SUMMARY
Electrochemical methods covered in this chapter include potentiometry, coulometry, and voltammetry. Potentiometric
methods are based on the measurement of an electrochemical cell’s potential when only a negligible current is allowed
to flow. In principle the Nernst equation can be used to calculate the concentration of species in the electrochemical cell
by measuring its potential and solving the Nernst equation; the presence of liquid junction potentials, however,
necessitates the use of an external standardization or the use of standard additions.
Potentiometric electrodes are divided into two classes: metallic electrodes and membrane electrodes. The smaller of
these classes are the metallic electrodes. Electrodes of the first kind respond to the concentration of their cation in
solution; thus the potential of an Ag wire is determined by the concentration of Ag + in solution. When another species
is present in solution and in equilibrium with the metal ion, then the electrode’s potential will respond to the
concentration of that ion. For example, an Ag wire in contact with a solution of Cl – will respond to the concentration of
Cl – since the relative concentrations of Ag + and Cl – are fixed by the solubility product for AgCl. Such electrodes are
called electrodes of the second kind.
The potential of a membrane electrode is determined by a difference in the composition of the solution on either side of
the membrane. Electrodes using a glass membrane respond to ions that bind to negatively charged sites on the glass
membrane’s surface. A pH electrode is one example of a glass membrane electrode. Other kinds of membrane
electrodes include those using insoluble crystalline solids and liquid ion exchangers incorporated in a hydrophobic
membrane. The F – ion-selective electrode, which uses a single crystal of LaF3 as the ion-selective membrane, is an
example of a solid-state electrode. The Ca 2+ ion-selective electrode, in which a chelating ligand such as di-(ndecyl)phosphate is immobilized in a PVC membrane, is an example of a liquid-based ion-selective electrode.
Potentiometric electrodes also can be designed to respond to molecules by incorporating a reaction producing an ion
whose concentration can be determined using a traditional ion-selective electrode. Gas-sensing electrodes, for
example, include a gas-permeable membrane that isolates the ion-selective electrode from the solution containing the
analyte. Diffusion of a dissolved gas across the membrane alters the composition of the inner solution in a manner that
can be followed with an ion-selective electrode. Enzyme electrodes operate in the same way.
Coulometric methods are based on Faraday’s law that the total charge or current passed during an electrolysis is
proportional to the amount of reactants and products in the redox reaction. If the electrolysis is 100% efficient, in that
only the analyte is oxidized or reduced, then the total charge or current can be used to determine the amount of analyte
in a sample. In controlled-potential coulometry, a constant potential is applied and the current is measured as a function
of time, whereas in controlled-current coulometry, the current is held constant and the time required to completely
oxidize or reduce the analyte is measured.
In voltammetry we measure the current in an electrochemical cell as a function of the applied potential. Individual
voltammetric methods differ in terms of the type of electrode used, how the applied potential is changed, and whether
the transport of material to the electrode’s surface is enhanced by stirring.
Polarography is a voltammetric experiment conducted at a mercury electrode under conditions in which the solution is
not stirred. Normal polarography uses a dropping mercury electrode (or a static mercury drop electrode) and a linear
potential scan in an unstirred solution. Other forms of polarography include normal pulse polarography, differential
pulse polarography, staircase polarography, and square-wave polarography, all of which apply a series of potential
pulses to the mercury electrode.
In hydrodynamic voltammetry the solution is stirred either by using a magnetic stir bar or by rotating the electrode.
Because the solution is stirred, a dropping mercury electrode cannot be used and is replaced with a solid electrode.
Both linear potential scans or potential pulses can be applied.
In stripping voltammetry the analyte is first deposited on the electrode, usually as the result of an oxidation or reduction
reaction. The potential is then scanned, either linearly or by using potential pulses, in a direction that removes the
analyte by a reduction or oxidation reaction.
Amperometry is a voltammetric method in which a constant potential is applied to the electrode and the resulting
current is measured. Amperometry is most often used in the construction of chemical sensors that, as with
potentiometric sensors, are used for the quantitative analysis of single analytes. One important example, for instance, is
the Clark O2 electrode, which responds to the concentration of dissolved O2 in solutions such as blood and
water.