Thermodynamic Treatment of Solubility

The solubilities of gases in liquids may be expressed in various ways. In the definition of the Bunsen coefficient the solubility is expressed as the volume of the ideal gas reduced to O C and 1 atm pressure soluble in a unit volume of liquid under the gas pressure of 1 atm and at the temperature of measurement. The Ostwald solubility ki — Vgl V oiy is defined as the ratio of the volume of absorbed gas to the volume of the absorbing liquid, measured at the same temperature. The Bunsen solubility coefficient a and the Ostwald coefficient L on the assumption of ideal gas behavior are related to the simple g-moles/liter concentration scale c by the relationships which are applicable when P = 1 atm  

Solubilities are also expressed as mole fractions, as gram-moles of gas per gram-mole of liquid, or as weight percent. 

Considering the solubility of a simple gaseous molecule Y in a molten salt, the equilibrium may be described by the equation 

For a rigorous thermodynamic treatment it is necessary to choose the reference state of the gas as the pure gas at unit fugacity and any temperature T, and for the gas in solution its state as a pure liquified gas A under atmospheric pressure and any temperature T. This method may be followed for systems which may be liquified at high temperatures and low pressures. 

AHt also represents the enthalpy change during removal of 1 g-mole of dissolved gas from the solution. 

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A rigorous thermodynamic treatment of solubility of anhydrous and hydrated solutes (electrolytes and nonelectrolytes) in water or in other solvent was given in 1944 by Williamson [88]. In a general form, the expression for the molar enthalpy of solution AH j of anhydrous or hydrated solute is... 

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