Sodium chloride thermodynamic properties

Calcium-sodium-chloride-type brines (which typically occur in deep-well-injection zones) require sophisticated electrolyte models to calculate their thermodynamic properties. Many parameters for characterizing the partial molal properties of the dissolved constituents in such brines have not been determined. (Molality is a measure of the relative number of solute and solvent particles in a solution and is expressed as the number of gram-molecular weights of solute in 1000 g of solvent.) Precise modeling is limited to relatively low salinities (where many parameters are unnecessary) or to chemically simple systems operating near 25°C. 

As shown in Fig. 2 [37], and also in the work of Barraclough and Hall [34], moisture uptake onto sodium chloride as a function of relative humidity is reversible as long as RH0 is not attained. This is evidence that actual dissolution of water-soluble crystalline substances does not occur below RH0. This is consistent with thermodynamic rationale that dissolution below RHo would require a supersaturated solution (i.e., an increased number of species in solution would be necessary to induce dissolution at a relative humidity below that of the saturated solution, RH0). In this regard, one should only need to consider the solid state properties of a purely crystalline material below RH0. As will be described, other considerations are warranted for a substance that contains amorphous material. 

Silvester, L. F. Pitzer, K. S. "Thermodynamics of Electrolytes. 8. High-Temperature Properties, Including Enthalpy and Heat Capacity with Application to Sodium Chloride" J. Phys. Chem., 1977, 81, 1822. 

Equations used to calculate L and 4>CP are taken from K. S. Pitzer, Ion interaction approach theory and data correlation , Chapter 3 in Activity Coefficients in Electrolyte Solutions, 2nd Edition, K. S. Pitzer, Editor, CRC Press, Boca Raton, Florida, 1991. Equations for calculating L, L2, Ju and J2 are summarized in K. S. Pitzer, J. C. Peiper, and R. H. Busey, Thermodynamic properties of aqueous sodium chloride solutions , J. Phys. Chem. Ref Data, 13, 1-102 (1984). 

L. F. Silvester and K. S. Pitzer, Thermodynamics of electrolytes. 8. High-temperature properties, including enthalpy and heat capacity, with application to sodium chloride , J. Phys. Chem., 81, 1822-1828 (1977). 

The thermodynamic treatment of systems in which at least one component is an electrolyte needs special comment. Such systems present the first case where we must choose between treating the system in terms of components or in terms of species. No decision can be based on thermodynamics alone. If we choose to work in terms of components, any effect of the presence of new species that are different from the components, would appear in the excess chemical potentials. No error would be involved, and the thermodynamic properties of the system expressed in terms of the excess chemical potentials and based on the components would be valid. It is only when we wish to explain the observed behavior of a system, to treat the system on the basis of some theoretical concept or, possibly, to obtain additional information concerning the molecular properties of the system, that we turn to the concept of species. For example, we can study the equilibrium between a dilute aqueous solution of sodium chloride and ice in terms of the components water and sodium chloride. However, we know that the observed effect of the lowering of the freezing point of water is approximately twice that expected for a nondissociable solute. This effect is explained in terms of the ionization. In any given case the choice of the species is dictated largely by our knowledge of the system obtained outside of the field of thermodynamics and, indeed, may be quite arbitrary. 

Figure 43. Thermodynamic transfer properties for sodium chloride on going from a solution in water to t-butyl alcohol + water mixtures at 298 K (Pointud et al., 1974).

Pilling MJ, Seakins PW (1995) Reaction Kinetics. Oxford University Press, Oxford, UK Pitzer KS, Peiper JC, Busey RH (1984) Thermodynamic properties of aqueous sodium chloride solutions. J Phys Chem Ref Data 13 1-102... 

The electrolyte in the measurements of the thermodynamic properties of bismuth sele-nide and telluride and of antimony telluride was the easily melted mixture of anhydrous zinc chloride (analytic purity) with sodium and potassium chlorides (chemical purity grade). The melting point of this mixture was Tmp — 208 C. The thermodynamic properties of antimony selenide were determined using a mixture of aluminum chloride (distilled twice in vacuum) and sodium chloride (chemical purity grade). The meltii point of this mixture was Tmp = 150-155°C. 

K. S. Pitzer, J. C. Peiper, R. H. Busey, "Thermodynamic Properties of Aqueous Sodium Chloride Solutions," /. Phys. Chem. Ref Data, 13,1 (1984). 

Ciiss, C. M. and Cobble, J. W., 1961, The thermodynamic properties of high temperature aqueous solutions. I. Standard partial molar heat capacities of sodium chloride and barium chloride from 0 to 100 °C. Jr. Amer. Chem. Soc., 83 3223-8. 

Cowley ER, Gong Z, Horton GK (1990) Theoretical study of the elastic and thermodynamic properties of sodium chloride under pressure. Phys Rev B 41 2150-2157 Creager KC (1992) Aninsotropy of the inner core from differential travel times of the phases PKP and PKIKP. Nature 356 309-314... 

Thermodynamic Properties of Aqueous Sodium Chloride Solutions Journal of Physical and Chemical Reference Data, in review... 

Povodyreveta/. (1997) have developedasix-term Landau expansion crossover scaling model to describe the thermodynamic properties of near-critical binary mixtures, based on the same model for pure fluids and the isomorphism principle of the critical phenomena. The model describes densities and concentrations at vapor-liquid equflibrium and isochoric heat capacities in the one-phase region. The description shows crossover from asymptotic Ising-hke critical behavior to classical (mean-field) behavior. This model was applied to aqueous solutions of sodium chloride. 

Lewis, J.W.E. and Singer, K. (1975) Thermodynamic properties and self-diffusion of molten sodium chloride. A molecular dynamics study. J. Chem. Soc., Faraday Trans. 2, 71, 41—53. 

The various examples that have just been presented show that it is possible, with limited experimental effort, to be able to prediet the evolution of the physical-chemical properties of liquid biological media or solid foods. It then becomes easy to be able to quickly calculate the elTeet of composition changes on the pH or the aw of the food, without going through tests expensive in time and money. Among the requests made by health authorities, the reduction of the quantities of sodium chloride has been included. The thermodynamics software allows us to quickly check the effect of such a reduction and to test the substitution of sodium chloride by potassium chloride or by any other salt or mixture of salts. It can be used to determine the optimal substitution rate which only requires the test of one or two solutions. Nonetheless, the most promising use of the thermodynamic model is its incorporation in process simulators. 

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