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

Peroxynitrous acid and peroxynitrite anion have been studied using pul se radiolysis of nitrite and nitrate solutions The formation rate cons tant is determined to be k(OH+NO2) = (4.5 +/- 1.0) x 10(9) M-1 s-1, an d the rate constant for the OH radical reaction with nitrite is determ ined to be k(OH+NO2-) = (6.0 +/- 1.0) x 10(9) M-1 s-1. In nitrate solu tions, the competing reaction between OH and NO32-is found to have a r ate constant of k(OH+NO32-) = (3.0 +/- 1.0) x 10(9) M-1 s-1. The inter mediate species in the nitrate system, NO32-, HNO3-, and H2NO3, decay into NO2 according to the first-order rate constants: (5.6 +/- 0.5) x 10(4), (2.0 +/- 0.5) X 10(5), and (7.0 +/- 2.0) x 10(5) s-1, respectiv ely. The rate constants k(H+NO3-) = (1.0 +/- 0.3) X 10(7) M-1 s-1 and k(H+NO2) = (1.0 +/- 0.2) x 10(10) M-1 s-I were also determined. The pK (a) of ONOOH is found to be 6.5 +/- 0.1 by absorption measurements, an d the maximum extinction coefficient at 240 nm is epsilon240(ONOOH) = 770 +/- 50 M-1 cm-1. The decay of peroxynitrous acid is determined to proceed through the first-order isomerization of ONOOH to HNO3 accordi ng to the rate equation k(obs) = k(iso)/(1 + K(a)/[H+]) with rate cons tants k(iso) = 1.0 +/- 0.2 s-1 and K(a) = (1.0 +/- 0.3) X 10(-7). A co mparison of all available literature values for the pK(a) and the deca y rate is reported.