Pt (s) | Cu2+ (aq), H+ (aq)
Note that the spectator ions, oxide and chloride, have been left out of the notation and that the anode is written to the far left.
The salt bridge or porous disk is shown in the notation as a double line ( || ). Therefore, a cell that undergoes the oxidation of magnesium by Al3+ could have the following cell notation if the anode is magnesium and the cathode is aluminum:
Mg (s) | Mg2+ (aq) || Al3+ (aq) | Al (s)
One can measure the cell potential, Ecell, in volts, of any galvanic cell with the aid of a potentiometer. However, it is impossible to directly measure the potential of each individual half-cell. Chemists, however, have devised a method to measure the ability of a chemical species to reduce another by compiling tables of standard reduction potentials, Eo (the o indicates that the reaction is at standard state). Arbitrarily assigning a value of exactly zero to the potential of the standard hydrogen electrode allows us to measure the Eo of any half- reaction. That measurement is made by constructing a galvanic cell between the SHE and the unknown half-cell at standard state conditions. For example, when the following cell is constructed (see Heading for a review of the line notation), an Eocell of 0.34 V is observed (note the setup of the SHE as the anode because Cu2+ has a greater reduction potential than H+):
Pt (s) | H2 (g) | H+ (aq) || Cu2+ (aq) | Cu (s)