• 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^–?? H_2?(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

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

• 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