Standard electrode potential
Measure of reducing power in electrochemical cells.
Standard electrode potential, denoted as E⊖ or Ered⊖, is a measure of the reducing power of an element or compound in electrochemistry. It is defined by IUPAC as the value of the standard electromotive force of a cell where molecular hydrogen under standard pressure is oxidized to solvated protons at the left-hand electrode. This concept is fundamental to understanding redox reactions in electrochemical cells, such as galvanic cells, where electricity is produced from the difference in electric potential between two electrodes. Every electrochemical cell is based on a redox reaction that can be split into two half-reactions: oxidation at the anode, where electrons are lost, and reduction at the cathode, where electrons are gained. The overall cell potential arises from the voltage difference between the two electrodes, but it is impossible to measure the absolute potential of a single electrode in isolation. To overcome this, the standard hydrogen electrode (SHE) is assigned a potential of exactly 0.00 V, serving as a reference. An unknown electrode can be paired with the SHE or another electrode of known potential to determine its relative value. Electrode potentials are conventionally defined as reduction potentials, and the sign of the potential for an electrode undergoing oxidation must be reversed when calculating the overall cell potential. These potentials are expressed in volts, representing energy per electron transferred, and can be combined directly even if the two half-reactions involve different numbers of electrons. For practical measurements, the electrode under study is connected to the positive terminal of an electrometer, while the SHE is connected to the negative terminal. A reversible electrode is one whose potential arises from changes of a reversible nature, requiring the system to be near chemical equilibrium and subjected to very small, slow perturbations. In practice, achieving true reversibility is difficult, as any finite perturbation forces the system out of equilibrium; electrodes used in electroplating, for instance, operate with high over-potential far from equilibrium. The magnitude of the standard reduction potential indicates ease of reduction: a larger positive value means the species is more easily reduced and is a better oxidizing agent. For example, fluorine (F₂) has a standard reduction potential of +2.87 V, making it a
- field
- Electrochemistry
- defined_by
- IUPAC Gold Book
- symbol
- E⊖ or Ered⊖
- reference_electrode
- Standard hydrogen electrode (SHE) at 0.00 V
- unit
- Volts
Lore & Background
The standard electrode potential is defined by IUPAC as the value of the standard electromotive force of a cell in which molecular hydrogen under standard pressure is oxidized to solvated protons at the left-hand electrode. This potential serves as a measure of the reducing power of any element or compound. Because the oxidation potential of a half-reaction is simply the negative of its reduction potential, standard electrode potentials are conventionally written and tabulated as standard reduction potentials. The magnitude of this value indicates the ease with which an element gains electrons: a larger positive value means the species is more readily reduced and thus acts as a stronger oxidizing agent, while a highly negative value indicates a strong reducing agent. For example, fluorine has a very positive standard reduction potential, making it an excellent oxidizing agent, whereas lithium has a very negative potential, meaning its metallic form readily undergoes oxidation. In a galvanic cell, a spontaneous reaction requires a positive overall cell potential, which is calculated by combining the standard reduction potential of the cathode with the reversed sign of the anode potential. The standard hydrogen electrode is assigned a potential of exactly 0.00 V and serves as the reference against which all other electrode potentials are measured. These potentials are independent of the number of electrons transferred, allowing direct combination of half-reaction values even when different electron counts are involved.
Reader's Guide
Standard electrode potential is crucial for predicting the spontaneity of redox reactions. The larger the value of the standard reduction potential, the easier it is for the element to be reduced, making it a better oxidizing agent. For example, F2 has a standard reduction potential of +2.87 V, indicating it is reduced easily, while Li+ has −3.05 V, meaning it is not easily reduced and Li(s) is a good reducing agent. In a galvanic cell, a spontaneous reaction requires a positive cell potential (Ecell⊖ > 0), which corresponds to a negative Gibbs free energy (ΔGcell⊖ < 0). The cell potential is calculated as Ecell⊖ = Ecathode⊖ − Eanode⊖, where the cathode potential is the standard reduction potential and the anode potential is the standard oxidation potential. This framework allows chemists to design batteries and understand corrosion and electrolysis processes.
Did You Know?
- The standard hydrogen electrode (SHE) is defined as having a potential of 0.00 V and serves as a reference for measuring other electrode potentials.
- The oxidation potential of a half-reaction is the negative of its reduction potential, so standard electrode potential is commonly written as standard reduction potential.
- Electrode potentials are independent of the number of electrons transferred; they are expressed in volts, which measure energy per electron transferred.
- A reversible electrode is one that owes its potential to changes of a reversible nature, requiring the system to be close to chemical equilibrium and subjected to very small, slow solicitations.
More in Physical Chemistry And Thermodynamics 1-20
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record