The chemistry of trioxodinitrates. Part 2. The effect of added nitrite on the stability of sodium trioxodinitrate in aqueous solution

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In solutions of pH ⩽ 3.29 added nitrite catalyses the decomposition of trioxodinitric acid to nitrogen mono-oxide. At 25 °C and I= 0.25 mol dm–3, the rate law is Rate = 1.14 × 104[H+][HNO2][H2N2O3], the reaction proceeding by initial nitrosation and a subsequent free-radical reaction. As the temperature is decreased, the rate law changes over a narrow temperature range to one involving a dependence on [HNO2] of order ca. 1.6 and having buffer-dependent and -independent terms. Further decrease in temperature has no effect on the rate of reaction. At pH 4.9, trioxodinitrate is stabilised by added nitrite. This may be understood in terms of an equlllbrium between [HN2O3]– and the products of nitrogen–nitrogen bond fission, HNO and [NO2]–. It is suggested that [HN2O3]– undergoes fast tautomerisation to [ON–N(OH)O]– which undergoes heterolytic cleavage in a slow step. Evidence is also presented for a previously unreported non-reversible decomposition path of [HN2O3]–. At intermediate pH, added nitrite results first in stabilisation of trioxodinitrate, and then, at higher [NO2–], in increased instability. These results confirm that the catalytic role of nitrous acid in the NO-producing decomposition of trioxodinitrate is via reaction with H2N2O3 rather than with NOH or [NOH2]+.