Infrared Absorption Spectra of Nitric Acid and Its Solutions

Abstract

Infrared spectra of reactive liquids such as anhydrous nitric and sulfuric acids were determined using silver chloride windows and, in some cases, Teflon spacers. The absorption frequencies of nitric acid agree well with those found from Raman spectra. A nitronium ion frequency was observed at 2360 cm—1. The intensity of this band is enhanced by the addition of nitronium fluoborate, sulfuric acid, phosphorous pentoxide, or small amounts of acetic anhydride. It is reduced by addition of sodium nitrate, potassium dihydrogen phosphate, water, or larger amounts of acetic anhydride. Its behavior in these media provides information about pertinent equilibria. A nitronium ion combination band was observed near 3745 cm—1 when the concentration of this ion was particularly large. Bands arising from unionized nitric acid are completely absent when a small amount of nitric acid is added to 100% H2SO4. Various bands found in aqueous nitric acid solutions are attributed to hydrogen-bonded structures, both because of their behavior in different media and by analogy with effects observed in Raman studies.