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 constant or the solubility of different salts. One of the main characteristics of a solution with dissolved ions is the ionic strength. Ionic strength can be molar (mol/L solution) or molal (mol/kg solvent) and to avoid confusion the units should be stated explicitly. The concept of ionic strength was first introduced by Lewis and Randall in 1921 while describing the activity coefficients of strong electrolytes.
Quantifying ionic strength.
The molar ionic strength, "I", of a solution is a function of the concentration of "all" ions present in that solution.
where one half is because we are including both cations and anions, "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, where each ion is singly-charged, the ionic strength is equal to the concentration. For the electrolyte MgSO4, however, each ion is doubly-charged, leading to an ionic strength that is four times higher than an equivalent concentration of sodium chloride:
Generally multivalent ions contribute strongly to the ionic strength.
Calculation example.
As a more complex example, the ionic strength of a mixed solution 0.050 M in Na2SO4 and 0.020 M in KCl is:
Non-ideal solutions.
Because in non-ideal solutions volumes are no longer strictly additive it is often preferable to work with molality "b" (mol/kg of H2O) rather than molarity "c" (mol/L). In that case, molal ionic strength is defined as: