Ionic strength

The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as the dissociation or the solubility of different salts. One of the main characteristics of a solution with dissolved ions is the ionic strength.

Quantifying ionic strength

The ionic strength, I, of a solution is a function of the concentration of all ions present in that solution.

where c_i is the molar concentration of ion i (M, mol/L), z_i is the charge number of that ion, and the sum is taken over all ions in the solution. For a 1:1 electrolyte such as sodium chloride, the ionic strength is equal to the concentration, but for MgSO₄ the ionic strength is four times higher. Generally multivalent ions contribute strongly to the ionic strength.

Calculation example

For example the ionic strength of a mixed 0.050 M in Na₂SO₄ and 0.020 M in KCl solution is: