The Hydrogen Electrode (HL) (HL IB Chemistry)

• The absolute value of a half-cell potential cannot be measured, only differences in potential between pairs of half-cells.
• For this reason, it is necessary to have a standard electrode against which all other half-cells can be compared
• The standard hydrogen electrode is a half-cell used as a reference electrode and consists of:
  • Hydrogen gas in equilibrium with H⁺ ions of concentration 1.00 mol dm^-3 (at 100 kPa)

2H⁺ (aq) + 2e^– ⇌ H₂ (g)

• • An inert platinum electrode that is in contact with the hydrogen gas and H⁺ ions

• 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 fact, the hydrogen electrode is rarely used in practice for a number of reasons:
  • The electrode reaction is slow
  • The electrodes are not easily portable
  • It is difficult to maintain a constant pressure
• Once one standard electrode potential has been measured relative to the standard hydrogen electrode, it is not necessary to use the standard hydrogen electrode again
• Any electrode whose electrode potential is known could be used to measure standard electrode potentials

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

• Since the hydrogen electrode is always on the left, the polarity of the half-cell measured is always with respect to hydrogen
• The half-reaction will therefore always be a reduction reaction, so that is why sometimes standard electrode potentials are termed standard reduction potentials
• Tables of standard electrode potentials have been compiled ranking half-cells from negative to positive values

Table of standard electrode potentials

Oxidised species  Reduced species	Eθ (V)
Li+ (aq) + e–  Li (s)	–3.04
K+ (aq) + e–  K (s)	–2.93
Ca2+ (aq) + 2e–  Ca (s)	–2.87
Na+ (aq) + e–  Na (s)	–2.71
Mg2+ (aq) + 2e–  Mg (s)	–2.37

• The more negative the value; the better the half-cell is at pushing electrons so the equilibrium lies to the left
• This means the more negative the half-cell; the better it can act as a reducing agent