3.4. Standard Electrode Potential and Reference Hydrogen Electrode

The standard electrode potential (denoted E° for a half‐cell) is a measure of a half‐cell’s tendency to be reduced under standard conditions: 1 molar concentrations of all aqueous species, 1 atmosphere pressure for any gases, and a temperature of 25 degrees Celsius (298 K).

Standard electrode potential (E°) indicates how likely a half-cell is to gain electrons and be reduced. It is measured under specific conditions: solutions should have 1 molar concentration, gases should be at 1 atmosphere pressure, and the temperature should be at 25 °C. This standardization allows scientists to compare the reduction abilities of different half-cells reliably.

Think of E° like a score in a sports game: just as scores help compare the performance of different teams under the same conditions, E° helps evaluate the ability of different half-cells to be reduced under standard conditions.

By convention, the half‐reaction for the standard hydrogen electrode (SHE) is assigned a potential of 0.00 volts. The SHE consists of a platinum electrode in contact with 1 molar H+ solution (acid) and hydrogen gas at 1 atmosphere bubbling over the platinum surface. The half‐reaction is:

2 H+(aq, 1 M) + 2 e− → H2(g, 1 atm)

The standard hydrogen electrode serves as a reference point for measuring other half-cell potentials and is assigned a value of 0.00 V. The SHE is formed with a platinum electrode submerged in a 1 molar solution of hydrogen ions, with hydrogen gas present at a pressure of 1 atm. The associated half-reaction describes how protons gain electrons to form hydrogen gas.

Imagine the SHE as the baseline for a graph at zero. Just like how you measure other scores or values against a baseline (like sea level for elevation), other half-cell potentials are compared against the hydrogen electrode’s starting point of 0 V.

Because the SHE is defined as zero, one can connect any other half‐cell to the SHE and measure the cell potential (voltage) to determine the unknown half‐cell’s standard potential. For a half‐reaction:

Oxidized form + n e− → Reduced form

the standard electrode potential E° (in volts) indicates how easily the reduced form is oxidized (reverse reaction) or the oxidized form is reduced (forward reaction).

By connecting any half-cell to the SHE, we can record the cell potential, which tells us the standard electrode potential (E°) for that cell. E° gives insight into the reactivity of the half-cell: a positive E° indicates a strong tendency for reduction while a negative E° suggests a greater tendency for oxidation.

Consider E° like a battery's voltage rating. A higher voltage means the battery can push more energy through the circuit, whereas a lower voltage means it does less work. Similarly, a more positive E° means that a half-cell is better at accepting electrons (reduction) compared to one with a lower or negative E°.