Fraction diagrams for components that may have negative concentrations:
When requesting a fraction diagram for H+, the plot
has Y-axis limits between 0 and 1 (0% to 100%).
However, negative values, or even values >1 are possible. Consider the
definition of fraction of H+, α(i),
bound to the different species in the system
H+/HCO3−:
|
HCO3− − H+
↔
CO32− |
|
α(CO32−) = − [CO32−]/[H+]TOT |
|
HCO3− + H+
↔
H2CO3 |
|
α(H2CO3) = [H2CO3]/[H+]TOT |
|
H2O − H+
↔ OH− |
|
α(OH−) = −[OH−]/[H+]TOT |
the value of [H+]TOT (the total concentration
of H+) might be positive (in acid solutions) or negative
(in alkaline solutions). Therefore in a fraction diagram for the component
H+, the fraction values might be negative
or even larger than one!
Activity ratio diagrams: a diagram that might seem strange (but correct),
is the case of the example input file EX7.DAT, where the activity
ratio for the solid phase Cu(OH)2(s) is greatest,
but the activity of the solid is nevertheless less than one, and the solid
is not present at equilibrium.
Predominance area diagrams: in some special cases, two solid phases
might coexist containing exactly the same amount of metal. In that case, no solid
predominates, and some parts of the lines will be missing in the predominance
area diagram. For example, on a system with H+,
e−, Fe2+, Ni2+ and
Cr3+, Fe3O4(s) and
NiFe2O4(s) might co-exist.