Redox couples and electrode processes

A redox couple is the oxidised and reduced forms of one substance taking part in a half reaction, written with the oxidised form first: Zn²⁺/Zn, Cu²⁺/Cu.

In the Daniell cell a zinc rod in ZnSO₄ solution and a copper rod in CuSO₄ solution are kept in separate beakers. A salt bridge, a U-tube of KCl or NH₄NO₃ solution set in agar, joins them and completes the circuit without letting the solutions mix.

When the two rods are joined by a wire through an ammeter and the switch is closed, zinc is oxidised (Zn → Zn²⁺ + 2e⁻) and copper ions are reduced (Cu²⁺ + 2e⁻ → Cu). Electrons flow from zinc to copper through the wire, while conventional current flows from copper to zinc.

The potential of an electrode in contact with its ions is its electrode potential. When all species are at unit concentration (gases at 1 atm) and 298 K it is the standard electrode potential, E°.

The hydrogen electrode is assigned E° = 0.00 V by convention, and other couples are measured against it.

A negative E° means the couple is a stronger reducing agent than the H⁺/H₂ couple; a positive E° means a weaker reducing agent, that is, a stronger oxidising agent than H⁺.

In the series fluorine sits at the top (E° = +2.87 V for F₂/F⁻), so F₂ is the strongest oxidant and F⁻ the weakest reductant. Lithium sits at the bottom (E° = −3.05 V for Li⁺/Li), so Li is the strongest reductant listed.

Other values worth recalling: MnO₄⁻/Mn²⁺ +1.51 V, Cl₂/Cl⁻ +1.36 V, Cr₂O₇²⁻/Cr³⁺ +1.33 V, Ag⁺/Ag +0.80 V, Cu²⁺/Cu +0.34 V, Zn²⁺/Zn −0.76 V, Na⁺/Na −2.71 V.