Two models of the extraction process have been proposed. In the first model it is assumed that the partitioning neutral species
is at first formed in the aqueous phase and then transferred into the organic phase. The second model is based on the assumption
that equivalent amounts of cations and anions are at first transferred from the aqueous into the organic phase and then associated
to form a neutral molecule. The role of the solubility parameter in extraction and the relation between the solubility of
liquid organic substances in water and the partition of complexes have been discussed. The extraction of simple complexes
and complexes with organic ligands has been discussed using the first model. Partition coefficients have been calculated theoretically
and compared with experimental values in some very simple cases. The extraction of ion pairs has been discussed using the
partition-association model and the concept of single-ion partition coefficients.