FORMATION AND DECAY OF PEROXYNITRIC ACID - A PULSE-RADIOLYSIS STUDY

Peroxynitric acid and peroxynitrate have been studied using pulse radi olysis of oxygenated nitrite and nitrate solutions, and the formation rate constants are determined to k(HO2+NO2) = (1.8 +/- 0.2) X 10(9) M- 1 s-1 and k(02- + NO2) = (4.5 +/- 1.0) x 10(9) M-1 s-1, respectively. An absorption spectrum is measured for both forms giving maximum extin ction coefficients Of epsilon245(O2NOOH) = 400 +/- 30 M-1 cm-1 and eps ilon285(O2NOO-) = 1650 +/- 100 M-1 cm-1. A rate constant for the oxida tion of nitrous acid by peroxynitric acid was determined to k(O2NOOH HNO2) = 12 +/- 2 M-1 s-1. A pK(a) for peroxynitric acid was measured from the pH-dependent absorbance of the anion of 290 nm, pK(a)(O2NOOH) = 5.85 +/- 0.10. From the decay kinetics a less accurate pK(a) = 5.65 +/- 0.20 was obtained. The decay mechanism for peroxynitric acid as w ell as for the anion is shown to be a unimolecular elimination of HNO2 /NO2- and 02 with a rate equation of k(obs) = k(a) + K(b)(K(a)/K(a) [H+])) where the rate constants k(a)(O2NOOH) = (7.0 +/- 2.0) X 10(-4) s-I and k(b)(O2NOO-) = 1.0 +/- 0.2 s-1, respectively. The results are compared with the scarce values in the literature.