Quantifying Ionic Strength
The ionic strength, I, of a solution is a function of the concentration of all ions present in that solution.
where ci is the molar concentration of ion i (mol·dm−3), zi 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
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