Raman spectroscopy of the reaction of sodium chloride with nitric acid: Sodium nitrate growth and effect of water exposure

The reaction of powdered NaCl with HNO3 was studied using Raman spectroscopy. NaNO3 growth was monitored as a function of HNO3 exposure in a flow cell. Mode-specific changes in the NO3- vibrational mode intensities with HNO3 exposure suggest a rearrangement of the NaNO3 film with coverage. In the absence of H2O, intensities of NaNO3 bands increase with HNO3 exposure until a capping layer of NaNO3 forms. The capping layer prevents subsequent HNO3 from reacting with the underlying NaCl. H2O exposure of NaNO3-capped NaCl results in hydration of the surface to form an aqueous solution containing NaNO3 and NaCl. Further exposure results in the formation of a thicker surface-adsorbed H2O layer that is consistent with an aqueous NaNO3 and NaCl solution. Upon exposure of hydrated NaCl to HNO3, molecular HNO3 and solid-state NaNO3 are observed in the thin H2O film.