Specific adsorption of iodide-ion at liquid (Ga-In)-electrode of eutectic composition from aqueous solutions with constant ionic strength

The adsorption behavior of iodide-ion at liquid (Ga-In)-electrode in aqueous electrolyte solutions at 305 K is studied by the electrochemical impedance spectroscopy, differential capacitance, and cyclic voltammetry. The equivalent circuit describing the experimental data in the presence and in the absence of the I⁻ ion is a series connection of a resistance and a capacitance that is frequency-independent over a ∼500 Hz to 100 kHz range. The experimental data were obtained by the mixed electrolyte method in electrolyte solutions acidified with HClO₄ down to pH 3, with excess of surface-inactive ion ClO₄⁻ and constant ionic strength (0.1 M). The analysis resulted in the determination of the charge σ₁ of the I⁻ ion specifically adsorbed at the liquid (Ga-In)-electrode at the adsorbate maximal concentration: σ_I = 7.73 μC/cm² in the case of analysis at σ = const (at the zero charge potential) or σ_I = 7.50 μC/cm² in the case of analysis at E = const. These values are characteristic of rather strong specific adsorption. The values of σ_I of the studied anion were used in the calculations of different isotherms with the purpose of the adsorption parameters determination. The obtained results were compared with literature data determined on other metals in the presence of specifically adsorbable I⁻ ion.