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Flash Generation and Decay Kinetics of the Nitrate Radical in Aqueous Nitric Acid Solutions' R. W. Glass and T. W. Martin2 Contributionfrom the Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37203. Received September 26, 1969
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Abstract: The generation and decay kinetics of the NO, radical produced at 25 j=1 by the flash photolysis of ceric ammonium nitrate in aqueous nitric acid solutions ranging from 1 to 15 Mare explained fully by the following steps: (I) CeIVN0,hv CeIII NO,; (ID CerlI NOS CeIVNO,-, ~ I I ;(111) NOa NO3 N206,ICIII; (IV) N206 2CeIV + 2N02+ O2 2CeII'; and (V) NOz+ NOaHzO~22HON02. Contrary to other workers, we argue that NO, decays only by second-order and not by first-order processes and that (I) is the only necessary and sufficient primary process to generate NOa under these conditions. In addition, we can show that steps I11 and IV are necessary to explain the production of 0 2 and the small amount of irreversible bleaching of CeIV upon photolysis. Although ICII is sensitiveto HONOzactivity, ~ I I isI constant with a value at 25" of (0.79 + 0.94) x 108 M-1 sec -1. Finally, the new value for the molar extinction coefficient of NO3in these solutions at 6350 A and 25' ( E N O , ~ ~ ~ O ) is 250 f 90 M-' cm-l.
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number of investigations concerning the generation and decay of the NO3 free radical in aqueous solutions have been reported in the literature. 3- lo Using a flash photolysis technique, Martin, et first showed that NO3 could be generated efficiently at room temperature in 6.0 M nitric acid (HON02) solutions of ceric ammonium nitrate, (NH4)2Ce(N03)a. They postulated the primary photochemical process in a generalized notation to be CeIYNOa- + hv --+ CelI1 + NO2 (1) and showed the major pathway for NO3 disappearance to be the bimolecular process kir
CeI"
+ NOa +CerVN03-
OH
+ HON02 +Hz0 + NOa
(2)
(1) Presented in part at the 156th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1968, Abstract PHYS-49. This work was made possible by the U. S. Atomic Energy Commission (Contract No. AT-(40-1)-2825), the National Science Foundation (Grant No. GP-2671), and Vanderbilt University. R. W. G. is indebted for his personal support to the U. S. Steel Foundation and to a Public Health Service Fellowship (No. F1-GM-28, 402) from the National Institute of General Medical Sciences. (2) To whom correspondence about this paper should be sent. Experimental details can be found in R. W. Glass, Ph.D. Thesis, Vanderbilt University, 1968. (3) T. W. Martin, A. Henshall, and R. C . Gross, J. Amer. Cfiem. Soc., 85,113 (1963). (4) T. W. Martin, R. E. Rummel, and R. C. Gross, ibid., 86, 2595 ( 1964). ( 5 ) E. Hayon and E. Saito, J . Chem. Phys., 43,43 14 (1965). . 71,3802 (1967). (6) L. Dogliotti and E. Hayon, J . P f i j , ~Cfiem., (7) M. Daniels, ibid., 70,3022 (1966). ( 8 ) M. Daniels, ibid., 73, 3710(1969). (9) R . I
