ABSTRACT
To avoid repetitive drawing of complicated cell diagrams, a common notation has been adopted for cells. All electrical contacts between half-cells are assumed, changes of phase (see Topic D4) are denoted by |, a salt bridge is shown as || and a junction between two different solutions (a liquid junction in a glass frit) by . If there are multiple species in the same phase these are separated by commas. The cell is shown starting at the left-hand electrode and moving to the right-hand electrode through the solutions. Examples of cells under standard conditions are:
Pt|H2(g, p=1 atm)|H+(aq, a=1)||Cl−(aq, a=1)|AgCl(s)|Ag(s) Pt|Fe3+(aq, a=1), Fe2+(aq, a=1)||Zn2+(aq, a=1)|Zn(s)
Activity, a, or concentration, c, and pressure, p, or fugacity values are not necessary and generally not quoted for standard cells (as they are defined), but are important away from the standard state (see Topic E5). Ions in the solution that take no part in the redox reaction are generally not included. In these cells, the half-cell on the left both experimentally and as written is called the left-hand (LH) electrode, while that on the right is called the right-hand (RH) electrode. Measured values of Ecell or are reported by convention as a positive or negative value, denoting that the right-hand (RH) electrode has a more positive or negative potential than the left-hand (LH) electrode:
For those cells with half-cells separated by a glass frit or porous ceramic, there is a small extra voltage associated with the liquid junction, which forms an extra component of . This complication is avoided by using a salt bridge, which has a negligible voltage as it contains two liquid junctions whose potential differences cancel.
