Pressure modulation, a new dynamic technique for the electrochemical determination of adsorption, reaction and activation volumes

Abstract

A new dynamic method for the measurement of pressure dependent kinetic and thermodynamic quantities is described and its successful operation demonstrated for two example systems. The pressure was modulated with an amplitude of only ±1 bar or less by means of a piezo-transducer. The small effect on the reaction rate, potential or charge of the electrode can be detected using the lock in technique. The determination of the reaction volume of the redox couple Fe(CN)₆⁴⁻/Fe(CN)₆³⁻ served as a control of the validity of the measurement and the reliability of the experimental approach. As a first model system the adsorption of hydrogen on polycrystalline Pt was studied. A volume of adsorption of 4.8 cm³ mol⁻¹ was found. The partial molar volume for protons in water (υ_H⁺ = −5.4 cm³ mol⁻¹) measured by Zana and Yeager, J. Phys. Chem., 1967, 71, 521–536, was used to establish that the partial molar volume of the adsorbed hydrogen was close to zero.