Thermodynamic Models for the (HClO₄ + NaClO₄){aq} and (HBr + NaBr){aq} Systems at 298.15 K and 0.1 MPa

Current Pitzer activity coefficient models for the (H + Na + ClO 4 ){aq} and (H + Na + Br){aq} systems are limited to ionic strengths <7 mol·kg –1 . To accurately describe activity coefficients and solid solubilities in these systems, new Pitzer model parameters β Na,a (0), β Na,a (1), C Na,a ϕ, θ H,Na s, and ψ H,Na,ClO 4 (a = ClO 4 –, Br – ) and standard state chemical potentials (μ i ° / RT ) were fit to the 298 K isopiestic and solid solubility data. The resulting NaClO 4 {aq} model reproduces the data, within their accuracy, to ionic strengths of approximately 18 mol·kg –1 . No activity coefficient model parameters were necessary for the neutral species. Our dissociation constant of 130.36 for NaClO 4 ·H 2 O is significantly different from the frequently cited value of 151.71. The θ H,Na s derived from the (H + Na + ClO 4 ){aq} system was tested for consistency against other acid systems containing soluble sodium salts, i.e., the (H + Na + Cl){aq}, (H + Na + Br){aq}, and (H + Na + NO 3 ){aq} systems. A new Pitzer model for the NaBr{aq} system, which reproduces the data within their accuracy to solid saturation at 9.179 mol·kg –1, was produced for this test. The fitted θ H,Na s parameter eliminated the need for the ψ H,Na,a parameter (a = Cl – or Br – ).