This document discusses various concepts related to electrogravimetric analysis including IR drop, overpotential, polarization, and coulometry. It defines electrogravimetry as quantitatively depositing a product on an electrode through electrolysis and weighing it. It describes IR drop as the potential drop due to resistance of current flow. Overpotential is the difference between theoretical and actual cell potentials. Polarization occurs when the electrode potential deviates from its theoretical value with current. Coulometry involves measuring the electric charge passed during an electrochemical reaction based on Faraday's laws of electrolysis. The document compares controlled potential and controlled current coulometry methods.

1. 1

2. IR DROP
, OVER POTENTIAL, POLARIZATION, CONTROLLED
CURRENT COULOMETRY, CONTROLLED POTENTIAL COULOMETRY
PRESENTED TO
DR. MUHAMMAD WASEEM MUMTAZ
PRESENTED BY
KAINAT
18551507-046
COURSE CODE
CHEM- 416
COURSE TITLE
ANALYTICAL CHEMISTRY-V (T)
BS CHEMISTRY VII-A
DEPARTMENT OF CHEMISTRY
UNIVERSITY OF GUJRAT
2

3. CONTENTS
SR# SLIDE #
1.1. ELECTROGRAVIMETRIC ANALYSIS………………………...04
2.1. IR DROP………………………………………………………..….06
3.1. TYPES OF ELECTROLYTIC CELL………………………...…08
3.1. OVERPOTENTIAL ……………………………………………....09
4.1. POLARISATION………………………………………………….10
4.2. CONCENTRATION POLARISATION………………...……….11
4.3. KINETIC POLARISATION……………………………………..14
5.1. CHARACTERISTICS OF GOOD DEPOSITS………………...16
6.1. COULOMETRY……………………………………………….....17
6.2. TYPES OF COULOMETRY………………………..........………20
6.3. CONTROLLED POTENTIAL COULOMETRY……........……20
6.4. CONTROLLED CURRENT COULOMETRY ………………..23
7.1. APPLICATIONS OF COULOMETRY………………….............27
8.1. REFERENCES……………………………………………………28
3

4. ELECTROGRAVIMETRIC ANALYSIS
 In electrogravimetry, product is deposited quantitatively on an electrode by
an electrolytic reaction and the amount of product is determined by weighing
the electrode before and after the electrolysis.
 The material is deposited on an electrode by the
application of potential.
 Hence named as electrogravimetry (weighing of
product after electrolysis).
 The analyte in solid form is deposited on
electrodes under application of electrical potential.
 For example electrodeposition of Cd from its solution involves the following
reduction event
Cd+2
(aq) + 2e- ↔ Cd(s)
Fig1: motion of ions through a solution
due to electric current passage
https://slideplayer.com/slide/14679336/
PRINCIPLE
4

5. Consider a cell of the type where copper is deposited at the cathode and oxygen is
evolved at the anode i.e. electrolysis of CuSO4 solution using Pt electrode.
 Ions present: Cu+2, H+ , SO4
2-, OH- ,
 At Cathode: Cu+2
(aq) + 2e- ↔ Cu(s)
 At Anode: 4OH-
(aq) ↔ O2(g) + H2O(l) + 4e-
 Cell Potential: ECell = Ecathode – Eanode
 Consider an electrolytic cell. A voltage
Eapplied is applied to the cell in such a way that current
flows through the cell.
 When Eapplied > Ecell , there will be a flow of current in the circuit.
 When there is a current, potential of the cell is less than thermodynamic potential
because of one of the following phenomena are operating:
1. IR DROP 2. Concentration Polarization 3. Kinetic polarization
Fig2: electrolysis of CuSO4 solution using
Pt electrode
https://www.aplustopper.com/electrolysis-
icse-solutions-class-10-chemistry/
5
EXPLANATION

6. OHMIC POTENTIAL: IR DROP
Electrochemical cells, like metallic conductors, resist the flow of charges. In both
type of conduction, Ohm’s law describes effect of this resistance. The product of
resistance R of a cell in ohms and the current, I in amperes is called as ohmic
potential or the IR drop of cell.
 When Eapplied = Ecell , no current flows through the cell. When applied potential
is increased gradually, a small current appears in circuit. This current through
cell encounters the resistance R resulting in potential drop of –IR volts.
 Thus, in the presence of current, a cell potential must be modified as
Eapplied = Ecell – IR
we know that ECell = Ecathode – Eanode
Eapplied = Ecathode – Eanode – IR
This equation can be rearranged as I = (Ecathode – Eanode ) – Eapplied / R
OR I = Ecell - Eapplied / R
Since Ecell is constant so I = - Eapplied / R
6
EXPLANATION

7.  Using three electrode systems
1. Reference electrode: maintains fixed potential
2. Working electrode: electrode of interest
3. Counter electrode/Auxilary electrode: electrode taking most of current flow.
 Having a very small cell resistance ( high ionic strength solution)
OHM’S LAW
Ohm's law states that the current through a conductor between two points
is directly proportional to the voltage across the two points and resistance
between the.
V=IR
Fig4: Graphical Representation of Ohm’s law
https://dipslab.com/ohms-law-statement-formula-limitations-applications/
7
HOW TO MINIMIZE IR DROP

