Abstract
Photoacoustic Spectroscopy (PAS) is now well-established as a useful technique for obtaining optical absorption spectra from strongly scattering and/or opaque materials which are difficult to study by conventional photometric methods. However, its quantitative application to solids in the UV-visible region has been somewhat restricted by the phenomenon of photoacoustic (PA) signal saturation, in which the intensity of the stronger bands in the spectrum becomes insensitive to changes in analyte concentration. By comparison, with the recent advent of Fourier Transform Infrared Photoacoustic Spectroscopy (FT-IR/PAS), this problem is expected to be much less serious in view of the low absorptivities characteristic of IR bands. Indeed, recent work in our laboratory has shown that when NH<sub>3</sub> is sorbed stepwise into the bulk of 12-tungstophosphoric acid, H<sub>3</sub>PW<sub>12</sub>O<sub>40</sub>, the base is quantitatively converted to NH<sub>4</sub><sup>+</sup> ion, and the calibration curve of the analyte peak area (standardized against a reference band of the substrate) vs. analyte concentration is virtually linear up to stoichiometric loading. With the use of this method, several salts of the parent acid were found to exist in partial (Brønsted) acid form, and adjusted molecular formulae thus calculated were consistent with those obtained in dependently by elemental analysis.
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