Isopiestic Molalities Research Articles

CaCl 2(aq) at elevated temperatures. Enthalpies of dilution, isopiestic molalities, and thermodynamic properties

Enthalpies of dilution of CaCI 2(aq) have been measured for molalities ranging from 0.008 mol·kg −1 to 7.26 mol · kg −1, at temperatures from about 298 K to 526 K, and for pressures of approximately 7 MPa to 40 M Pa. Isopiestic molalities of CaCI 2(aq) were measured over the temperature range of 444 K to 524 K at molalities ranging from 0.55 mol · kg −1 to 4.8 mol · kg −1 with NaCl(aq) serving as the reference electrolyte. The results can be described quite well within the framework of the ion-interaction model. Our results have been combined with other thermodynamic results for CaCl 2(aq), including heat capacities and densities, to produce a general model for the thermodynamic properties of CaCl 2(aq) which is valid for molalities to 4.6 mol · kg −1 and pressures to 40 MPa over the temperature range 270 K to 526 K. The model is based on the ion-interaction treatment of Pitzer and allows for a small amount of ion association at the highest temperatures by using the β (2) parameter in the second virial coefficient and making the α 1 parameter linearly dependent on temperature. For temperatures greater than 523 K, a description of the thermodynamic properties of CaCl 2(aq) will require explicit consideration of ion association.

Isopiestic studies of NaHSO 4(aq) at elevated temperatures. Thermodynamic properties

Isopiestic molalities of NaHSO 4(aq) have been measured over the temperature range of 383.46 K to 498.19 K with NaCl(aq) serving as the reference electrolyte for the calculation of excess thermodynamic quantities. Molalities of the reference electrolyte ranged from about 0.5 mol·kg -1 to greater than 6 mol·kg -1. The ion-interaction model gave an excellent description of the experimental results when the solutions were treated as mixed-electrolyte systems resulting from partial hydrolysis of HSO - 4. A simple model for the temperature dependence is capable of reproducing the available experimental results over the temperature range of 298.15 K to 498.19 K.

Corrected values of osmotic and activity coefficients of aqueous NaTcO4 and HTcO4 at 25�C

Published isopiestic molalities for aqueous NaTcO4 and HTcO4 solutions at 25°C have been reexamined. Our calculations indicate that published smoothed values of the osmotic coefficients ϕ and mean molal activity coefficients γ± of NaTcO4 are substantially in error, by up to 100% for ϕ and 300% for γ±. Published smoothed values of ϕ and γ± for HTcO4 solutions are in somewhat better agreement with the input data, but the reported ϕ are sometimes higher and sometimes lower than experimental values, by up to 6%. Consequently, we have reanalyzed isopiestic data for these two systems, and find that errors arose in representing the data mathematically. We report parameters for an extended form of the specific ion interaction equations and for Pitzer's equations, and corrected values of ϕ and γ± are tabulated for aqueous NaTcO4 and HTcO4.

Isopiestic studies of aqueous solutions at elevated temperatures IX. {(1 − y)NaCl + yLiCl}

Isopiestic molalities for the mixed electrolyte solutions {(1 − y)NaCl + yLiCl}(aq) have been measured over the temperature range 383.19 to 523.17 K. NaCl(aq) was used as the isopiestic standard for the calculation of osmotic coefficients. The ion-interaction model for mixedelectrolyte solutions was fitted to the experimental results with acceptable standard deviations. One mixing parameter (in addition to the pure-electrolyte parameters) was adequate to give a satisfactory description of the solutions. The importance of the mixing parameter increased with temperature, contributing as much as 20 per cent to the activity coefficient at 523 K. Excess thermodynamic functions were either independent of temperature or had a simple dependence on temperature.