Near-infrared spectroscopy in the wavelength region 800−1100 nm has been applied to the measurement of
aqueous solutions of hydrofluoric acid over the concentration range 0.01−2.65 M and the pH range 2−13. The
analysis is based on the detection of subtle perturbations
of the water spectrum which result from the presence of
the acid and other species in solution. Spectral
characterization of the hydrofluoric acid system was performed
using a spectrophotometric titration and multivariate
model-based regression. Excellent agreement was observed between the spectra and a simple equilibrium
model describing the acid's dissociation. Five
species,
including hydrofluoric acid, fluoride, bifluoride, hydronium, and hydroxide ions, were quantitated with estimation errors below 10 mM using this procedure. In
addition, the species' spectral profiles were recovered,
and optimum estimates of the system's equilibrium
constants were determined.