Answer: the standard electrode potential (\(E^\circ\)) of an electrode is its potential measured relative to the standard hydrogen electrode (SHE, taken as 0.00 V) when all ions are at unit concentration (1 M, strictly unit activity), gases are at 1 bar (1 atm) pressure and the temperature is 298 K (25 °C).
Key points:
- By IUPAC convention it is written as a reduction potential, for example \(\mathrm{Cu^{2+} + 2e^- \rightarrow Cu}\), \(E^\circ = +0.34\ \mathrm{V}\) and \(\mathrm{Zn^{2+} + 2e^- \rightarrow Zn}\), \(E^\circ = -0.76\ \mathrm{V}\).
- It is measured by making a cell of the electrode with the SHE and reading the EMF, since a single electrode potential cannot be measured alone.
- A more positive \(E^\circ\) means a greater tendency to be reduced (stronger oxidising agent); a more negative \(E^\circ\) means a greater tendency to be oxidised.
- Arranging electrodes in order of \(E^\circ\) gives the electrochemical series. The EMF of a cell is \(E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}\).