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Chapitre 5. Carrying on
entire precipitation process or to disable precipitation of one or several solids. Disabling precipitation of specific species is different from excluding these species : the former option only skips
the test on the saturation state of the mineral, while keyword exclude eliminates a species from
the database used by the run. Accordingly, disabling precipitation allows to examine saturation
states by means of the saturation indices.
Note : Although colloidal and mineral hydrous ferric oxides have identical formation constants,
colloidal species will precipitate first. This choice of CHESS has been made by purpose, since
we expect that colloidal phases are formed prior to minerals. One may dislike this choice and
alter CHESS’ behavior by slightly changing the formation constants of one of the solids in the
Database panel.
5.3 Redox disequilibrium
By default, CHESS assumes no redox equilibrium, since CHESS has its origins in modelling
of laboratory systems where redox equilibrium is not likely to be reached. Redox is enabled,
however, as soon as the Eh or pe is set. But more sophisticated management of the redox state
is possible. The assumption of thermodynamic equilibrium in the case of redox-sensitive species
of a geochemical system is not always justified in natural waters, especially at low temperature.
In addition, an Eh measurement may reflect the predominant redox couple, or at least the fastest
reacting couple during the measurement, but it does not forcedly represent the redox potential
for all the other redox couples. For each solution, a detailed management of the redox behavior
is proposed. You can set the redox potential manually if, for example, the Eh is unknown but you
know the concentration of one or several redox-sensitive species.
5.3.1 Pyrite dissolution
For example, we might be interested in knowing the equilibrium state of pyrite in pure water. We
introduce 1 gram of mineral Pyrite in the main solution and enable redox reactions. Run CHESS
and study the general report. Here is a part of it :
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