Ionic Strength - Quantifying 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 (mol·dm−3), 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.

For example the ionic strength of a mixed 0.050 mol dm−3 in Na₂SO₄ and 0.020 mol dm−3 in NaCl solution is:

I = 1/2((2 × (+1)2 × 0.050) + (1 × (−1)2 × 0.020) + (1 × (−2)2 × 0.050) + (1 × (+1)2 × 0.020)) = 0.17 mol·dm-3