Characterization of Nitrogen-Containing Species Produced From Nitric Acid/Water Systems

Abstract The speciation of gases formed in the vapor phase above HNO3-H2O mixtures at room temperature is measured and characterized using Fourier-transform infrared (FTIR) spectroscopy. Although studies of speciation of compounds have been performed in the solution phase, this study focuses solely on the gas phase species formed over HNO3-H2O mixtures and their concentrations as a function of time. The nitric acid concentration in the experiments range from 14 to 4 M. The NO2, NO, and N2O (collectively known at NOx) gases are measured by FTIR at 1585, 1850, and 2208 wavenumbers, respectively. The concentrations of these NOx species ranges from 0 to 3 ppm and change over time as a function of acid concentration. Two purities of commercially available HNO3 are used to prepare the HNO3-H2O mixtures. An analysis of thermodynamic data shows that the formation of NOx gases without the presence of a catalyst are thermodynamically favorable.