p. C. WILHELMSEN, R. LUMRY AND H. EYRING 91 



of O,. corrected to S.T.P. plotted as the In k/T versus 1/T. The slope 

 of this line gives a AH* of 19 kcal using the rate equation. Without 

 some way of obtaining absolute values for k\ it is impossible to cal- 

 culate the entropy of formation of the activated complex. 



Information Available about Mechanism 



Kinetic Data 



The data that have been presented are suggestive with respect to the 

 mechanism for chemiluminescence. Inasmuch as the reaction with 

 oxygen alone proceeds very slowly and that with oxygen and an 

 additional oxidant proceeds much more rapidly, it can be reasoned 

 that the mechanism has as its slow step, an oxidation by oxygen which, 

 however, proceeds more rapidly with ferricyanide. Furthermore, it has 

 been demonstrated that oxygen is required for efficient chemilumines- 

 cence of the DPD's even when ferricyanide is present. Bremer's 

 experiments, in which she found that light was produced when 

 oxygen was added to the anaerobic mixture of luminol and ferricya- 

 nide, might indicate that the initial oxidation precedes the reaction 

 with molecular oxygen (1953). The kinetic data also indicate that 

 the reaction is first order with respect to both ferricyanide and oxy- 

 gen concentration. 



Energy Considerations 



The energy of the light emitted by the DPD's during chemilumines- 

 cence is another source of information about the mechanism. Luminol 

 and all the DPD's emit radiation consisting of broad bands with the 

 maxima of the intensities at about 4000 A (Bremer, 1953). This cor- 

 responds to about 60 kcal per einstein. The radiation emitted includes 

 some shorter wavelengths that correspond to 75 or more kcal per 

 einstein. A valid mechanism for the reaction must provide a way for 

 getting this sort of energy into the excited state of a molecule and 

 certainly the reaction must evolve energy of at least this magnitude. 

 However, inasmuch as visible hght can be produced from hydrazine 

 and ferricyanide, it is probable that there will be sufficient energy 

 available from the oxidation to form nitrogen of the DPD's to produce 

 the observed light. 



