Direct determination of the Gibbs' energy of formation of peroxynitrous acid

The kinetics of decomposition of peroxynitrous acid (ONOOH) was investigated in the presence of 0.1-0.75 M HClO₄ and at a constant ionic strength. The decay rate of ONOOH decreased in the presence of H₂O₂, approaching a limiting value well below 75 mM H₂O₂. It also decreased in the presence of relatively low [HNO₂] but did not approach a lower limiting value, since ONOOH reacts directly with HNO₂. The latter reaction corresponds to a HNO₂- and H⁺-catalyzed isomerization of ONOOH to nitrate, and its third-order rate constant was determined to be 520 ± 30 M^-2 s^-1. The mechanism of formation of O₂NOOH from ONOOH in the presence of H₂0₂ was also scrutinized. The results demonstrated that in the presence of 0.1-0.75 M HClO₄ and 75 mM H₂O₂ the formation of O₂NOOH is insignificant. The most important finding in this work is the reversibility of the reaction ONOOH + H₂O ⇌ HNO₂ + H₂O₂ and its equilibrium constant was determined to be (7.5 ± 0.4) × 10^-4 M. Using this value, the Gibbs' energy of formation of ONOOH was calculated to be 7.1 + ± 0.2 kcal/mol. This figure is in good agreement with the value determined previously from kinetic data using parameters for radicals formed during homolysis of peroxynitrite.