Standard potentials

The electrode potential and the cell potential depend on the concentrations of solutions, the pressures of gases and the temperature. To facilitate comparison of different galvanic cells, it is necessary to measure the cell voltage under a given set of standard conditions of concentration and temperature.

The standard conditions chosen are 1 M concentration of solution, 1 atm pressure for gases, solids and liquids in pure form, and $25\,^0\mathrm{C}$. The voltage measured under these conditions is called the standard potential, designated $E^0$. The standard cell potential is the algebraic sum of the standard electrode potentials, similar to Eq. (5.22):

Here $E^0_{oxi}$ is the standard oxidation potential and $E^0_{red}$ is the standard reduction potential.

According to IUPAC convention, the standard potential of an electrode is taken as the standard reduction potential. It must be realised that the standard oxidation potential of any electrode is numerically equal to its standard reduction potential with the reversal of sign. For example, if the standard oxidation potential of the $\mathrm{Zn^{2+}\,(1M)\ \vert\ Zn}$ electrode is 0.76 V, its standard reduction potential is -0.76 V. Hereafter the standard reduction potential will be called the standard potential — the voltage associated with a reduction reaction.

It follows that the standard cell potential (emf) is written in terms of the standard potentials of the electrodes. In Eq. (5.23), $E^0_{oxi}$(anode) is replaced by $-E^0_{red}$(anode); we then write $E^0_{cell} = -E^0_{red}\,\text{(anode)} + E^0_{red}\,\text{(cathode)}$. Omitting the subscript $red$, we have …