Potentiometric acid-base titration data for three hematite samples that differed on the basis of specific surface area (17.4, 33, 83 m(2)/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/nm(2). In general, little dependence in the quality of fit was determined, irrespective of the value of N-s. Values of the electrolyte adsorption equilibrium constants (log K-cation(0) and log K-anion(0)) steadily increased with decreasing N-s. cation anion 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 (C-1) were varied during these optimizations and increased with decreasing N-s, particularly below 5 sites/nm(2). The optimized C-1 values exhibited an apparent inverse relationship with specific surface area (i.e., C-1 for hematite A > C-1 for hematite B > C-1 for hematite C). The magnitude of change in C-1 with respect to N-s depended upon the magnitude of C-1 for each hematite as the higher the C-1 value, the greater was the change with respect to N-s. 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 C-1 should be accounted for, particularly for low specific surface area samples that have a high C-1. (C) 2007 Elsevier Inc. All rights reserved.