Example 6. Calculated Eh
Data for the chemical system H+, e−,
Fe2+ and HS−
was extracted using DATABASE. Then it was modified in
SPANA to exchange components for reactions,
in this case e− was exchanged for O2(g),
Fe2+ for FeS2(cr),
and HS− for H2S
(without a phase indicator, it means that H2S is aqueous, dissolved).
The calculated Eh is plotted against
the amount of added O2 (from 0
to 0.05M) in a closed system that
initially contains some pyrite (0.01 mol/L of FeS2)
and some H2S (10−4 M). Both pyrite
and sulfide will be oxidised by oxygen.
Important: to calculate the Eh
you can not give a value of Eh (or pe); instead the total concentration
of e− must be given. In this example the
total concentration of O2 is given.
The overall reaction
2 FeS2(cr) + 7.5 O2(g)
=
Fe2O3(cr) + 4 H+ + 4 HSO4−
is shown in a logarithmic diagram:
.gif "06_Eh_calculated_(log).plt")
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