Standard Electrode Potential: Definition, Electrochemical Series, Measurement, Applications, and Examples in Chemistry

According to IUPAC, electrode potential is defined as the electromotive force (EMF) of a galvanic cell constructed using two electrodes.

Electrode potential is represented by the symbol E. The absolute value of a single electrode potential cannot be measured directly. Only the potential difference between two electrodes can be determined experimentally. To measure the electrode potential of an unknown electrode, it is paired with a reference electrode of known potential in a galvanic cell. The cell potential obtained is the sum of the potentials of both electrodes.

                  E_Cell = E_Cathode + E_Anode

The cell potential (E_cell) is measured experimentally using a voltmeter. Since the electrode potential of one electrode (the reference electrode) is known, the electrode potential of the other (unknown) electrode can be calculated.

The hydrogen electrode

•           The absolute value of a half-cell (electrode) potential cannot be measured; only the potential difference between two half-cells can be determined.

•           A standard reference electrode is needed to compare electrode potentials reliably.

•           The standard hydrogen electrode (SHE) serves as the reference electrode and consists of:

• Hydrogen gas at 100 kPa
  • In equilibrium with H⁺ ions at 1.00 mol dm^-3 
  • An inert platinum electrode in contact with the hydrogen gas and H⁺ ions
• The half-equation for the standard hydrogen electrode is:

2H⁺ (aq) + 2e^– ⇌ H₂ (g)

• It is given an arbitrary value of E^θ = 0.00 volts
• When the standard hydrogen electrode is connected to another half-cell, the standard electrode potential of that half-cell can be read from a high-resistance voltmeter.

Standard hydrogen electrode diagram

The standard electrode potential of a half-cell can be determined by connecting it to a standard hydrogen electrode.

•In practice, the standard hydrogen electrode (SHE) is rarely used due to several limitations.

      •   The electrode reaction is slow.

      •   The setup is not easily portable.

      •    Maintaining a constant pressure of hydrogen gas can be challenging.

• However, once the standard electrode potential of a half-cell is established relative to the SHE, that electrode can serve as a secondary reference. This allows other electrode potentials to be measured more conveniently using the known reference.

• So, any half-cell with a known potential can then serve as a secondary reference

Measurements using the hydrogen electrode         

• If a hydrogen electrode is used to measure the electrode potentials of zinc and copper half reactions, the conventional cell diagrams would be:

Pt 丨H₂(g), 2H⁺(aq)  ∥  Zn²⁺(aq), Zn(s)          E^θ = -0.76 V

Pt 丨H₂(g), 2H⁺(aq)  ∥  Cu²⁺(aq), Cu(s)           E^θ = +0.34 V

•           The hydrogen electrode is always written on the left-hand side of the cell diagram by convention.

•           The polarity of the other half-cell is measured relative to the hydrogen electrode, which is assigned a standard electrode potential of 0.00 V.

 •          The half-reactions in standard electrode potential tables are always written as reductions, which is why these values are known as standard reduction potentials.

•           Standard reduction potentials are listed in order from most negative to most positive, indicating the tendency of species to gain electrons.

 •          The IB Chemistry data booklet (Section 19) includes a table of standard reduction potentials measured at 298.15 K (25°C).

Table of standard electrode potentials

Oxidised species   Reduced species	Eθ (V)
Li+ (aq) + e– ⇌ Li (s)	–3.04
Al3+ (aq) + 3e– ⇌ Al (s)	–1.66
Pb2+ (aq) + 2e– ⇌ Pb (s)	–0.13
Fe3+ (aq) + e– ⇌ Fe2+ (aq)	+0.77
F2 (g) + 2e– ⇌ F– (aq)	+2.87

Interpreting electrode potential values

•           The electrode potential of a half-cell indicates how easily the species involved in the half-equation undergo reduction or oxidation.

•           For instance, in the half-equation:

  Cu²⁺ + 2e⁻ ⇌ Cu(s)

Cu²⁺ is the species being reduced (gains electrons), while Cu(s) is the species being oxidised (loses electrons).

•           The electrode potential reflects the tendency of this equilibrium to shift toward gaining electrons (reduction) or losing electrons (oxidation), indicating how readily electrons are accepted or donated.

A more negative standard electrode potential (E°) implies that:

  • The species has a greater tendency to lose electrons (i.e., undergo oxidation).

  • The redox equilibrium lies toward the left, favoring the oxidized form.

  • The half-cell favors oxidation over reduction.

  • The species behaves as a stronger reducing agent.

A more positive standard electrode potential (E°) indicates that:

  • The species has a greater tendency to gain electrons (i.e., undergo reduction).

  • The redox equilibrium lies toward the right, favoring the reduced form.

  • The half-cell favors reduction over oxidation.

  • The species acts as a stronger oxidizing agent.

Standard Electrode Potential

The standard electrode potential (E°) of a half-cell (or half-reaction) is the potential measured against the standard hydrogen electrode (SHE) under standard conditions. These conditions include:

Temperature: 298 K (25°C)

Pressure: 1 atmosphere (1 atm)

Concentration: 1 mol dm⁻³ (1 M) of all aqueous species

The standard hydrogen electrode (SHE) is a gas-ion electrode that serves as the reference electrode for measuring standard electrode potentials of other half-cells. It can function as either an anode or a cathode, depending on the cell it is connected to.

By definition, the standard reduction potential and oxidation potential of SHE is 0.00 V at 298 K. This zero value forms the basis of the thermodynamic scale for measuring oxidation-reduction (redox) potentials of various electrodes.

The standard electrode potential is denoted by E°. It refers to the potential of a half-cell measured under standard conditions using the standard hydrogen electrode (SHE) as the reference.For any electrode, either the standard reduction potential or the standard oxidation potential can be determined relative to the SHE.The standard cell potential (E°cell) is calculated as the difference between the standard reduction potentials of the two half-cells:This value indicates the overall driving force of the redox reaction in an electrochemical cell under standard conditions. It can be represented as:

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