Ladder Diagrams for Oxidation-Reduction Equilibria

For potentials more positive than the standard-state potential, the predominate species is Fe +, whereas Fe + predominates for potentials more negative than E°. When coupled with the step for the Sn +/Sn + half-reaction, we see that Sn + can be used to reduce Fe +. If an excess of Sn + is added, the potential of the resulting solution will be near -1-0.154 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 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 

Ladder diagram for Zn +, Zn(NH3)32+, and Zn(NH3)4 +, showing how cumulative formation constants are included. 

From this equation we see that the areas of predominance for U02 and U + are defined by a step whose potential is 

Ladder diagram showing the effect of a change in pH on the areas of predominance for the U02 +/U + half-reaction. 

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