On the derivation and extension of the uniquac equation

The UNIQUAC equation is derived by phenomenological arguments based on a two-fluid theory. This derivation is free of the inconsistencies which arise when a lattice one-fluid theory is used to derive UNIQUAC or any other local-composition equation. The essential step in the derivation is the adoption of Wilson's assumption that local compositions can be related to overall compositions through Boltzmann factors.In UNIQUAC, the energy terms in the Boltzmann factors are assumed identical to those in the excess energy of mixing. When the assumption of identity is replaced by the much less stringent assumption of proportionality, a three-parameter UNIQUAC equation is derived. This equation often gives a better fit of experimental data whenever all three binary paramters are freely adjusted. Upon assigning a universal value to the third parameter, only marginal improvement is obtained.