where one half is because we are including both cations and anions, ci is the molar concentration of ion i (M, mol/L), 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, 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.
As a more complex example, the ionic strength of a mixed solution 0.050 M in Na2SO4 and 0.020 M in KCl is:
Because in non-ideal solutions volumes are no longer strictly additive it is often preferable to work with molalityb (mol/kg of H2O) rather than molarity c (mol/L). In that case, molal ionic strength is defined as:
Media of high ionic strength are used in stability constant determination in order to minimize changes, during a titration, in the activity quotient of solutes at lower concentrations. Natural waters such as mineral water and seawater have often a non-negligible ionic strength due to the presence of dissolved salts which significantly affects their properties.
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