SPANA / Modify Chem. System

Example of a solubility diagram for a specific solid phase.

If the the solubility of a solid, for example AlOOH (boehmite), has to be calculated, it is necessary to set the activity = 1 (log activity = 0) for this solid. Note that the most stable Al solid at 25°C is instead Al(OH)₃ (gibbsite). Unless the activity of AlOOH is fixed equal to one, Al(OH)₃ would precipitate in the calculations.

See also:

• Solubility of solids: dissolution or precipitation?
• Solubility diagram for Al(OH)₃ (gibbsite)

To set the activity = 1 for a solid you must first exchange a component for a reaction; in this case, exchange Al³⁺ with AlOOH(s).

The diagram shows the total dissolved concentration of Al (on a logarithmic scale) against H⁺:

This diagram was obtained with the following input file:

 2 8 3 1
H+,  AlOOH(s)
Al(OH)2+     , -2.337   1  1  Al(OH)3      , -7.057   0  1
Al(OH)4-     , -14.387 -1  1  Al2(OH)2 4+  ,  9.106   4  2
Al3(OH)4+5   ,  11.269  5  3  AlOH+2       ,  3.433   2  1
OH-          , -14.002 -1  0  Al 3+        ,  8.403   3  1
Al(OH)3(am)  , -2.397   0  1  Al(OH)3(cr)  ,  0.703   0  1
Al2O3(cr)    , -2.844   0  2  LS,-9, 1, H+,
LAV, -2 -12  LA, 0