engleski

Surface Potential of Hematite in Aqueous Electrolyte Solution. Hysteresis and Equilibration at the Interface

Electrostatic potential at the inner plane of hematite aqueous interface, i.e. surface potential, was measured by means of a single-crystal hematite electrode. Acidic solutions were titrated with base and then back-titrated with acid. Surface potentials were evaluated from electrode potentials by setting the zero-value at the isoelectric point. In the case of fast titrations the equilibration time was approximately 10 minutes, and a significant hysteresis was obtained, more pronounced at higher electrolyte concentrations. Hysteresis disappeared in slow titration runs when the equilibration time was extended up to 120 minutes, and also when ultrasound was applied. Hysteresis was observed in the pH region close to neutrality where the concentrations of potential determining H+ and OH^- ions are low. Equilibration was fast in acidic and basic regions. These results are explained on the basis of kinetics of surface reactions, supported by following rate of single-crystal electrode equilibration. It is concluded that the equilibration rate at the interface is specific for a given system and is not a general phenomenon. As several systems may undergo fast equilibration, such data may be regarded as equilibrium data and interpreted by the Surface Complexation Model. In other cases, one should perform kinetic tests and apply extended equilibration times.

surface potential; hematite; hysteresis

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