Ionic strength

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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 related to it.

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

With

  • : Molar concentration of the ion type (in mol / l)
  • : Molality of the ion type (in mol / kg)
  • : Number of charges of the type of ion .

Since the ions charge in the square in the ionic strength is received, a doubly charged ion provides four times the contribution to the ionic strength in comparison with a monovalent ion at the same concentration.

Examples

In the case of singly charged ions, the ionic strength of completely dissociated electrolytes is equal to the salt concentration. For a saline solution with (NaCl) = 0.001 mol / l, the concentration of the two types of ions Na + and Cl - is also 0.001 mol / l. The ionic strength is due to (Na + ) = 1 and (Cl - ) = −1:

In the case of a 1: 2-valent or 2: 1-valent electrolyte, for example calcium chloride , the ionic strength is three times the salt concentration. For example, for calcium chloride with the charge numbers (Ca 2+ ) = 2 and (Cl - ) = −1 and the stoichiometric ratios (CaCl 2 ) = (Ca 2+ ) = (Cl - ):

meaning

The ionic strength was introduced as a practicable quantity in electrochemistry when the Debye-Hückel theory was developed. This theory shows that the mean activity coefficients in dilute solutions depend on the root of the ionic strength and, for example, gives the following formula for dilute aqueous solutions at 25 ° C :

With

literature

  • Application of the ionic strength I in the Kohlrausch square root equation for the calculation of equivalent conductivities as well as the calculation of activity coefficients of ions in salt solutions; In: Kunze / Schwedt: Fundamentals of qualitative and quantitative analysis, Thieme Verlag Stuttgart, 1996, p. 270 and 47, ISBN 3-13-585804-9

Individual evidence

  1. ^ GN Lewis, M. Randall, J. Am. Chem. Soc. , 43, 1921 , 1112.
  2. ^ S. Glasstone, An Introduction To Electrochemistry , 2007 , 140.
  3. Pure Appl. Chem. , 68 (4), 1996 , 957.