10047
J. Phys. Chem. 1993,97, 10047-10052
Formation and Decay of Peroxynitric Acid: A Pulse Radiolysis Study T. Lorgager and K. Sehested' Section of Chemical Reactivity, Environmental Science and Technology Department, Rise National Laboratory, DK-4000 Roskilde, Denmark Received: May 20, 1993; In Final Form: July 19, 1993"
Peroxynitric acid and peroxynitrate have been studied using pulse radiolysis of oxygenated nitrite and nitrate solutions, and the formation rate constants are determined to k(H02+N02) = (1.8 f 0.2) X lo9 M-1 s-1 and k(02-+N02) = (4.5 f 1.0) X lo9 M-l s-I, respectively. An absorption spectrum is measured for both forms giving maximum extinction coefficients of c245(02NOOH) = 400 f 30 M-l cm-l and E Z E ~ ( O ~ N O O =- 1650 ) f 100 M-' cm-l. A rate constant for the oxidation of nitrous acid by peroxynitric acid was determined to k(O2NOOH+HN02) = 12 f 2 M-' s-l. A pKa for peroxynitric acid was measured from the pH-dependent absorbance of the anion of 290 nm, pKa(02NOOH) = 5.85 f 0.10. From the decay kinetics a less accurate pKa = 5.65 f 0.20 was obtained. The decay mechanism for peroxynitric acid as well as for the anion is shown to be a unimolecular elimination of H N 0 2 / N 0 2 - and 0 2 with a rate equation of kob = k, Kb(Ka/(Ka [H+])) where the rate constants ka(02NOOH) = (7.0 f 2.0) X lo" s-l and kb(02N00-) = 1.0 f 0.2 s-l, respectively. The results are compared with the scarce values in the literature.
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Introduction Peroxynitric acid has potential importance in aqueous atmosphere chemistry, and recently interest about its chemistry in water has increased.1.2 It is a very difficult species to study due to the extreme conditions used to generate it in solution: >90% H 2 0 2 and 70% HN03,1,390% H202 and N02BF4,3 or pulse radiolysis of Orsaturated tetranitr~methane.~Pulse radiolysis of Orsaturated nitrite solution was studied in 1965 by Adams et al.,s and a characteristic spectrum was obtained. Later, a similar spectrum was obtained by Broszkiewicz,6who postulated it to be a peroxide of the nitrite ion based on studies by Daniels and Wigg7 and Anbar.8 In this work we applied pulse radiolysis tonitriteandnitratesolutionsfor thestudyoftheUV-vis spectrum of peroxynitric acid and its anion, the pK,, and the pH-dependent decay mechanism.
Experimental Section Materials and Solutions. All solutions were prepared using triply distilled water. NaNO2 (Riedel-de HaEn p.a.), N a N 0 3 , NazHP04, NaH2P04, NaOH (Merck, p.a.), HC104 (Frederik Smith Co, doubly distilled from Vycor), 02 (N48), and H2 (N60) were used as supplied. Orsaturated (0.1-1 .O) X M NaNO2 and 02-saturated (0.1-1.0) X 10-2 M NaNO3 solutions were adjusted to pH using NaOH or HClO, or buffered using M phosphate in the pH range 5-8. Note that in solutions of pH < 3.5 (pKJHN02) = 3.2) the evaporation of HNO2 is substantial when saturating a solution with a gas by bubbling. The evaporated concentration depends on the total bubbling time; e.g., in a M HN02, pH 2 solution, about 20 and 50% of the nitrous acid (measured at 370 nm) is lost after 20 and 50 min of bubbling, respectively. The pK, of peroxynitric acid was studied at 290 nm, 0.8 ms after the pulse, as was the yield of 0 2 N 0 0 - , while the yield of 02NOOH was studied at 250 nm. The formation and spectra of 02NOOH and OzN00- were studied in the 235400-nm wavelength range, and the decays at 245,260 and 280, 290 nm, respectively, using 230- or 280-nmlight filters to minimize solute and peroxynitric acid photolysis in the time scales
