Electrochemical Detection - Waterborne Pathogens: Detection Methods and Applications - page 145

working electrode, the ratio between the different redox species has to be
adjusted giving rise to the faradaic current. The magnitude of the current
is determined by the rate of the oxidation or reduction reaction. It depends
on the rate at which the reactants and products are transported to and from
the electrode and the rate at which electrons pass between the electrode and
the redox species in solution. The mass transport is the result of a diffusion
mechanism due to the existence of a concentration gradient for the redox
molecules. Electrodynamic effects can also be involved in mass transport.
Additionally, hydrodynamic effects have to be taken into account if stir-
ring is used or if temperature gradient exists in the solution. This convec-
tion phenomenon has a direct impact on the shape of the voltammogram
(i.e. existence of a peak current or limiting current). The electron transfer
can exhibit two extreme cases. If it is fast, the redox reaction is at equilib-
rium and the reaction process is reversible. On the contrary, if it is slow, the
Nernst equation is not verified and the reaction process is irreversible.
Apart from this faradaic current, a capacitive charging current can also
be temporarily recorded due to the application of the potential on the elec-
trode. Indeed, when applying the potential, we modify the charges on the
electrode. In order to maintain electroneutrality at the solid-liquid interface,
charges of opposite sign migrate from the solution to the interface, and
charges of the same sign migrate away from this interface. This flow of ions
is called the capacitive charging current and cannot be distinguished from
the electron flow. This phenomenon is responsible for the creation of the
so-called electrical double layer.
There are many different voltammetric techniques depending on how
the voltage is applied at the working electrode, when the current is mea-
sured, and whether the solution is stirred. In polarography, a dropping
mercury electrode is chosen as the working electrode. The current flowing
through the system is measured while applying a linear potential ramp
or a series of potential pulses for better sensitivity. Polarography is used
for the analysis of metal ions, inorganic anions, such as NO 3 and some
organic compounds (e.g. carboxylic acids). Hydrodynamic voltammetry is
very similar to polarography (i.e. same applied voltage solicitations) except
that the liquid working electrode is replaced by a solid-state electrode
and the electrolyte is stirred to reproduce the distribution of electroactive
species found in polarography at the liquid-liquid interface. Anodic strip-
ping voltammetry involves the preconcentration of a metal phase onto a
solid electrode surface or into Hg (liquid) at negative potentials through
an electrolysis process. Metal deposition is usually enhanced by stirring.