Ionic strength
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The ionic strength of a solution is a function of the concentration of all ions present in a solution.
where ci is the molarity concentration of ion i, zi is the charge 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.
Because in non-ideal solutions volumes are no longer strictly additive it is often preferable to work with molality rather than molarity. In that case we define:
[edit] Importance
The ionic strength plays a central role in the Debye-Hückel theory that describes the strong deviations from ideality typically encountered in ionic solutions. It is also important for the description of colloids
[edit] Ionic replacement
In the study of solution reaction kinetics it is often desirable to change the concentration of an active species without changing the ionic strength, because the latter affects the value of the rate constant k. This can often be done by replacement. If for example the [S2O82-] is to be lowered, the solution is diluted with a solution containing the same concentration of a non-active species of equal charge e.g. SO42- rather than just solvent. This produces a solution with a lower [S2O82-] but of the same ionic strength.
