Cell Notation
Drawing a full galvanic cell diagram every time would be exhausting. Cell notation is a shorthand that captures the same information in a single line of text. The MCAT uses this notation in passages, so you need to read it fluently.
The Rules
Cell notation always reads left to right, anode to cathode - the same direction electrons flow through the external circuit.
| Symbol | Meaning |
|---|---|
| Single line | | Phase boundary (solid/solution or solution/gas interface) |
| Double line || | Salt bridge separating the two half-cells |
| Commas , | Separate species in the same phase |
| (s), (l), (aq), (g) | Phase labels |
The Daniell Cell in Cell Notation
Zn(s) | Zn2+(1 M) || Cu2+(1 M) | Cu(s)
Reading from left to right:
- Zn(s) - solid zinc anode (the electrode)
- | - phase boundary between the solid electrode and the solution
- Zn2+(1 M) - zinc ion solution in the anode compartment
- || - salt bridge
- Cu2+(1 M) - copper ion solution in the cathode compartment
- | - phase boundary between solution and solid electrode
- Cu(s) - solid copper cathode
Examples with Different Setups
A cell with an inert electrode and a gas:
Pt(s) | H2(g) | H+(aq) || Ag+(aq) | Ag(s)
Here, the anode is a platinum electrode where H2 gas is oxidized to H+ ions. The platinum does not react - it just provides a surface.
A cell where both species are in solution (requires inert electrode):
Pt(s) | Fe2+(aq), Fe3+(aq) || MnO4-(aq), Mn2+(aq), H+(aq) | Pt(s)
A comma separates species in the same phase. Fe2+ and Fe3+ are both in the anode solution. Multiple species in the cathode solution are also comma-separated.
Common Mistakes to Avoid
- Do not reverse the order. Anode is always on the left, cathode on the right.
- Do not confuse | and ||. A single line is a phase boundary within a half-cell. The double line is the salt bridge between half-cells.
- Do not forget phase labels. The MCAT expects you to know that electrodes are typically (s), solutions are (aq), and some half-reactions involve (g).