Abstract
The adsorption of HSO4-, Cl- and I- from aqueous solutions was examined using radiotracer, FTIR spectroscopy and ellipsometry techniques. Adsorption dependence was measured as a function of time, concentration and potential. For HSO4-, some measurements were made as a function of temperature at constant potential. The dependence on time corresponded neither to the "ln Γ[t] - t" nor "Γ[t] - ln t" relations expected from previously deduced theoretical models. Up to -1/4r, θ[t] was proportional to the adsorption time. The dependence of adsorption on concentration was done over an extended range: 10^-9 to 10^-2M. Up to 10^-5M it followed the Henry isotherm and at concentrations up to 10^-3M, the results followed a logarithmic law. ([partial]θ/[partial]log c)[V,T] was about the same for HSO4- and Cl- but around twice higher for I-. θ[max] of HSO4- and Cl- were reached at ~0.25 and ~1 for I-. The dependence on potential at constant concentration was essentially linear until oxide formation for HSO4- and Cl-. I- results showed little variation with potential before oxide formation. The dependence of (θ)[v,c] as a function of temperature for HSO4- cannot be expressed as a simple law because of the low number of experiments successfully performed; nevertheless, approximate values of ΔG°, ΔH° and ΔS° were determined. An isotherm was derived such that the heterogeneity of the surface, ion-ion repulsion through a simple imaging model, and ion-ion dispersive forces were considered. Electrostriction effects on ion size were computed. Evidence for partial charge transfer was found: 0.2 (HSO4-), 0.6 (Cl-) and 0.9 (I-). Evidence of the displacement of solvent molecules from the surface was consistent with the desorption of an entity represented by a dimer. For HSO4-, ΔG°=-32 kJ/mol (25°C), ΔH°=-57 kJ/mol and ΔS°=-78 J/mo l°K (θ[ss]=0.22). From these values, the Pt-0 bond energy was estimated as ~1/2 of that expected for isolated Pt-O bonds in the gas phase. The entropy was consistent with immobile adsorbed ions with hindered rotation. The concentration dependence of SO4= adsorption on bare iron electrodes followed a logarithmic trend, ΔG°=-28 kJ mol^-1. The values of θ>>1 observed on iron-oxide were explained in terms of formation of iron-sulfate complexes.
Gamboa-Aldeco, Maria E. (1992). In situ studies of adsorption of inorganic anions on metallic electrodes. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1293034.