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 NaNU₃ film with coverage. In the absence of H₂O, intensities of NaNO₃ bands increase with HNOj 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.