electrochemical methods of analysis of complex samples

ELECTROCHEMICAL
METHODS
SAN 405

Introduction
• 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 method 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:
1. Potentiometry,
2. Voltammetry,
3. Coulometry,
4. Conductometry, and
5. Dielectrometry.

Electrochemical techniques
The electrochemical techniques is divided into two:
1.Static technique: It is when the current is not passed 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.
2.Dynamic technique: 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 variable potential.

Controlling and Measuring Current and Potential
• 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.

• 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 electrode.
• 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 potential, E. (E =iR)
• Each of these experimental designs uses a different
electrochemical technique

Types 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 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.

Potentiometric titration
• It 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
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 quantitative method.

Potentiometric Measurements:
• It 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 half cells, 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, connect 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 reaction; thus
• Zn(s) ↔ Zn2+
(aq) +2e-
• The electrode on the right is the cathode, where the reduction
reaction occurs
• Ag+
(aq) + e−
↔ Ag (s)

electrochemical methods of analysis of complex samples

Types 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) Acid-Base Titration
b) Complexometric Titration
c) Oxidation-Reduction Titration
d) Precipitation Titration

Location 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
endpoint 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.

Analytical or Derivative Method:
• The end point can be more precisely
located from the first or second
derivative curves.
• The first derivative curve involves plot of
slope of the titration curve (ΔE/ΔV-ratio
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
the titration.

• 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 the indicator electrode potential.

Potentiostat
• A potentiostat is an electronic instrument that controls the voltage between two electrodes
Two Electrode Configurations
• This configuration consists of a Working Electrode where the chemistry of interest occurs and a
Counter Electrode which acts as the other half of the cell. The applied potential (EA) is measured
between the working and counter electrode and the resulting current is measured in the working
or counter electrode lead.
• The counter electrode in the two electrode set up serves two functions.
‐
• It completes the circuit allowing charge to flow through the cell, and
• it also maintains a constant interfacial potential, regardless of current.
• Fulfilling both of these requirements is an impossible task under most conditions. In a two
electrode system, it’s very difficult to maintain a constant counter electrode potential (eC) while
current is flowing. This fact, along with a lack of compensation for the voltage drop across the
solution (iRS) leads to poor control of the working electrode potential (eW) with a two electrode
system.
• The roles of passing current and maintaining a reference voltage are better served by two
separate electrodes.

Three Electrode Configuration
• The three electrode system remedies many of the issues of the two electrode
configuration. It consists of a working electrode, counter electrode, and reference
electrode.
• The reference electrode’s role is to act as a reference in measuring and controlling the
working electrode potential, without passing any current.
• The reference electrode should have a constant electrochemical potential at low current density.
• Additionally, since the reference electrode passes negligible current, the iR drop between the
reference and working electrode (iRU) is often very small.
• Thus with the three electrode system, the reference potential is much more stable, and there is
compensation for iR drop across the solution.
• This translates into superior control over working electrode potential.
• The most common lab reference electrodes are the Saturated Calomel Electrode and the Ag/AgCl
electrode.
• In the three electrode configuration, the only role of the counter electrode is to pass
all the current needed to balance the current observed at the working electrode. The
counter electrode will often swing to extreme potentials in order to accomplish this
task.

Advantages of potentiometric titrations over 'classical'
visual indicator methods are:
1. Can be used for coloured, turbid or fluorescent
analyte solution.
2. Can be used if there is no suitable indicator or the
colour change is difficult to ascertain.
3. Can be used in the titration of polyprotic acids,
mixtures of acids, mixtures of bases or mixtures of
halides.

2. Voltammetry method:
• It is based on the application of 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, cyclic voltammetry, polarography and
amperometry).

3. 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;

4. Coulometry methods:
• It is 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.

electrochemical methods of analysis of complex samples

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