Determination of Adsorption Isotherms of Hydrogen on Titanium in Sulfuric Acid Solution Using the Phase-Shift Method and Correlation Constants

Abstract

The phase-shift method and correlation constants, i.e., the unique electrochemical impedance spectroscopy (EIS) techniques for studying the linear relationship between the behavior (−φ vs E) of the phase shift (90° ≥ −φ ≥ 0°) for the optimum intermediate frequency and that (θ vs E) of the fractional surface coverage (0 ≤ θ ≤ 1), have been proposed and verified to determine the Langmuir, Frumkin, and Temkin adsorption isotherms of H and related electrode kinetic and thermodynamic parameters on noble metals (alloys) in aqueous solutions. On Ti in 0.5 M H2SO4 aqueous solution, the Frumkin and Temkin adsorption isotherms (θ vs E), equilibrium constants (K = 8.3·10−12 exp(−6.6θ) mol−1 for the Frumkin, and K = 8.3·10−11 exp(−11.2θ) mol−1 for the Temkin adsorption isotherm), interaction parameters (g = 6.6 for the Frumkin and g = 11.2 for the Temkin adsorption isotherm), rates of change of the standard free energy (r = 16.4 kJ·mol−1 for g = 6.6 and r = 27.8 kJ·mol−1 for g = 11.2) of H with θ, and standard free energies [(63.2 ≤ ΔGθ0 ≤ 79.6) kJ·mol−1 for K = 8.3·10−12 exp(−6.6θ) mol−1 and 0 ≤ θ ≤ 1 and (63.1 < ΔGθ0 < 79.6) kJ·mol−1 for K = 8.3·10−11 exp(−11.2θ) mol−1 and 0.2 < θ < 0.8] of H are determined using the phase-shift method and correlation constants. At 0.2 < θ < 0.8, the Temkin adsorption isotherm correlating with the Frumkin adsorption isotherm, and vice versa, is readily determined using the correlation constants. The two different adsorption isotherms appear to fit the same data regardless of their adsorption conditions. The phase-shift method and correlation constants are probably the most accurate, useful, and effective ways to determine the adsorption isotherms of H and related electrode kinetic and thermodynamic parameters on highly corrosion-resistant metals in aqueous solutions.