A substantial amount of effort has been expended in developing the theory of electrolyte solutions. Debye and Huckel successfully addressed this issue. A more thorough treatment of this is postponed to a different course. In many relationships, the activity term for a solute is replaced by the concentration of the solute. Activity takes into account deviations from ideal behavior that concentration does not. For ideal solutions, activities are equal to the concentration. The activity of the solute is related to the concentration as:
activity = concentration X activity coefficient
Note that the activity coefficient is not a constant, but depends upon the concentration of the chemical itself, plus all other chemicals in the solution.
For very dilute aqueous solutions at 25 °C, the mean ionic activity coefficient, f±, is related to the ionic strength as:
In this equation, the constant (-0.509) incorporates information about the temperature, dielectric constant of water, and base-10 logarithms among other things. The constant is different based upon solvent and temperature. (Help and example calculations)
At higher concentrations (>0.01 m), a more complex relationship that includes an empirically determined constant is used for activity coefficients.
