2. Physico-chemical concepts

2.1 Chemical equilibria

One of the main tasks of hydrogeochemical software is to calculate the spectrum of aqueous species for a given chemical configuration, with the concentration of each species depending on solution characteristics such as pH, redox potential, alkalinity, ionic strength or temperature. The first software programs used the thermodynamic equilibrium approach. Indeed, many chemical reactions in the aqueous phase are rapid on the time scale of the industrial process or the natural environment, and can therefore be considered to be at equilibrium.

There are, however, a number of specific reactions (redox reactions, precipitation of solids, etc.) which are controlled by kinetics over the reference period. Some designers have therefore decided...