Hydra  DATABASE

Solids: Solubility

The solubility of oxides, hydroxides or oxihydroxides for many transition metals is strongly dependent on the particle size and crystallinity of the solid.

For Fe(III), Al(III), etc, there is a large difference between the solubilities of crystalline oxides and amorphous hydroxides. The amorphous solids precipitate when the oversaturation is high, as when mixing solutions in the laboratory. The amorphous solids will slowly re-crystallise and form more insoluble (more stable) solids.

The following diagram shows that for Fe(III) the difference is a factor of almost 105:

Diagr_Solub_Fe(III)
The endings “(am)” and “(cr)” in the formulas are phase indicators. The diagram shows that [Fe(III)]TOT10−12.3 when haematite (Fe2O3(cr)) is in equilibrium with an aqueous solution of pH = 8. The diagram also shows that you can have a 10−5 mM solution of Fe(III) at pH 2 without precipitation: the solution is metastable. If the pH is increased, amorphous Fe(OH)3 precipitates at pH about 4.3

Dissolution or precipitation?

To model the precipitation of Fe(III), the diagram above shows that the amorphous hydroxide, Fe(OH)3(am), should be included in the calcualtions, but the crystalline phases should be excluded (Fe2O3(cr) and FeOOH(cr)). On the other hand, to model a dissolution reaction the dissolving solid must be included in the calculations.

To exclude solids, they can be deactivated in the calculations, see deactivate spcies in SPANA. Alternatively, you can exclude “(cr)”-solids from the search in DATABASE.

DATABASE: Search preferences for solids

The menu “Options / Preferences / Solids” will display a secondary window where the user can choose between:

See also