Ionic strength

The ionic strength (symbol, in the older literature also μ ) of a solution is a measure of the electric field strength due to dissolved ions. The chemical activity of dissolved ions and the conductivity of electrolyte solutions are with her in context.

According to the recommendations of the IUPAC, the ionic strength can be defined both on the molar concentration and the molality of the dissolved ions:

With

• : Molar concentration of the ion species ( in mol / l)
• : Molality of the ion species ( in mol / kg)
• : Charge number of the ion species.

Since the ion charge received in the ionic strength squared, provides a doubly charged ion as compared to a monovalent ion at the same concentration, four times the contribution to ionic strength.

Examples

For singly charged ions, the ionic strength is in completely dissociated electrolyte equal to the salt concentration. A saline (NaCl ) = 0.001 mol / l, the concentration of the ionic species Na and Cl - are also 0.001 mol / l The ionic strength is due to (Na ) = 1 and (Cl - ) = -1:

In a 1:2 or 2:1 valent valent electrolytes, such as calcium chloride, the ionic strength is equal to three times the salt concentration. For example, for the calcium chloride ( (Ca2 ) = 2 and ( Cl ) = -1):

Importance

The ionic strength was introduced in the development of the Debye- Hückel theory as a viable size in the electrochemistry. This theory shows that the mean activity coefficients depend in dilute solutions of the root of the ionic strength, and provides, for example, dilute aqueous solutions at 25 ° C the following formula:

With