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 (N(s)) 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 Ns. Values of the electrolyte adsorption equilibrium constants (log K cation 0 and log K anion 0) steadily increased with decreasing Ns. 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 (log K cation theta and log K anion theta) a single value for each respective constant was determined. Values of the inner-layer capacitance (C1) were varied during these optimizations and increased with decreasing Ns, particularly below 5 sites/nm2. The optimized C1 values exhibited an apparent inverse relationship with specific surface area (i.e., C1 for hematite A>C1 for hematite B>C1 for hematite C). The magnitude of change in C1 with respect to Ns depended upon the magnitude of C1 for each hematite as the higher the C1 value, the greater was the change with respect to Ns. 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 C1 should be accounted for, particularly for low specific surface area samples that have a high C1.