An Ftir Study of Phosphoric Acid on Platinum and Gold

The paper represents an FTIR study of the adsorption from aqueous solutions of phosphoric acid onto Pt and Au electrodes at various concentrations and potentials. The experimental method utilizes parallel polarized light; the light path through the solution is around 10^–4cm. The signal-to-noise ratio was made acceptable by subtracting spectra obtained in regions in which H₃PO₄was known from radiotracer measurements not to be adsorbed from the spectra at potentials at which adsorption occurs. It was found that there is one main peak at 1074 cm^-1which varies parabolically with potential. Values of the electrode coverage, 0, were obtained by calibrating the peak heights against the results of parallel radiotracer measurements. Plots of 0 against log C are linear. The adsorption maximum on Au was ca. 300 mV more positive than that on Pt. The validity of the measurements is supported by the rational trends of the data obtained, in particular, the relation to the spectroscopic data separately obtained for H₃PO₄in solution. H₃PO₄in solution is 99% molecular and 1% ionic. It is shown that the coverage data can be evaluated in such a way (solvation of H₃PO₄) that 0.2 < θ < 0.6, so that a Temkin isotherm may be applicable. The difference in results on Au and Pt may not be interpreted unambiguously in terms of displacement by oxide: accounting for water bonding appears necessary. The method offers an alternative to SERS.