Electrochemical Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 293

1 coulomb¼1As) relates to the current (I) according to the following equations
depending on whether the cell has a constant current

Q ¼ It ðconstant currentÞ ð11 : 6Þ

ð t

Q ¼ I dt ðcontrolled potentialÞ ð11 : 7Þ

0

where the current (I) has a unit of ampere (A) or ''amp'' for short and t is the time in
seconds(s). Two classes of coulometric techniques, therefore, are common: constant
current techniques and controlled potential techniques. In constant current
techniques (also known as coulometric titrations), a reagent is generated at one
electrode, this reagent reacts stoichiometrically with the analyte referred to as the
titrant. In controlled potential techniques, the current resulting from a complete
electrochemical reaction of the analyte at the electrode is monitored with respect to
time.
The underlying equation relating charge (Q) and the quantity of analtye (m)is
the Faraday's law of electrolysis, which states that the mass (m) of the analyte
reduced or oxidized during electrolysis is proportional to the number of moles of
electrons transferred at that electrode. Mathematically, it can be written as

Q
F

M
n

m ¼ ð11 : 8Þ

where F, M, and n are constants representing, respectively, the Faraday's constant
(96,485 coulombs per mole electron), molecular weight of the analyte (g per mole
analyte), and the number of mole of electrons per mole of analyte. For example, if a
constant current of 0.5 A was used to deposit all Cu 2þ in 10 min in the electrolytic
cell described in Figure 11.1, then the amount of copper (Cu) in the solution can be
determined as

0 : 5 A10 min60 s = min 63 : 5gCu = mol Cu

mol Cu ¼ 9:8710 2 gCu

m ¼

mol e 96 ; 485 coulombs = 2 mol e =

Voltammetry

Voltammetry is a dynamic electrochemical method, where current (I) is monitored as
a function of the changing potential (E) over a period. A voltammetric cell is
an electrolytic cell consisting of three electrodes, a micro indicator electrode, a
reference electrode, and an auxiliary counter electrode (Fig. 11.2). A changing
potential is applied on the indicator electrode to drive a redox reaction that can not
occur spontaneously, and the counter electrode serves to conduct electricity between
two electrodes. The reference electrode has a constant potential throughout the
experiment. This unique three-electrode system instead of the two-electrode system
allows for an accurate application of potential functions and the measurement of the
resultant current. There are various voltammetric techniques. They are distinguished