Ionic strength is a measure of the concentration of dissociated ions dissolved in a polar solvent; in the context of the life sciences, this almost always means aqueous solutions. The two most common dimensions used to express ionic strength are the same as those used to express solute concentration more generally: molarity (moles per liter of solution) and molality (moles per kilogram of solvent).
What distinguishes ionic strength from the sum of the concentrations of the ions present in solution? Ionic strength accounts for the charge of each ion, in addition to the concentration of that ion. The general formula for the molar ionic strength of an ideal solution is:
where I is the molar ionic strength, ci is the molar concentration of a particular ion i, and zi is the charge of that ion.
For instance, an ideal 1 M solution of NaCl has an ionic strength of 1 M, whereas an ideal 1 M solution of MgCl2 has an ionic strength of 3 M, and an ideal 1 M solution of MgSO4 has an ionic strength of 4 M. The ionic strength contributions of multiple salts in an ideal solution are additive, so a solution that is 1 M with respect to NaCl and 1 M with respect to MgSO4 has an ionic strength of 5 M.
These are very simple cases of ionic strength, since they deal with ideal solutions of strong electrolytes. Weak electrolytes, which are critical to biochemistry in their role as pH buffers, are more complicated, and require that dissociation constants be taken into consideration (not to mention temperature and the presence of strong electrolytes in the same solution). Situations involving substantial non-ideal behavior are handled using models such as the Debye-Hückel or Poisson-Boltzmann equations.
In the context of bioprocessing, and particularly in downstream processing, ionic strength is often a critical parameter. In conjunction with pH, ionic strength can have profound effects on protein folding and aggregation. Extreme changes in the salt concentration of a buffer, and consequently in its ionic strength, can cause proteins to precipitate out of solution. In ion exchange chromatography, higher ionic strength buffers disfavor protein binding because dissolved ions disrupt the electrostatic interactions between protein molecules and the charged surface of the resin; this is why high-salt buffers are commonly used as eluants.