For potentials more positive than the standard-state potential, the predominate
species is Fe3+, whereas Fe2+ predominates for potentials more negative than E°.
When coupled with the step for the Sn4+/Sn2+ half-reaction, we see that Sn2+ can be
used to reduce Fe3+. If an excess of Sn2+ is added, the potential of the resulting solution
will be near +0.154 V.
Using standard-state potentials to construct a ladder diagram can
present problems if solutes are not at their standard-state concentrations.
Because the concentrations of the reduced and oxidized species
are in a logarithmic term, deviations from standard-state concentrations
can usually be ignored if the steps being compared are separated
by at least 0.3 V.1b A trickier problem occurs when a half-reaction’s potential
is affected by the concentration of another species. For example,
the potential for the following half-reaction