Electrochemical Detection - Waterborne Pathogens: Detection Methods and Applications - page 141

for the redox reaction at the cathode and the anode. The reduction potential
depends linearly on the logarithm of the ionic concentration according to the
formulation introduced by Walther Nernst in 1889 ( Eqn (6.1) ).
E = E ° −(R T / n F) * ln( a Red / a Ox ) (6.1)
where E ° is the standard-state potential, R is the gas constant, T is the tem-
perature in Kelvin, n is the number of electrons in the redox reaction, F is
Faraday's constant, and a Red / a Ox is the reaction quotient (ratio between the
reductant and oxidant electrochemical activities).
Indicator electrodes can be metallic types or membrane versions, which
are also called ion-selective electrodes (ISEs). Metal electrodes can be sub-
divided into four kinds. In the first kind, the metal electrode is in direct
contact with the electrolyte. If ions of this metal are contained in the system,
then equilibrium is obtained at the metal surface depending on the con-
centration of the metal ions in the solution. Metal ions are accepted by the
metal surface and simultaneously released into the electrolyte. However, this
type of electrode suffers from poor selectivity because it can react to any,
more easily reduced cations. A metal electrode of the second kind consists
of a metal either coated with, or immersed in, one of its soluble salts (e.g.
AgCl). This electrode reacts to the anions of the salt. A metal electrode of
the third kind uses two equilibrium reactions to respond to a cation other
than that of the metal electrode. If the electrolyte does not contain any ions
of the corresponding metal, then metal electrodes can still form an oxida-
tion/reduction potential if a redox reaction occurs in the electrolyte. The
electrode surface is inert to the redox reaction. No metal ions are released
from the metal; in this case, the metal surface only acts as a catalyst for the
electrons. Typically, gold or platinum are used for the metal indicator elec-
trode as they are chemically inert (do not contribute to the reaction). This
electrode is the fourth kind of metal electrodes.
Nowadays, potentiometry usually uses electrodes made selectively sensi-
tive to the ion of interest, such as a fluoride-selective electrode. Membrane
electrodes have a filling solution sealed inside. This solution contains ions to
which the membrane is selective. If there is a difference in activity of these
ions on the two sides of the membrane, ions will enter the membrane from
the side where the activity is higher, and they will exit the membrane on
the other side. This ion flow alters the electrochemical properties of the
membrane and causes a change in potential. A perfect selectivity to one
type of ion is almost never possible. Most ion-sensitive electrodes often
react with ions with similar chemical properties or a similar structure. These