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 or the solubility of different salts. One of the main characteristics of a solution with dissolved ions is the ionic strength.

Quantifying ionic strength

The ionic strength, I, of a solution is a function of the concentration of all ions present in that solution.

$I = \begin{matrix}\frac{1}{2}\end{matrix}\sum_{{\rm i}=1}^{n} c_{\rm i}z_{\rm i}^{2}$

where c_i is the molar concentration of ion i (mol·dm^-3), 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, 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.

For example the ionic strength of a mixed 0.050 mol dm^-3 in Na₂SO₄ and 0.020 mol dm^-3 in NaCl solution is:

I = 1/2((2 × (+1)² × 0.050) + (+1)² × 0.020 + (−2)² × 0.050 + (−1)² × 0.020) = 0.17 mol·dm^-3

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{H₂O}) rather than molarity c (mol/L). In that case, ionic strength is defined as:

$I = \begin{matrix}\frac{1}{2}\end{matrix}\sum_{{\rm i}=1}^{n} b_{\rm i}z_{\rm i}^{2}$

i = individual element z = charge of element

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 theory of double layer and related electrokinetic phenomena and electroacoustic phenomena in colloids and other heterogeneous systems. That is, the Debye length, which is the inverse of the Debye parameter (κ), is inversely proportional to the square root of the ionic strength. Debye length is characteristic of the Double layer thickness. Increasing the concentration or valence of the counterions compresses the double layer and increases the electrical potential gradient.

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 seawater have a non-zero ionic strength due to the presence of dissolved salts which significantly affects their properties.

See also

• Activity (chemistry)
• Activity coefficient
• Double layer (interfacial)
• Double layer (electrode)
• Electrical double layer

External links

• Ionic strength
• pH calculation — ionic strength and activity coefficients

References

• IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "ionic strength, I".
• Debye, P.; Hückel, E. (1923). "Zur Theorie der Elektrolyte. I. Gefrierpunktserniedrigung und verwandte Erscheinungen [The theory of electrolytes. I. Lowering of freezing point and related phenomena]" (PDF). Zeitschrift für Physik 24: 305–324. http://electrochem.cwru.edu/estir/hist/hist-12-Debye-1.pdf. 
• Skoog, D.A.; West D.M., Holler F.J., Crouch S.R. (2004). Fundamentals of analytical chemistry. Brooks/Cole Pub Co. ISBN 0030584590.