Pitzers Equations at Elevated Temperatures

We have applied Pitzer s equations at T= 298.15 K, but they are not limited to that temperature and can be applied at any temperature where the coefficients are known.k Table 18.1 (and Table A7.1 of Appendix 7) gives the Debye-Hiickel coefficients AA7, Ah, and Aj as a function of temperature, but the coefficients specific to the electrolyte are tabulated in Appendix 7 only at T = 298.15 K. The usual solution to this problem is to express the coefficients as 

Using the values of the coefficients given by Silvester and Pitzer while holding b and a constant at the T= 298.15 K values, the thermodynamic properties of NaCl solutions as a function of molality are reproduced within experimental error over the temperature range from 273.15 K to 573.15 K. 

The problem with this procedure is that it adds more parameters, although not as many as might be expected, since derivatives of equations (18.46) to (18.48) using equations (18.33), (18.36), and (18.37) give the thermal coefficients / 0 L, /7(I)L, Bj x, and C ,x. Silvester and Pitzer use a total of 16 parameters to represent the thermodynamic properties of NaCl(aq) as a function of the temperature from 273.15 to 573.15 K and molalities from 0 to that of the saturated solution at p = 0.1 MPa (or the saturation vapor pressure). 

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