Ion association

Ion association is a concept which was introduced by Niels Bjerrum and describes that mutually charged ions come very close in solutions due to their electrostatic attraction and thereby form a kind of unit, the Bjerrum ion pair. Bjerrum characterizes two ions (arbitrarily) as associated when they are closer than they come (with the valences of the two ions and the Bjerrum length of the surrounding medium). Bjerrum introduces a reaction constant for the dissociation of an ion pair , which describes the following reaction of the association to an ion pair: ${\displaystyle R=z_{1}z_{2}\lambda _{B}}$ ${\displaystyle z_{i}}$ ${\displaystyle \lambda _{B}}$ ${\displaystyle K}$

A ^{n +} + B ^{m -} AB ^{( n - m ) +} ${\displaystyle \rightleftharpoons }$

In the case of aqueous solutions of salts, the phenomenon of ion association must always be considered. This applies all the more to solvents with a lower dielectric constant than water, since the electrostatic interaction is stronger there.

Furthermore, in Lars Onsager's analyzes of the second Wien effect, Bjerrum ion pairs and thus ion association play a role.

Demarcation

A similar concept of bound ions is counterion condensation . However, counterion condensation does not describe the interaction of two ions (like ion association), like ion association, but the interaction between a charged rod and its counterions.

Individual evidence

1. ↑ Niels Bjerrum: STUDIES ON ION ASSOCIATION. I. 1926, accessed in 2018 . 
2. ↑ Niels Bjerrum: STUDIES ON ION ASSOCIATION. I. 1926, accessed in 2018 .