concentration or chemical composition. It is caused by different rates of migration of cations and anions
across the boundary thereby leading to a charge separation. Its value is often several hundredths of a
volt and variable, but it can be minimized by using a salt bridge connection, e.g. an agar gel saturated
with KCl or NH 4 NO 3 for which the potential is only 1–2 mV.
Ohmic Drop, IR
A potential developed when a current I flows in an electro-chemical cell. It is a consequence of the cell
resistance R and is given by the product IR. It is always subtracted from the theoretical cell potential
and therefore reduces that of a galvanic cell and increases the potential required to operate an
electrolysis cell.
Activation Overpotential (Overvoltage)
The additional potential required to cause some electrode reactions to proceed at an appreciable rate.
The result of an 'energy barrier' to the electrode reaction concerned, it is substantial for gas evolution
and for electrodes made of soft metals, e.g. Hg, Pb, Sn and Zn. It increases with current density and
decreases with increasing temperature, but its magnitude is variable and indeterminate. It is negligible
for the deposition of metals and for changes in oxidation state.
Concentration Overpotential or Concentration Polarization
The additional potential required to maintain a current flowing in a cell when the concentration of the
electroactive species at the electrode surface is less than that in the bulk solution. In extreme cases, the
cell current reaches a limiting value determined by the rate of transport of the electroactive species to
the electrode surface from the bulk solution. The current is then independent of cell potential and the
electrode or cell is said to be completely polarized. Concentration overpotential decreases with stirring
and with increasing electrode area, temperature and ionic strength.
Activity Dependence of Electrode Potentials – The Nernst Equation
Electrode and therefore cell potentials are very important analytically as their magnitudes are
determined by the activities of the reactants and products involved in the electrode reactions. The
relation between such activities and the electrode potential is given by the Nernst equation. For a
general half-cell reaction written as a reduction, i.e. aA + bB +... ne = xX + yY +... , the equation is
of the form
where E is the electrode potential, the standard potential, [ ] the activities of reactants and products
at the electrode surface, R the gas constant, T the thermodynamic temperature, F the Faraday constant
(96