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

Principal factor analysis has been applied to Raman spectra of 26 sulfuric acid/water mixtures covering the 0–100% H 2 SO 4 concentration range. The analysis greatly facilitates the identification of peaks due to different species. The results show that SO 4 2− ions and "free" HSO 4 − ions do not co-exist with undissociated H 2 SO 4 molecules in solution and that two water molecules rather than one are required for the first ionization of H 2 SO 4 . A species with the composition H 2 SO 4 •2H 2 O, assigned a hydrated ion pair structure, reaches maximum concentration at the same medium composition at which SO 4 2− , free HSO 4 − , and H 2 SO 4 are at concentration minima, about 75% w/w. The only species apparent in the more concentrated solutions are the ion pair and undissociated H 2 SO 4 , which could be taken to mean that H 2 SO 4 is a weaker acid than H 3 O + , but a stronger one than H 3 O + •H 2 O (or H 5 O 2 + , if this entity has a real existence). Separate peaks due to the postulated H 3 O + •H 2 SO 4 (or H 5 SO 5 + ) were not observed.