Solids in Chemical Equilibria
Solids have names ending with either:
- (cr) or (c) for crystalline;
- (am) for amorphous; or
- (s) for solid (unknown crystallinity).
For solids, like for aqueous complexes, the logK-value for
the formation reaction is used. Note that this is the inverse of the
solubility product, i.e.: logKf =
−logKsp
Owing to the Gibbs phase rule, the number of solids that may be present
simultaneously in a chemical system is limited.
Unlike aqueous species, which have concentrations that may become gradually
small, solid phases either are stable, or not. The concentration usually changes
in a narrow range of conditions between the maximum
possible amount, and zero.
If a solid is present in a system, its composition is constant, and its
activity may be set equal to one (log activity = 0).
Its concentration in mol/(L of solution) may be calculated and plotted.
If the solid is not present, its concentration is zero, and its activity is undefined.
In that case the programs calculate the saturation index
(SI) instead of the activity:
SI = logKf°
− log IAP
where Kf is the equilibrium constant for the
formation of the solid, and IAP is the
ion activity product for the solid (the actual product of
activities as defined in the equilibrium constant of formation).