text: 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 is the molar concentration of ion i (M, mol/L), z 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 MgSO, 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.