Nernst distribution law:

The Nernst distribution law is a useful approximation for the distribution of a species in the two phases. Careful experiments reveal two serious shortcomings. The first shortcoming is encountered when the distributing species is involved in chemical reaction such as dissociation or association and is changed in either phase.

A very typical case in this regard is the distribution of benzoic acid between water and benzene. In the aqueous phase, benzoic acid is partly ionized while in benzene it is partly dimerized by hydrogen bonding in the carboxylic group. The pertinent equilibria dealing with this case will be discussed in the next sub-section.

In the extraction of metal ions, the existence of a single species can be properly accounted for while considering the distribution of the species in the two phases. They definitely add complexity to the distribution expression but this does not represent any essential inadequacy of the distribution law itself.