Standard redox potentials

Standard Redox Potentials

The standard reduction potential E° is the central quantity used to compare the oxidising or reducing power of a redox couple. It is defined relative to the standard hydrogen electrode (SHE), assigned E° = 0 V by international convention (IUPAC).

Definition and Measurement of E°

A standard potential E°(Ox/Red) is measured by constructing a cell between the half-cell of interest and the SHE under standard conditions (unit activities, 25 °C, 1 bar). The value is positive if the couple is a stronger oxidant than H⁺/H₂, negative otherwise.

Key couples (at 25 °C):

Predicting Spontaneous Reactions: the γ Rule

To predict whether a redox reaction is spontaneous, apply the activity series (γ rule):

A spontaneous reaction pairs the strongest oxidant (highest E°) with the strongest reductant (lowest E°).

Formally, E°_cell = E°_cathode − E°_anode > 0 for a spontaneous reaction.

Example: Does Cu²⁺ (E° = +0.34 V) oxidise Zn (Zn²⁺/Zn, E° = −0.76 V)?

E°_cell = 0.34 − (−0.76) = +1.10 V > 0 → spontaneous

Conversely, Copper (Cu) is not dissolved by H⁺ because E°(Cu²⁺/Cu) > E°(H⁺/H₂).

Couples Useful in Laboratory Practice

Effect of Conditions on E: Nernst Equation

Away from standard conditions, the actual half-cell potential is given by:

E = E° − (0.0592/n) log ([Red]/[Ox]) (at 25 °C)

A change in pH shifts E for couples involving protons (e.g. MnO₄⁻/Mn²⁺: E decreases by 0.0592 × 8/5 ≈ 0.095 V per pH unit). This is why permanganate titrations are performed in acidic media.

Mastery of the E° table is a powerful predictive tool, complemented by Pourbaix diagrams for pH-dependent systems.