Re-Evaluation of the Thermodynamic Activity Quantities in Aqueous Rubidium and Cesium Chloride Solutions at 25 °C

Abstract

The Hückel equation used in this study to correlate the experimental activities of dilute RbCl and CsCl solutions up to a molality of about 3.5 mol·kg−1 contains two parameters being dependent on the electrolyte: B [that is related closely to the ion-size parameter (a*) in the Debye−Hückel equation] and b 1 (this parameter is the coefficient of the linear term with respect to the molality, and this coefficient is related to hydration numbers of the ions of the electrolyte). In more concentrated solutions up to the saturated molality of RbCl (= 7.78 mol·kg−1) and up to a molality of about 8 mol·kg−1 for CsCl, an extended Hückel equation was used. It contains additionally a quadratic term with respect to the molality, and the coefficient of this term is parameter b 2. All parameter values for the Hückel equations of RbCl were determined from the isopiestic data measured by Rard for NaCl and RbCl solutions (J. Chem. Eng. Data 1984, 29, 443−450) and all parameters for CsCl from the isopiestic data measured by Rard and Miller for NaCl and CsCl solutions (J. Chem. Eng. Data 1982, 27, 169−173). In these estimations, the Hückel parameters determined recently for NaCl solutions (J. Chem. Eng. Data 2009, 54, 208−219) were used. The resulting parameter values were tested with the cell potential, vapor pressure, and isopiestic data existing in the literature for RbCl and CsCl solutions. Most of these data can be reproduced within experimental error by means of the extended Hückel equation up to a molality of about 8.0 mol·kg−1. Reliable activity and osmotic coefficients for RbCl and CsCl solutions can, therefore, be calculated by using the new Hückel equations, and they have been tabulated here at rounded molalities. The activity and osmotic coefficients obtained from these equations were compared to the values suggested by Rard (RbCl, see citation above), Rard and Miller (CsCl, see citation above), and Robinson and Stokes (Electrolyte Solutions, 2nd ed.; Butterworths Scientific Publications: London, 1959). These values were also compared to those calculated by using the Pitzer equations with the parameters of Pitzer and Mayorga (J. Phys. Chem. 1973, 77, 2300−2308) and Pitzer (Activity Coefficients in Electrolyte Solutions, 2nd ed.; CRC Press: Boca Raton, 2000; pp 100−101) and to those calculated by using the extended Hückel equation of Hamer and Wu (J. Phys. Chem. Ref. Data 1972, 1, 1047−1099).

Re-evaluation of the thermodynamic activity quantities in aqueous alkali metal nitrate solutions at T= 298.15 K

“…In the previous studies, it was found that the following Hückel equations apply very well to the thermodynamic properties of NaCl [11], KCl [11], LiCl [19], RbCl [20], CsCl [20], and alkali metal bromide [21] solutions at least up to a molality of 1 mol Á kg À1 :…”

“…In more concentrated solutions, the following extended Hückel equations were used here as earlier [11,[18][19][20][21] for the activity and osmotic coefficients:…”

“…These values together with equation (5) and (6)) of alkali metal nitrates by the least-squares fitting using equation (19) for LiNO 3 , NaNO 3 , and KNO 3 data and using equation (20) for RbNO 3 data. Taken from table 3 …”

“…The vapour pressure of an alkali metal chloride or nitrate solution is thus compared to that of the potassium chloride solution at the same molality. The recommended osmotic coefficients were taken for NaCl and KCl solutions from reference [11], for LiCl solutions from reference [19], and for RbCl and CsCl solutions from reference [20]. The deviations for CsNO 3 solutions (values are given only up to 1.4 mol Á kg À1 ) are not in this figure discernible from those of RbNO 3 or KNO 3 solutions.…”

“…These activity and osmotic coefficients were compared to those of the previous investigations (some of which, in addition to Robinson and Stokes' values [2], have achieved wide acceptance). Activity coefficient deviations in this comparison are presented as the cell-potential deviations for galvanic cells without a liquid junction (in the same way as in references [8,11,18]) and the osmotic coefficient deviations are presented as vapour pressure deviations (as in references [11,[19][20][21]). …”