Observations on the interpretation and analysis of sulfuric acid hydrate infrared spectra

New infrared spectra of pure bulk molecular sulfuric acid and its mono- and tetrahydrates under equilibrium water partial pressure conditions have been obtained using an internal reflection geometry. For each of the species present (H2SO4, HSO4- and SO42-), a well-defined set of core SO4 vibrations has been identified and assigned to individual vibrational modes. Using these data, earlier literature data has been reexamined. Existing methods for the quantification of composition and ion speciation in H2SO4/H2O mixtures are examined and the need for a more sophisticated method for the determination of sulfate aerosol composition is proposed.