Presentation electroanalytical

PRESENTATION
ELECTRO ANALYTICAL TECHNIQUES

• PRESENTED TO
SIR MOHSIN
• PRESENTED BY
AROOJ ANWAAR
• CLASS
MSC IV
• TOPIC
CONTROL POTENTIAL COULOMETRY 2
Douglas A. Skoog, Donald M. West, F. James Holler, Stanley R. Crouch - Fundamentals of Analytical Chemistry-Cengage Learning (2013).

PRESENTATION SCHEME
• INTRODUCTION TO ELECTROANALYTICAL TECHNIQUES
• TYPES
• COULOMETRY
• PRINCIPLE
• CONTROL POTENTATIL COULOMETRY
• APPLICATION
• CONCLUSION
• REFERENCES
3

INTRODUCTION
Electroanalytical techniques include a group of instrumental techniques like potentiometry,
voltammetry, conductometry, coulometry, electrogravimetry in all of those techniques there is
interaction of electricity with matter and in all the above mentioned techniques we measure the
electrochemical properties like potential, current, charge etc with help of different instruments like
potentiometer, pH meter, conductometer, voltammeter etc,. The use of electrical measurements for
analytical purposes has found large range of applications including analytical, environmental
monitoring, industrial quality control & biomedical analysis.
4
Douglas A. Skoog, Donald M. West, F. James Holler, Stanley R. Crouch - Fundamentals of Analytical Chemistry-Cengage Learning
(2013).

CLASSIFICATION OF ELECTROANALYTICAL TECHNIQUES
 POTENTIOMETRY
 AMPEROMETRY
5

6

COULOMETRIC METHOD
In coulometric methods, the quantity of electrical charge required to convert a
sample of an analyte quantitatively to a different oxidation state is measured.
7

HISTORY
8

ELECTRICAL CHARGE
Electrical charge with symbol Cis basis of other electrical quantites
such as
Current
Power
Voltage
9

PRINCIPLE
During an electrolysis, the total charge, Q, in coulombs, passing
through the electrochemical cell is proportional to the absolute
amount of analyte by faraday’s law
Q = nFNA
where n is the number of electrons per mole of analyte, F is
Faraday’s constant (96487Cmol-1), and NA is the moles of analyte
10

A rate of charge flow equal to one coulomb per second is the
definition of one ampere (A) of current. Thus, a coulomb can be
considered as that charge carried by a constant current of one
ampere for one second. The charge Q that results from a constant
current of I amperes operated for t seconds is
Q = It
11

to calculate Q. Knowing the total charge, we then use equation (1)
to determine the moles of analyte.
To obtain an accurate value for NA, all the current must be used to
oxidize or reduce the analyte.
 In other words, coulometry requires 100% current efficiency
12

Two methods have been developed that are based on measuring the
quantity of charge
controlled-potential (potentiostatic) coulometry
 controlled-current coulometry, often called coulometric titrimetry.
13

CONTROLLED-POTENTIAL COULOMETRY
PRINCIPLE
In controlled-potential coulometry, the potential of the working
electrode is maintained at a constant level such that only the analyte
is responsible for conducting charge across the electrode/solution
interface. The charge required to convert the analyte to its reaction
product is then determined by recording and integrating the current-
versus-time curve during the electrolysis.
14

As electrolysis progresses the
analyte’s concentration decreases, as
does the current. The resulting current-
versus-time profile for controlled-
potential coulometry is shown in the
15

Methods for determining the total charge;
1. One method is to monitor the current as a function of time and
determine the area under the curve, as shown in the above figure.
2. Modern instruments use electronic integration to monitor charge
as a function of time. The total charge at the end of the electrolysis
is read directly from a digital readout
16

SELECTING A CONSTANT POTENTIAL
In controlled-potential coulometry, the potential is selected so that
the desired oxidationn or redn reaction goes to completion without
interference from redox reaction involving other components of the
sample matrix
17

MINIMIZING ELECTROLYSIS TIME
• The current-time curve for controlled-potential coulometry in Fig.
shows that the current decreases continuously throughout
electrolysis.
• An exhaustive electrolysis, therefore, may require a long time.
• Since time is an important consideration in choosing and designing
analytical methods, the factors that determine the analysis time
need to be considered.
18

INSTRUMENTATION
Three-electrode potentiostat is used to set the potential in
controlled potential coulometry. The working electrodes is usually
one of two types: a cylindrical Pt electrode manufactured from
platinum-gauze or a Hg pool electrode
Platinum is the working electrode of choice when we need to apply
a positive potential while Hg is the electrode of choice for an
analyte requiring a negative potential..
19

The auxiliary electrode, which is often a Pt wire, is separated by a salt
bridge from the analytical solution. This is necessary to prevent the
electrolysis products generated at the auxiliary electrode from reacting
with the analyte and interfering in the analysis.
 A saturated calomel or Ag/AgCl electrode serves as the reference
electrode.
20

PLATINUM-GAUZE TYPE
21

• It consists of a platinum-gauze working electrode, a platinum-wire counter electrode,
and a saturated calomel reference electrode.
• The counter electrode is separated from the analyte solution by a salt bridge that usually
contains the same electrolyte as the solution being analyzed.
• The salt bridge is needed to prevent the reaction products formed at the counter
electrode from diffusing into the analyte solution and interfering. For example, hydrogen
gas is a common product at a cathodic counter electrode. Unless hydrogen is physically
isolated from that analyte solution by the bridge, it will react directly with many of the
analytes that are determined by oxidation at the working anode
22

Mercury pool type cell
23

• The second type of cell, is a mercury-pool type. A mercury cathode is particularly
useful for separating easily reduced elements as a preliminary step in an analysis.
• In addition, however, it has found considerable use for the coulometric
determination of several metallic cations that form metals soluble in mercury. In
these applications, little or no hydrogen evolution occurs even at high applied
potentials because of the large overvoltage of hydrogen on mercury.
• A coulometric cell such as that is also useful for the coulometric determination of
certain types of organic compounds.
24

APPLICATION
• Coulometry is used for the quantitative analysis of both inorganic
and organic analytes.
• The majority of controlled-potential coulometric analyses involve
the determination of inorganic cations and anions, including trace
metals and halides ions.
• It is also used for the quantitative analysis of organic compounds,
example is the reduction of trichloroacetate to dichloroacetate, and
of dichloroacetate to monochloroacetate.
25

The technique is widely adopted for the determination of uranium
and pluotinum and thus finds extensive use in the nuclear energy
field.
This technique is especially useful for the determination of small
amounts of analyte (0.01– l mg) with an accuracy
26

DISADVANTAGES
• Difficult to ensure 100% current efficincy
• Need a long time
27

CONCLUSION
• Coulometry is an analytical method for measuring an unknown concentration of
an analyte in solution by completely converting the analyte from one oxidation
state to another. Coulometry is an absolute measurement similar to gravimetry
or titration and requires no chemical standards or calibration.
28

REFERENCES
• Douglas A. Skoog, Donald M. West, F. James Holler, Stanley R. Crouch -
Fundamentals of Analytical Chemistry-Cengage Learning (2013).
29

Douglas A. Skoog, Donald M. West, F. James Holler, Stanley R. Crouch - Fundamentals of Analytical Chemistry-Cengage Learning (2013). 30

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