Electrochemical Detection - Waterborne Pathogens: Detection Methods and Applications - page 149

Controlled-potential coulometric analyses have been used to determine
the concentration of more than 50 chemical elements, including trace met-
als and halides ions. These analyses are frequently used for the determina-
tion of uranium and plutonium, as well as for the measurement of oxygen
content. However, controlled-current coulometry is more versatile and thus
used in a wider range of applications. It can determine concentrations of
water on the order of milligrams per liter in substances such as butter, sugar,
cheese, paper, and petroleum. It is also used to measure the thickness of
metallic coatings.
6.5. IMPEDANCE SPECTROSCOPY
Small signal electrochemical impedance spectroscopy is a valuable
tool for studying transport properties in electrolytes and electrochemical
reactions at the interface between the solid and the electrolyte. The value
of impedance spectroscopy is derived from the effectiveness of the tech-
nique in isolating individual phenomenon in a multistep process. Assuming
that each phenomenon has a unique associated time constant, the various
phenomena can be separated in the frequency domain. Although any kind
of perturbation can be applied, most reported studies apply a small sinusoi-
dal current perturbation to an equilibrium system and measure the corre-
sponding voltage response.
Typically, alternative current impedance experiments are carried out
over a wide range of frequencies (several millihertz to several megahertz),
and the interpretation of the resulting spectra is performed by extracting an
equivalent analog circuit made of resistors and capacitors. 14 In general, the
equivalent circuits are not unique. One has to choose a physically plausible
circuit containing a minimal number of components. From the value of
each component, some meaningful insight in the system properties can be
drawn.
The current flowing at an electrified interface due to an electrochemical
reaction is made of faradaic and nonfaradaic components. When the charge
transfer takes place at the interface, the mass transports of the reactant and
product determine the rate of electron transfer. To eliminate or minimize
the effects of capacitive currents during transient electrochemical experi-
ments and identify the various transport processes, voltammetric (Section
6.3 ) and coulometric (Section 6.4) have been developed. 15 However, even
if they give good performance in terms of accuracy and rapidity, most often,
they are not able to give a complete picture of the mechanisms involved