An ideal solution may be defined as one in which the interactions between all the species present are equal (equivalent to saying ΔH of mixing is zero).
Ionic solutions are strongly non-ideal, primarily due to the strong, long-range Coulombic interactions between ions.
To compensate for the non-ideality, we introduce the activity of the solute as an effective concentration in all thermodynamic calculations. The concept of activity was first introduced in the discussion of the chemistry of solutions, here, and it is this section that should be consulted for further details.
Recall that the activity, a, of a solute of molality b is given by a = γb/bº
where bº is the molality of the standard state, numerically equal to 1 mol kg-1 , and γ is the activity coefficient for the solution at that particular temperature and molality.
For any ionic solution, there are activity coefficients γ+ and γ– for the cations and anions respectively, the deviation from ideality of the solution being related to the product of these two quantities. There is no experimental way of separating the product γ+γ– into distinct contributions from the cations and anions, so a compromise is adopted whereby a mean activity coefficient, γ± , is defined to assign responsibility for the non-ideality equally to both types of ion. For a compound MpXq which dissolves in solution to give p cations and q anions for each formula unit, we define