Electrochemical Detection - Waterborne Pathogens: Detection Methods and Applications - page 147

by voltammetry techniques. Concentrations of trace metals in blood, urine,
and tissue can be measured by voltammetry. Choline, ethanol, formalde-
hyde, glucose, and lactate are other examples of molecules measured in
clinical samples by voltammetric techniques. These techniques also find
applications in the food and pharmaceutical industries. On a more funda-
mental aspect, voltammetry is a technique of choice for the study of the
electrochemical behaviors (i.e. equilibrium constant, reversibility, etc. …)
of complex electrolytes. Indeed, if the analytes behave independently, the
voltammogram of a multicomponent solution is the sum of each individual
voltammogram.
6.4. COULOMETRY
Coulometry uses either an applied current or potential to exhaus-
tively convert an analyte from one oxidation state to another at the working
electrode. In these experiments, the total current passed is measured directly
or indirectly to determine the number of electrons passed. Knowing the
number of electrons passed, we can extract the concentration of the analyte
using faraday's law:
Q = n F N A (6.2)
where n is the number of electrons per mole of analyte, F is Faraday's
constant (96,487 C mol −1 ), and N A is the moles of analyte. To obtain an
accurate value for N A , 100% of the current must be used to oxidize or to
reduce the analyte.
There are two forms of coulometry: controlled-potential coulometry
and controlled-current coulometry. A three-electrode potentiostat is used to
set the potential in controlled potential coulometry ( Fig. 6.4 ). The working
electrode is either a platinum or a liquid mercury electrode. The auxiliary
electrode is very often made of platinum and is separated by a salt bridge
from the analytical solution. A saturated calomel or silver chloride electrode
serves as the reference electrode. To measure the total charge, the potentiostat
monitors the current as a function of time and uses electronic integration to
calculate the corresponding charge as a function of time. During the elec-
trolysis process, the analyte's concentration in the solution decreases, and as
a consequence the electrolysis rate decreases. This situation leads to a rather
long analysis time (i.e. between 30 and 60 min), which can be problematic for
industrial applications. Controlled-current coulometry has the advantage of
a constant electrolysis current and thus a shorter analysis time (i.e. <10 min).