Gas dissolution

The second example of the application of Henry's law involves the dissolution of the so-called permanent gases in condensed phases. Examples are helium solution in glass and oxygen dissolution in blood. To describe these equilibria, the chemical potentials of the gas (species A) in the two phases are equated. In the gas phase, the chemical potential of A is given by Eq (8.4). In applying Eq (8.3) to A in the condensed phase, however, the assignment of the reference free energy gA(s or l) poses a difficulty. It cannot refer to pure A as a solid or liquid because A is gaseous at all temperatures of interest. Consequently, gA(s or l) in Eq (8.3) is replaced by the molar free energy of A in its normal gaseous state at 1 atm pressure. That is, gA(s or l) is replaced by gA(g). In so doing, the characteristic behavior yA ^ 1 as xA ^ 1 is lost, but this is of no practical consequence because high concentrations of the permanent gases in the condensed phases cannot be attained. With this modification in Eq (8.3), equating with Eq (8.4) yields:

pa Y a where kHA is the Henry's law constant for gas A in the solid or liquid. Note that the definition of kHA is the inverse of that defined in Eq (8.8) for condensable vapors.

The activity coefficient in Eq (8.10), has lost its original meaning as a deviation from solution ideality. yA would not equal 1 even if A and B satisfied the ideality condition in the condensed phase (Sect. 7.3). In the end, all that thermodynamics has been able to elucidate for this case is the proportionality of xA and pA

Table 8.1 lists the Henry's-law constants for atmospheric gases in water. Note that the concentration units are moles per liter (molarity) rather than mole fraction. The former is the customary concentration unit in aqueous solutions

Table 8.1 Henry's-law constants for several gases in water at 25oC (Ref. 2)


Partial Pressure in air, atm

H, atm/(moles/lit)

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Solar Panel Basics

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