# Programming guide for the Extended UNIQUAC activity coefficient model

## Free Programming guide for the Extended UNIQUAC model

A programming guide showing step by step how to implement The Extended UNIQUAC activity coefficient model can be downloaded as pdf file by clicking the button below or the image to the right.

The guide includes the extended Debye-Hückel term and the combinatorial and residual terms of the UNIQUAC model.

Activity coefficients are derived as molar derivatives of the excess Gibbs energy function. The composition derivatives of the activity coefficients are also given.

Enthalpy terms are derived as temperature derivatives of the activity coefficients. Heat capacity terms are second temperature derivatives of the activity coefficient terms.

### The activity coefficient model is only the first step

While the activity coefficient model is a fundamental requirement for calculation of salt precipitation, it is not sufficient. Properties like osmotic coefficients and mean activity coefficients are described on the “Properties of salt solutions” page. Such properties can be calculated with the model under the assumption of complete dissociation. When partly dissociated salts like carbonates, phosphates, borates, and even sulfates are considered, a speciation equilibrium calculation is required before any properties can be calculated.

In order to perform a speciation equilibrium calculation, a solid-liquid equilibrium calculation or a vapor-liquid equilibrium calculation, standard state properties at the relevant temperatures need to be calculated. One method for performing these equilibrium calculations was described in the PhD thesis of Kaj Thomsen: https://doi.org/10.11581/dtu:00000074