Solubility of solids:
Dissolution or precipitation?
The solubility of oxides, hydroxides or oxyhydroxides 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. See for example:
D Langmuir, Aqueous Environmental Geochemistry. Prentice-Hall, Inc., 1997.
Sect.7.7 and Chapter 12, diagrams on
p. 255 & 438.
The following diagram shows that for Fe(III) the difference is a factor
of almost 105:
The endings “(am)” and “(cr)” in the formulas are
phase indicators.
The diagram shows that [Fe(III)]TOT ≈
10−12.3 when haematite
(Fe2O3) 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
Modelling 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.
Exclude solids from 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 as explained below.
For advanced users:
Search preferences for solids in DATABASE:
If you edit the ini-file DataBase.ini with a text editor,
and change the entry All_Solids_Ask to true (or set advancedMenu = true),
and then re-start DATABASE, the menu “Options / Preferences / Solids”
will be available. Selecting this menu option will show a secondary window where
you can choose between:
- Include all solids when searching the databases. This option is the default.
This should be choosen to model processes where solids are added and their
dissolution process is being studied.
- Exclude solid names ending with “(cr)”. This option
should be chosen when modelling precipitation processes.
See also
