Mean Activity Coefficients and Osmotic Coefficients in Aqueous Solutions of Salts of Ammonium Ions with Univalent Anions at 25 °C

Abstract

The Hückel equation used in this study to describe the thermodynamic properties of dilute solutions of the ammonium salts of NH4Cl, NH4Br, NH4I, NH4NO3, NH4SCN, and NH4H2PO4 up to 1.5 mol·kg–1 is consisted of two electrolyte-dependent parameters B and b1. Parameter B is linearly related to the ion-size parameter a* in the Debye–Hückel equation, and parameter b1 is the coefficient of the linear correction term with respect to the molality. This coefficient is associated with the hydration numbers of the ions forming the electrolyte. For NH4ClO4 solutions, the estimated Hückel equation applies up to 2.1 mol·kg–1, and the equation was obtained from the isopiestic data measured by Esval and Tyree (J. Phys. Chem.1962, 66, 940–942) against KCl solutions. For molalities above 1.5 mol·kg–1 (in the best case up to 10 mol·kg–1), an extended Hückel equation was used. For this equation, the Hückel equation is supplemented with a quadratic term in molality, and the coefficient of this term is parameter b2. All of the parameters for the Hückel equations of NH4Cl and NH4NO3 solutions were estimated from isopiestic data of Wishaw and Stokes (Trans. Faraday Soc.1953, 49, 27–31). The former data were measured against KCl solutions and the latter against NaCl solutions. The Hückel parameters for NH4Br, NH4I, NH4SCN, and NH4H2PO4 solutions were estimated from the data measured by Covington and Irish (J. Chem. Eng. Data1972, 17, 175–176), Bonner (J. Chem. Eng. Data1976, 21, 498–499), Covington and Matheson (J. Solution Chem.1977, 6, 263–267), and Filippov et al. (J. Appl. Chem., U.S.S.R., 1985, 58, 1807–1811), respectively, using the same experimental technique against NaCl solutions. In all of these estimations, the Hückel parameters of a recent KCl and NaCl study (J. Chem. Eng. Data2009, 54, 208–219) were used for the solutions of the reference electrolyte. In the tests of the new parameter values, the cell potential difference, vapor pressure, and isopiestic data available in the literature were used. These data support well the tested Hückel parameters in most cases at least up to 3.5 mol·kg–1. Reliable thermodynamic activity quantities for ammonium salt solutions are, therefore, obtained using the new Hückel parameters. The activity coefficients, osmotic coefficients, and the vapor pressures obtained using these equations are tabulated here at rounded molalities. These values were compared to those suggested by Robinson and Stokes (Electrolyte Solutions, 2nd ed.; Butterworths Scientific Publications: London, 1959), to those obtained using Pitzer equations (Activity Coefficients in Electrolyte Solutions, 2nd ed.; CRC Press: Boca Raton, 1991; pp 100–101), and to those obtained using the extended Hückel equations presented by Hamer and Wu (J. Phys. Chem. Ref. Data1972, 1, 1047–1099).