Metal/metal ion half-cells involve a metal electrode in a solution of its aqueous metal ion, e.g., the Cu/Cu²⁺ half-cell shown above (left).

Ion/ion half-cells involve an inert platinum electrode in a solution containing ions of the same element in different oxidation states, e.g., the Fe²⁺/Fe³⁺ half-cell shown above (right).

The standard electrode potential (E^⦵) of a half-cell is the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode.

The standard hydrogen electrode.

The standard hydrogen elecrode has an E^⦵ value of zero, by definition.

1. The concentration of H⁺_(aq) ions is 1.00 mol dm^-3.
2. The temperature is 298 K.
3. The pressure is 100 kPa.

The experimental set-up requires:

1. A zinc electrode in an aqueous solution of Zn²⁺ ions.
2. A copper electrode in an aqueous solution of Cu²⁺ ions.
3. A voltmeter connecting the two electrodes.
4. A salt bridge connecting the two solutions.

A salt bridge is a component that connects two half-cells, allowing ion flow while preventing the mixing of solutions.

A salt bridge is necessary to maintain charge neutrality in the cell.

A salt bridge typically contains a concentrated solution of an electrolyte that does not react with either solution, e.g., a strip of filter paper soaked in KNO_3(aq).

The more negative (or less positive) the E^⦵ value, the greater the tendency of a redox system to lose electrons and undergo oxidation.

Conversely, the more positive (or less negative) the E^⦵ value, the greater the tendency of a redox system to gain electrons and undergo reduction.

V

E