Standard Electrode Potential


Meaning of electrode potential
Potential difference produced when an equilibrium is established between metal M and the aqueous solution containing metal \(M^{n+}\)ions in a half-cell
  • Electrode potential cannot be measured directly.
  • The value is determined based on the difference of electrode potential between two half-cells.
  • The electrode potential can be measured by pairing up the electrode to the standard reference electrode system.
  • The international consensus is the standard hydrogen electrode.
Standard condition for the standard electrode potential measurement
  • The concentration of ions in an aqueous solution is \(\text{1 mol dm}^{-3}\)
  • The gas pressure of 1 atm or 101 kPa
  • The temperature at 25°C or 298K
  • Platinum is used as an inert electrode when a half-cell is not a metal electrode


Standard Hydrogen Electrode


Standard Hydrogen Electrode

  • Function of platinum(IV) oxide, \(PtO_2\): To increase the surface area of platinum, Pt to adsorb hydrogen gas, \(H_2\).
  • This is done so that hydrogen molecules are in closer contact with hydrogen ions, \(H^+\) in the solution.
  • Half equation of the hydrogen half-cell;
    • \(2H^+ (aq) +2e^- \rightleftharpoons H_2(g)\)
  • The standard hydrogen electrode potential, Eº is given the value of 0.00 V:
    • \(H^+ (aq) +e^- \rightleftharpoons \frac{1}{2}H_2(g) \hspace{1cm}E^0=0.00V\)

Standard Electrode Potential Value, Eº

Apparatus set-up to determine standard electrode potential, Eº of zinc

  • Since the standard hydrogen potential is 0.00 V, the voltmeter reading of 0.76 V shows the electrode potential of zinc.
  • Zinc, Zn has a greater tendency to release electrons compared to hydrogen.
  • Hence, zinc, Zn becomes the negative terminal.
  • Electrons move from zinc electrode, Zn (negative terminal) to platinum electrode, Pt (positive terminal) through the connecting wires.
Electrode Zinc Hydrogen
Terminal Negative Positive
Half equation \(Zn(s) \rightarrow Zn^{2+} (aq) +2e^-\) \(2H^{+} (aq) +2e^-\rightarrow H_2(g) \)
Reaction Oxidation Reduction
Standard electrode potential for half-cell

\(Zn^{2+} (aq) +2e^- \rightleftharpoons Zn(s)\\ \,\\ E^0=-0.76V\)

The negative symbol shows that zinc electrode is the negative terminal when connected to the standard hydrogen electrode



Oxidising Agents and Reducing Agents Based on the Value of Standard Electrode Potential

  • The standard electrode potential, Eº is also known as the standard reduction potential.
  • All half-cell equations are written as reduction.
  • Eº value is a measure of the tendency of a substance to accept or donate electrons.
    • \(\text{Oxidising agent }+ \text{electron} \rightleftharpoons \text{Reducing agent}\)
Standard Electrode Potential Series


  • A comparison of standard electrode potential, Eº value is used to determine whether silver, Ag or magnesium, Mg is an oxidising agent or reducing agent.


When Eº value of Ag is more positive Eº value of Mg is more negative
  • Silver ion, \(Ag^+\) on the left side is a stronger oxidising agent.
  • It is easier for \(Ag^+\) ion to receive electrons and undergo reduction.
  • Conversely, silver atom, Ag on the right side is difficult to release electrons.
  • Magnesium atom, Mg on the right side is a stronger reducing agent.
  • It is easier for magnesium atom, Mg to release electrons and undergo oxidation.
  • Conversely, magnesium ion, \(Mg^{2+}\) on the left side is difficult to accept electrons.


The relationship between Eº value to the strength of a substance as an oxidizing agent and a reducing agent

Molecules or ions with a more positive or less negative standard electrode potential value Eº Atoms or ions with a more negative or less positive standard electrode potential value Eº
Has a greater tendency to accept electrons Has a greater tendency to release electrons
Easier to undergo reduction reaction Easier to undergo oxidation reaction
A stronger oxidising agent A stronger reducing agent