Having defined the half-reaction and a potential associated with the reduction depicted by the half-reaction, we need to determine what the potential is.

There need to be two half-cells to make up the whole cell, just as there are two half reactions which make up the full redox reaction. In defining potentials, they are made relative to the H+/H2 redox couple, which has a reduction potential of zero. A reference electrode for determining the potential of other couples is therefore the standard hydrogen electrode. This has been (arbitrarily) chosen for reporting electrode potentials, which are determined for the half-cell when measured against it.

The Standard Hydrogen Electrode

The standard hydrogen is being used to measure the potential of the Fe2+/Fe3+ couple.

Some important features of the standard hydrogen electrode are:

In the standard hydrogen electrode, the pressure of the hydrogen gas is fixed at one atmosphere, and the concentration of the hydrochloric acid is 1.18M. This corresponds to a proton activity of one.

The potential between the two electrodes is measured using a digital voltmeter. This draws a negligible current, and so no electrolysis occurs during the potential measurement, and there is no change in the concentrations of H+ and H2, or of Fe2+ and Fe3+.

The reference electrode is made from platinized platinum, rather than a smooth platinum surface. This is to ensure the presence of catalytic, defect sites, and thus fast electrode kinetics, where the equilibrium, between electrons at the metal surface and aqueous hydrogen ions with the hydrogen gas produced, is rapidly established.

When the hydrogen electrode is operating under standard conditions, and the activities of all the ions present are one, the value of the potential measured is known as the standard electrode potential of the Fe2+/Fe3+ couple.

An important feature of the structure of the cell used with the standard electrode is that the two half-cells are separated by a salt bridge.  The salt bridge is a tube containing an aqueous solution of potassium chloride, and its purpose is to keep the two half cells in electrical contact, whilst preventing the solutions from the two different half cells from mixing. Any mixing would result in the potential being recorded being disturbed.

The Calomel reference electrode

The standard hydrogen electrode is that against which the potentials of all other half-cells are made. However, the standard hydrogen electrode is often inconvenient to use as a reference electrode experimentally. It has to have freshly made platinum electrodes, a solution of HCl with a very accurately determined concentration corresponding to unit activity of hydrogen ions, and the safe provision of a hydrogen gas source at 1 atmosphere pressure.

It is generally much more convenient to use a calomel electrode as the reference electrode.

The saturated Calomel reference electrode
A calomel electrode used as a reference electrode eliminates liquid junction potentials

The reduction reaction occurring at the calomel electrode corresponds to the reduction of mercury (I).

The potential of the calomel electrode depends on the chloride ion activity, but this is kept constant, and saturated, due to the present of excess Cl in the undissolved KCl crystals. The electrode is in constant with the test solution by means of the porous frit in the bottom of the electrode, which acts as the equivalent of a salt bridge.

To calculate the reduction potential for the test solution, it is measured in a cell as above. To adjust this so that it is compared to the standard hydrogen electrode, we need to know the standard electrode potential of the saturated calomel electrode: this is 0.242 V at 25 oC. The standard electrode potential for the test solution is then given by: