Activity coefficients in concentrated electrolytes: a comparison of the Brunauer-Emmett-Teller (BET) model with experimental values

Electrolyte mean ionic activity coefficients, γ_±, are calculated from the Stokes-Robinson application of the Brunauer-Emmett-Teller (BET) adsorption model for seven electrolytes (NaOH, HCl, KOH, CaCl₂, LiCl, LiBr and Ca(NO₂)₃). Only two model parameters are needed, which are derived from water vapor pressure measurements. Results were compared with experimental mean ionic activity coefficients (γ_±). Because the standard state for the BET model is the anhydrous electrolyte rather than the infinitely dilute solution, it is necessary to adjust for the differing standard states by comparing the BET and experimental γ_± values at one ‘anchoring’ concentration. The higher this ‘anchoring’ concentration, the better is the agreement between the BET and experimental mean ionic activity coefficients over the entire concentration (molality, mol kg⁻¹) range except at the two extremes of nearly pure water and nearly pure electrolyte. This is because the BET model lacks a finite limit at infinite dilution, and the experimental data are referenced to the infinite dilute solution. The BET model is well-behaved and in good agreement with experimental data on γ_±.