Nitric/Nitrous Acid Equilibria in Supercritical Water

UV−vis spectroscopy was utilized to measure the decomposition of aqueous HNO3 solutions above 300 °C, in some cases with added NaOH, H2O2, and/or NaNO2, to form NO2, HNO2, NO, N2O, and oxygen. Individual bands corresponding to HNO2 and NO2 were deconvoluted from the spectra up to 400 °C. With constrained nonlinear optimization, areas of these bands were used to determine extinction coefficients of NO2 and HNO2 and equilibrium constants for the reactions: HNO3 ⇌ H+ + NO3 -; 2HNO3 ⇌ H2O + 2NO2 + (1/2)O2; 2NO2 + H2O ⇌ HNO3 + HNO2; 2HNO2 ⇌ H2O + 2NO + (1/2)O2; 2NO ⇌ N2O + (1/2)O2; (Na+)(NO3 -) ⇌ Na+ + NO3 -.