The standard hydrogen electrode is shown below.

mtaimg/images/topics/21/21-2-1.png/mtaimg

As the hydrogen gas flows over the platinum catalyst, an equilibrium is set up between hydrogen molecules and hydrogen ions in solution.

(2H⁺{(aq)} + 2e^- ⇌ H{2 (g)} )

This is the equilibrium that all other systems are compared with.

bStandard conditions/b

Because the position of equlibrium is affected by certain conditions, these must also be standardised so that a fair comparison can be made.

The standard conditions are a temperature of 298K (25°C), a gas pressure of 100 kPa (1 bar or 1 atmosphere) and ionic concentration in solution of 1 mol dmsup-3/sup.