Re-Evaluation of the Thermodynamic Activity Quantities in Aqueous Alkali Metal Bromide Solutions at 25 °C

Abstract

The Hückel equation used in this study to correlate the experimental activities of dilute alkali metal bromide solutions up to a molality of about 1.5 mol·kg−1 contains two parameters that are dependent on the electrolyte: B [that is related closely to the ion-size parameter (a*) in the Debye−Hückel equation] and b1 (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 a molality of about 5 mol·kg−1, an extended Hückel equation was used, and it contains additionally a quadratic term with respect to the molality, and the coefficient of this term is parameter b2. All parameter values for the Hückel equations of LiBr, KBr, RbBr, and CsBr were determined from the isopiestic data measured by Robinson for solutions of these salts against KCl solutions (J. Am. Chem Soc. 1935, 57, 1161−1165), and all parameters for NaBr were determined from the isopiestic data measured by Robinson for KCl and NaBr solutions (Trans. Faraday Soc. 1939, 35, 1217−1220). In these estimations, the Hückel parameters determined recently for KCl 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 alkali metal bromide solutions. Most of these data can be reproduced within experimental error by means of the extended Hückel equations up to a molality of about 5.0 mol·kg−1. Reliable activity and osmotic coefficients for alkali metal bromide 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 Robinson and Stokes (Electrolyte Solutions, 2nd ed.; Butterworths Scientific Publications: London, 1959), to those calculated by using the Pitzer equations with the parameter values of Pitzer and Mayorga (J. Phys. Chem. 1973, 77, 2300−2308), and to those calculated by using the extended Hückel equations of Hamer and Wu (J. Phys. Chem. Ref. Data 1972, 1, 1047−1099).