What are Cell diagrams?

Consider the galvanic cell depicted in Figure. This cell has a hydrogen electrode at the left and a silver-silver chloride electrode at the right. The hydrogen electrode is a strip of platinum in contact with hydrogen gas and with aqueous hydrochloric acid, which is the ionic conductor. In the type of hydrogen electrode shown in the figure, hydrogen gas is introduced through a side tube into a closed-end glass jacket that surrounds the platinum strip and is immersed in the hydrochloric acid; the gas bubbles out through holes near the bottom of the tube. The silver-silver chloride electrode is a silver strip or wire that dips into the hydrochloric acid and is coated with solid silver chloride.

The cell in Figure is compactly described by the following cell diagram:

Cu |Pt | H₂ (g) | H⁺ (aq), Cl^-(aq) | AgCl(s) | Ag | Cu

A cell diagram indicates which electrode is at the left and which is at the right, and shows the reactants and products of the two electrode reactions. A single vertical bar represents a phase boundary. Commas are used to separate different species in the same phase.

The same cell can be described by a slightly different cell diagram that omits the copper terminals seen in the figure and shows the electrolyte solute instead of the ion species:

Pt | H₂ (g) | HCl (aq) | AgCl (s) | Ag

The reason it is not necessary to include the terminals is that the property whose value we seek, the zero-current cell potential, is the same regardless of the metal used for the terminals.