Predominance Area Diagrams

A predominance area diagram shows the species formed with a given “main” component as a function of two variables. The following rules are used:

A solid is always shown on top of aqueous species, even when the solid has a very small concentration,
If more than one solid are formed, the solid with the largest fraction of the main component is shown,
If no solid is formed, the diagram shows the aqueous species which has the largest fraction of the main component.

A special type of predominance area diagram is the potential-pH diagram, also called a Pourbaix diagram. Note that electrons (e⁻) have to be selected as one of the components in the chemical system. The following figure shows a potential-pH diagram for iron (also called Pourbaix diagram):

This diagram was obtained with the following input file:

 3 21 8 0
H+, e-, Fe 2+
Fe 3+         , -13.051    0 -1  1  Fe(OH)2       , -20.8     -2  0  1
Fe(OH)3-      , -33.4     -3  0  1  Fe(OH)4-2     , -46.35    -4  0  1
FeOH+         , -9.1      -1  0  1  H2            , -3.083     2  2  0
H2(g)         ,  0         2  2  0  H2O2          , -59.61    -2 -2  0
O2            , -85.989   -4 -4  0  O2(g)         , -83.091   -4 -4  0
O3            , -155.14   -6 -6  0  O3(g)         , -153.23   -6 -6  0
OH-           , -14.002   -1  0  0  Fe(OH)2+      , -17.851   -2 -1  1
Fe(OH)3       , -25.051   -3 -1  1  Fe(OH)4-      , -34.651   -4 -1  1
Fe2(OH)2+4    , -28.922   -2 -2  2  Fe3(OH)4+5    , -45.453   -4 -3  3
FeO4-2        , -125.901  -8 -4  1  FeOH 2+       , -15.201   -1 -1  1
HO2-          , -71.26    -3 -2  0
Fe(cr)        , -15.89     0  2  1  Fe(OH)2(am)   , -13.5     -2  0  1
Fe(OH)2(cr)   , -12.996   -2  0  1  Fe(OH)3(s)    , -16.081   -3 -1  1
Fe0.932O(cr)  , -13.51    -2 -0.136  0.932 /wüstite
Fe3O4(cr)     , -36.44    -8 -2  3 /magnetite
Fe2O3(cr)     , -26.002   -6 -2  2 /haematite
FeOOH(cr)     , -13.221   -3 -1  1 /goethite
EH, e-, H+, Fe 2+,
LAV,  0 -13  LAV,  16.9 -18.6  T, 1E-6