The titration of sulphuric acid by infrared spectroscopy using the attenuat
ed total reflection (ATR) technique was made in the 0 to 14 pH range for so
lutions ranging from 1.89 to 0.01 M. The subtraction of the water spectrum
was made using acidic, neutral, and basic waters which exhibit different sp
ectra. The results gave the spectra of mixtures of the HSO4- and SO42- ioni
c species in solutions. For each concentration, factor analysis (FA) sorted
the spectra of the pure ionic species and gave through the multiplying fac
tors (MF) the distribution of the species as a function of pH. This distrib
ution is the same as that obtained from the equilibrium equations. The IR m
easurements gave directly the ratios of the activity coefficients of HSO4-
and SO42- ions. This ratio varies with the total concentration of both ions
but is independent of the relative concentration. The fitting of the exper
imental and theoretical volumetric titration curves in the high and low pH
regions gave the mean activity coefficients of the other species present in
the solutions. The values obtained are comparable to those obtained by ele
ctrochemical measurements. These results show that it is possible to use IR
spectroscopy to study aqueous solutions of inorganic acids from low to hig
h concentrations.