Cell Diagrams and IUPAC Conventions

In electrochemical cell notation, a single vertical bar denotes a phase interface, such as between a solid electrode and an aqueous solution. A double vertical line represents a salt bridge or liquid junction where junction potentials in galvanic cells are assumed to be eliminated. If the junction potential is not negligible, a dotted or dashed line is used instead.

According to IUPAC conventions, cell electromotive force (emf) is defined as E = ΦR − ΦL, where ΦR and ΦL are the open-circuit potentials of the right and left electrodes, respectively. A positive emf indicates that the corresponding cell reaction will spontaneously occur when the cell is connected to an external load.

In electrochemical cell notation, the left electrode is designated as the anode where oxidation occurs, and the right electrode is the cathode where reduction occurs. If the overall cell reaction proceeds in this direction, the emf is positive and the reaction is spontaneous under standard conditions.

A positive cell electromotive force indicates that the corresponding cell reaction will spontaneously occur when the cell is connected to an external load. If the right electrode acts as the cathode and the left as the anode, the reaction proceeds spontaneously. Conversely, a negative emf means the reverse reaction is spontaneous.

When multiple species are present in the same phase, they are separated by commas in electrochemical cell notation. This allows the compact representation of complex electrochemical systems without requiring detailed diagrams, making it easier to communicate cell structure and reaction pathways.

If the right electrode is the cathode, the cell reaction is spontaneous and emf is positive. If the left electrode is the cathode, the reverse cell reaction is spontaneous and emf is negative. The overall cell reaction is determined by adding the two half-reactions: oxidation at the left electrode and reduction at the right electrode.

A dotted or dashed line is used instead of a double vertical line when the junction potential between miscible liquids or at a liquid junction is not negligible. The double vertical line assumes the junction potential is eliminated, simplifying emf measurements, but when this assumption is invalid, the alternative notation indicates that junction effects must be considered.