Thermodynamic Properties of Aqueous HClO4 to 373 K and at Low Pressure: A Comprehensive Pitzer Ion-Interaction Model

Abstract

We present a Pitzer ion-interaction model fit to most of the available isopiestic, dilution enthalpy, heat capacity, and volumetric data for HClO4(aq). Osmotic and activity coefficients are accurately reproduced for molalities between infinite dilution and 8 mol kg-1 at 298 K. Isopiestic data at temperatures other than 298 K do not appear to be reliable. The dilution enthalpy data at 298 K are also reproducible to 8 mol kg-1; consequently, HClO4(aq) activities may be calculated to 8 mol kg-1 at 298 K and at somewhat higher or lower temperatures with only minor losses in confidence. Heat capacity data do not extend beyond 1.7 mol kg-1 at 298 K and 1.0 mol kg-1 at T > 328 K, and interlaboratory agreement is poor. Consequently, activity coefficients and thermal properties may only be calculated to about 1 mol kg-1 at 373 K. There are numerous volumetric data for molalities between 0.02 and 18 mol kg-1 and at 298 K; however, there are large variances between datasets at m > 5 mol kg-1. Volumetric data at T > 348 K are limited to one study and m < 1 mol kg-1. The volumetric form of the model is simple but due to disagreement between datasets is considered to have large uncertainties.