METHODS OF ANALYSIS
Electrochemistry is branch of chemistry
concern with the interaction of electrical and
A large part of this field deals with the study of
chemical changes caused by the passage of an
electrical current and the production of
electrical energy by chemical reaction.
It is named electrochemistry because its
originated from the study of the movement of
electrons in an oxidation–reduction reaction.
Electrochemical methods: are analytical
techniques that use a measurement of
potential, charge, or current to determine an
analyte’s concentration or to characterize an
analyte’s chemical reactivity.
It is a qualitative and quantitative methods of
analysis based on electrochemical phenomena
occurring within a medium or at the phase
boundary and related to changes in the structure,
chemical composition, or concentration of the
compound being analyzed.
These methods are divided into five major
groups: potentiometry, voltammetry, coulometry,
conductometry, and dielectrometry.
Obtaining thermodynamic data about a
To generate an unstable intermediate such as
radical ion and study its rate of decay or it is
They use to analyze a solution for trace
amount of metal ions or organic species.
4. The electrochemical properties of the
system themselves are of primary interest,
for example, in the design of a new power
source or for the electrochemical methods
have been developed.
Type of electrochemical techniques
1. Bulk techniques, in which we measure a
property of the solution in the
electrochemical cell. An example is the
measurement of a solution’s conductivity,
which is proportional to the total
concentration of dissolved ions,
2. Interfacial techniques, in which the
potential, charge, or current depends on the
species present at the interface between an
electrode and the solution in which it sits.
An example is the determination of pH
using a pH electrode.
Despite the difference in instrumentation, all
electrochemical techniques share several
(1) The electrode’s potential determines the
analyte’s form at the electrode’s surface
(2) The concentration of analyte at the
electrode’s surface may not be the same as its
concentration in bulk solution;
(3) Current is a measure of the rate of the
analyte’s oxidation or reduction; and
(4) We cannot simultaneously control
current and potential.
Interfacial Electrochemical Techniques
The interfacial electrochemical techniques is
divided into Static techniques and dynamic
Static technique the current is not pass through
the analyte’s solution. Potentiometry, in which
we measure the potential of an electrochemical
cell under static conditions, is one of the most
important quantitative electrochemical methods
Dynamic techniques, in which we allow current
to flow through the analyte’s solution, it
comprise the largest group of interfacial
electrochemical techniques e.g. Coulometry, in
which we measure current as a function of time,
Amperometry and voltammetry, in which we
measure current as a function of a fixed or
Measuring Current and
we cannot simultaneously control both
current and potential
if we choose to control the potential, then we
must accept the resulting current, and we
must accept the resulting potential if we
choose to control the current.
The second electrode, which we call the
counter electrode, completes the electrical
circuit and provides a reference potential
against which we measure the working
electrodes potential. Ideally the counter
electrode’s potential remains constant so that
we can assign to the working electrode any
change in the overall cell potential.
Electrochemical measurements are made in an
electrochemical cell consisting of two or more
electrodes and the electronic circuitry for controlling
and measuring the current and the potential.
The simplest electrochemical cell uses two
electrodes. The potential of one electrode is
sensitive to the analyst’s concentration, and is
called the working electrode or the indicator
If the counter electrode’s potential is not
constant, we replace it with two electrodes: a
reference electrode whose potential remains
constant and an auxiliary electrode that
completes the electrical circuit.
Because we cannot simultaneously control the
current and the potential, there are only three
basic experimental designs
(1) Measure the potential when the current is zero,
(2) Measure the potential while controlling the current,
(3) Measure the current while controlling the potential
Each of these experimental designs relies on Ohm’s
law, which states that a current, i, passing through an
electrical circuit of resistance, R, generates a
Each of these experimental designs uses a different
type of instrument
of Electrochemical Methods
1. Potentiometry methods: it measures the
potential of a solution between two electrodes.
The potential is then related to the concentration
of one or more analytes. The cell structure used
is often referred to as an electrode even though it
actually contains two electrodes: an indicator
electrode and a reference electrode.
Potentiometry usually uses electrodes made
selectively sensitive to the ion of interest,
such as a fluoride-selective electrode. The
most common potentiometric electrode is the
glass-membrane electrode used in a pH meter.
