Galvanic Cells and Cell Notation

Oxidation occurs at the anode and reduction at the cathode, in every electrochemical cell without exception. In a galvanic cell the anode is the source of electrons and is therefore labeled negative, while the cathode is positive. The anode metal typically loses mass as it dissolves; the cathode typically gains mass as metal plates onto it.

For the Zn/Cu galvanic cell, oxidation is Zn(s) → Zn²⁺ + 2 e⁻ and reduction is Cu²⁺ + 2 e⁻ → Cu(s). Electron flow in the external metal circuit is Zn → Cu; conventional current is opposite. The negative anode and positive cathode labels describe this discharging galvanic cell. Oxidation/reduction definitions remain valid in electrolysis, where an external supply gives the opposite polarity. Metal mass change is a useful clue only for metal dissolution or plating; gas and all-dissolved redox electrodes need not gain or lose electrode mass.

As oxidation proceeds, positive ions build up in the anode compartment; as reduction proceeds, positive charge is consumed in the cathode compartment. Without relief, the charge imbalance would stop the reaction almost immediately. The salt bridge, usually a gel of an inert electrolyte such as KNO₃, releases anions toward the anode and cations toward the cathode to keep both solutions neutral.

For every mole Zn dissolving in the Daniell-cell example, two positive charge equivalents enter the anode solution, requiring compensating negative charge movement. At the cathode, removing one mole Cu²⁺ leaves its counteranions behind, requiring positive-ion compensation. Electrons travel through electronic conductors, while ions carry current through solution and the bridge. Disconnecting either path prevents sustained current. Choose bridge ions that do not precipitate, complex strongly or react with the half-cell species; KCl would be unsuitable for an Ag⁺ half-cell because AgCl precipitates.

The convention is anode | anode solution || cathode solution | cathode. A single vertical line marks a phase boundary, a double vertical line marks the salt bridge, and a comma separates two species in the same phase. Concentrations and pressures are written in parentheses when they are not standard.

Read Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s) as two phase interfaces separated by an ionic junction. In Pt(s) | Fe²⁺(aq), Fe³⁺(aq) || Ag⁺(aq) | Ag(s), the comma means the iron ions share one solution; Pt provides the conducting surface for their interconversion. A pure metal participating in its own redox couple needs no additional Pt electrode. Notation records a defined cell orientation; under altered concentrations a signed cell voltage can reverse, so the spontaneous direction must still agree with the actual conditions.

Some half-reactions have no solid conductor of their own. The Fe³⁺/Fe²⁺ couple is entirely dissolved, and the hydrogen electrode involves a gas. In those cases a platinum or graphite electrode provides the surface for electron transfer without taking part chemically, and it is written explicitly at the outer edge of the notation.

“Inert” means the chosen conductor is not included in that intended half-reaction. It is still a physical electrode and can affect kinetics; graphite or Pt is not chemically inert under every possible potential. For 2 H⁺ + 2 e⁻ ⇌ H₂, write an appropriate gas–solution interface at Pt and specify gas pressure and ion activity when quantitative potentials are needed.