Resolution of Raman spectra of aqueous sulfuric acid mixtures using principal factor analysis

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Abstract
Principal factor analysis has been applied to Raman spectra of 26 sulfuric acid/water mixtures covering the 0–100% H2SO4 concentration range. The analysis greatly facilitates the identification of peaks due to different species. The results show that SO42− ions and "free" HSO4− ions do not co-exist with undissociated H2SO4 molecules in solution and that two water molecules rather than one are required for the first ionization of H2SO4. A species with the composition H2SO4•2H2O, assigned a hydrated ion pair structure, reaches maximum concentration at the same medium composition at which SO42−, free HSO4−, and H2SO4 are at concentration minima, about 75% w/w. The only species apparent in the more concentrated solutions are the ion pair and undissociated H2SO4, which could be taken to mean that H2SO4 is a weaker acid than H3O+, but a stronger one than H3O+•H2O (or H5O2+, if this entity has a real existence). Separate peaks due to the postulated H3O+•H2SO4 (or H5SO5+) were not observed.

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