2. Voltammetry method: is based on the applies a
constant and/or varying potential at an
electrode's surface and measures the resulting
current with a three electrode system.
Voltammetry, with its variety of methods,
constitutes the largest group of electrochemical
methods of analysis and is commonly used for
the determination of compounds in solutions
(for example, polarography and amperometry).
3. Coulometry methods: based on the
measurement of the amount of material
deposited on an electrode in the course of an
electrochemical reaction in accordance with
Faraday’s laws. A distinction is made between
coulometry at constant potential and
coulometry at constant current.
Coulometry uses applied current or potential to
completely convert an analyte from one
oxidation state to another. 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
can indicate the concentration of the analyte or,
when the concentration is known, the number of
electrons transferred in the redox reaction.
4. Conductometry methods: in which
the electrical conductivity of electrolytes
(aqueous and non-aqueous solutions, colloid
systems and solids) is measured
It is based on the change in the concentration
of a compound or the chemical composition
of a medium in the interelectrode space;
Is a technique similar to direct titration of a
redox reaction. No indicator is used, instead
the potential across the analyte, typically an
electrolyte solution is measured. To do this,
two electrodes are used, an indicator
electrode and a reference electrode.
In potentiometry we measure the potential of
an electrochemical cell under static conditions.
Because no current—or only a negligible
current—flows through the electrochemical
cell, its composition remains unchanged. For
this reason, potentiometry is a useful
A potentiometer is used to determine the
difference between the potential of two
electrodes. The potential of one electrode—the
working or indicator electrode—responds to
the analyte’s activity, and the other electrode—
the counter or reference electrode—has a
known, fixed potential.
Potentiometric Electrochemical Cells
The electrochemical cell consists of two halfcells, each containing an electrode immersed in
a solution of ions whose activities determine
the electrode’s potential. A salt bridge
containing an inert electrolyte, such as KCl,
connects the two half-cells.
The ends of the salt bridge are fixed with
porous frits, allowing the electrolyte ions to
move freely between the half-cells and the salt
bridge. This movement of ions in the salt
bridge completes the electrical circuit as
shown in the Figure below.
By convention, we identify the electrode on the
left as the anode and assign to it the oxidation
Zn(s) ↔ Zn2 (aq) +2e −
The electrode on the right is the cathode, where
the reduction reaction occurs
Ag +(aq) + e− ↔ Ag (s)
of potentiometric titrations over
'classical' visual indicator methods are:
Can be used for coloured, turbid or
fluorescent analyte solution.
Can be used if there is no suitable indicator
or the colour change is difficult to ascertain.
Can be used in the titration of polyprotic
acids, mixtures of acids, mixtures of bases or
mixtures of halides.
of Potentiometric Titration
Depending on the type of the reactions involved to
which potential measurement can be applied for end
point detection, potentiometric titrations can be
classified into followings:
A cid-Base Titration
of the End Point
Titration Curve: It is obtained by plotting the
successive values of the cell emf on y=axis and
corresponding values of volume of titrant added
on the x-axis. This gives an S-shaped curve. The
central portion of this curve which shows the
steeply rising portion corresponds to the volume
for the end point of the titration.
When there is a small potential change at
the end point like in the titration of weak
acid with strong base, titration of very
dilute solution etc, it is difficult to locate
end point by this method.
b) Analytical or Derivative Method: The
end point can be more precisely located from
the first or second derivative curves. The first
derivative curve involves the plot of slope of
the titration curve (ΔE/ΔV-ration of change
in emf and change in volume added) against
the volume of the titrant added.
Most frequently ΔE/ΔV is plotted against
the average volume of titrant added
corresponding to the values of emf taken.
Volume on the x- axis corresponding to
the peak of the curve is the end point of
In second derivative curve we plot the slope of
first derivative curve (Δ2E/ΔV) against volume.
The point on volume axis where the curve cuts
through zero on the ordinate gives the end point.
This point corresponds to the largest steepest
point on titration curve and maximum slope of
the ΔE/ΔV curve.
This mentioned methods need values of
potential corresponding to very small change
in volume of titrant added near the end point
for good result. In the immediate area of the
end point the concentration of the original
reactant becomes very small, and it usually
becomes impossible for the ions to control
the indicator electrode potential.