The dependence of hematite site-occupancy standard state triple-layer model parameters on inner-layer capacitance

Potentiometric acid–base titration data for three hematite samples that differed on the basis of specific surface area (17.4, 33, 83 m2/g for hematite A, B, and C, respectively) was analyzed using the triple-layer model (TLM). The sensitivity of the TLM fits of the data to the choice of site density (NsNs) was evaluated from 1.5 to 22 sites/nm2. In general, little dependence in the quality of fit was determined, irrespective of the value of NsNs. Values of the electrolyte adsorption equilibrium constants (logKcation0 and logKanion0) steadily increased with decreasing NsNs. These constants are consistent with the commonly used 1.0 M standard state and when converted into comparable constants consistent with the site-occupancy standard state (logKcationθ and logKanionθ) a single value for each respective constant was determined. Values of the inner-layer capacitance (C1C1) were varied during these optimizations and increased with decreasing NsNs, particularly below 5 sites/nm2. The optimized C1C1 values exhibited an apparent inverse relationship with specific surface area (i.e., C1C1 for hematite A>C1A>C1 for hematite B>C1B>C1 for hematite C). The magnitude of change in C1C1 with respect to NsNs depended upon the magnitude of C1C1 for each hematite as the higher the C1C1 value, the greater was the change with respect to NsNs. These results suggest when the site-occupancy standard state parameters are used to predict constants at different site density values without re-regression of titration data that variations in C1C1 should be accounted for, particularly for low specific surface area samples that have a high C1C1.

New site-occupancy equilibrium constants are independent of site density as long as variations in C1C1 are taken into account, particularly for low specific surface area samples with high C1C1 values.Figure optionsDownload as PowerPoint slide