Composition of a gaseous mixture in chemical equilibrium as studied by Raman spectroscopy: Gas phase acetic acid

Abstract

AbstractAn experimental method for the determination by Raman spectroscopy of the composition of a gas mixture in which a chemical equilibrium occurs, and the related processing of the data are discussed. Acetic acid vapour, for which none of the components of the equilibrium can be isolated, is chosen as an example. The intensities of the ν(CC) bands are taken as monitors for the concentrations of monomer and dimer species. The mixture has been studied at 120°C and 155°C, in the pressure of c. 0.2–1.5 atm. Using gas density data, a treatment in terms of molal concentrations leads to consistent results, the ratio of Raman scattering cross‐sections at the wavenumbers corresponding to the ν(CC) vibrations (dimer/monomer) being found the same for both isotherms and equal to 1.64. The other significant results are: Kc app = 0.48 atm. at 120°C and 2.67 atm. at 155°C. ΔH (dissociation of one mole of dimer) = 16.3 kcal (range 120–155°C). An alternative treatment in which an ideal behaviour of the gas phase is assumed shows that Raman measurements are accurate enough to emphasize the inadequacy of this approximation in the temperature and pressure range considered. The Raman spectrum of gaseous acetic acid (range 100–3700 cm−1) at ∼220°C and ∼2 atm is presented.