8. TYPES OF ELECTROLYTIC CELL
There are two types of electrolytic cells based on cell polarity
a) Non-Polarised Cells
b) Polarized Cells
1. Non-Polarised Cells
 Cells that exhibit linear relationship between current and voltage are
said to be non-polarised cells.
 The more non-polarised cell, the greater will be the efficiency of cell.
2. Polarised Cells
 Cells that exhibit non-linear relationship between current and voltage
are said to be polarised cells.
 The degree of polarisation is given by over potential or over voltage.
 Polarisation requires the application of a potential greater than
theoretical value to give a current of expected magnitude.
8

9. OVERPOTENTIAL
Overvoltage is the potential difference between the theoretical cell
potential and actual current potential at a level of given current.
 On the other hand polarisation is the departure of the electrode potential
from its theoretical value on passage of current.
Eapplied = Ecell – IR – overvoltage
Fig5:Current Potential curve
https://youtu.be/U71-Bswu0lA
9
GRAPHICAL REPRESENTATION
overpotential

10. POLARISATION
 After the increase of Eapplied to overcome IR Drop and over potential, if
we further increase Eapplied , then current becomes independent of Eapplied .
 This give rise to the limiting current.
 At this situation, electrode is said to be
completely polarised since its potential
can be changed widely without
affecting the current.
Fig6:Current Potential curve
Chap#05, Page#30
POLARISATION
Concentration Polarisation Kinetic Polarisation
10

11. 1. Concentration Polarisation
 When ions are discharged and deposited on an electrode as are result of
passage of current, the area in the immediate vicinity of the electrode is
depleted of ions and layer offers resistance to the passage of current.
 This type of resistance due to changes in concentration of the electrolyte
around the electrode is known as concentration polarisation.
 Concentration polarization occurs when ions do not arrive at cathode or the
product species do not leave anode fast enough to maintain the desired
current.
 Reactants are transported to an electrode surface by three mechanisms a)
diffusion b) migration c) convection.
 Concentration Polarisation arises when effects of diffusion, migration and
convection are insufficient to transport the ions at a rate that produce a current
of desired magnitude.
 If the depleted area in the vicinity of electrode is not supplied with ions by
increased diffusion or convection, the cathode will assume more and more
negative potential as the applied potential is increased.
11

12. CONTD……
 Hence, concentration polarisation required applied potentials that are larger than
the theoretical potential to maintain a given current in an electrolytic cell.
Example#01:Cathodic Polarisation (concentration)
 Suppose that there are concentration effects near electrode surface for hydrogen
reduction reactions.
 If reactant hydrogen is too slow to diffuse at electrode surface then electrons
again can accumulate at the metal side of interface.
 As a result electrode potential become more and more negative due to
concentration polarisation.
Fig7:Cathodic polarisation (concentration)
https://slidetodoc.com/electrochemical-polarization-materials-
engineering-dr-lubna-ghalib-electrochemical/
12

13. Example #02: Anodic polarisation (concentration)
 Suppose oxidation of Fe atoms to Fe2+ ions is to slow to diffuse away
the metal surface.
 The surface become more positive due to accumulation of Fe2+ ions.
 The electrode potential E become more positive due to concentration
gradient.
Fig8:Anodic polarisation (concentration)
https://slidetodoc.com/electrochemical-polarization-materials-
engineering-dr-lubna-ghalib-electrochemical/
13

14. 2. Kinetic Polarisation
 Kinetic polarisation may occur due to changes in chemical nature of
depositing analyte.
 May also be caused by deposition of films on electrode
 Coating of electrode with some gaseous products e.g. oxygen or
hydrogen is called as GAS PLOARISATION or GAS
OVERPOTENTIAL.
1. Electrode material: If electrode material is rough then there will be a
decrease in gas overpotential.
2. Current density: Current density ∝ gas overpotential
3. Temperature: Temperature ∝ 1/ gas overpotential
4. pH
Factors on which Gas overpotential depends
14

15. How to minimize polarisation?
The following are the factors which can minimize polarisation:
1. If electrolyte is constantly stirred concentration change in vicinity of
electrode can be avoided and if there is no increase in concentration
in vicinity of electrode and polarisation does not take place.
2. Electrodes can be brushed off to remove the deposited gases.
3. Deposition of gases on surface of electrode can be minimized by
using black platinic chloride PtCl4 on the surface of Pt electrodes
and electrodes are known as platinized Pt electrodes.
4. Polarisation phenomena can be minimized by using some strong
oxidizing agents like HNO3, chromic acid, MnO2 etc. so that as soon
as H2 is formed it must be oxidized to water. Such substances which
minimize the polarisation are known as Depolarizers.
15

16. CHARACTERISTICS OF GOOD DEPOSITS
Good Analyte deposit should be
 Compact
 Dense
 Inherent
 Fine
 Smooth
 Pure
16

17. COULOMETRY
Coulometric methods of analysis are based on the measurement of
quantity of electric charge that passes through a solution during an
electrochemical reaction
 It is electroanalytical technique in which analyte is characterized by
measuring electric charge.
 Based on Faraday’s Laws of Electrolysis.
1. Faraday’s First Law of Electrolysis
The mass of substance liberated at the electrodes during electrolysis is directly
proportional to the quantity of electric charge that passed through the electrolyte.
Mass of deposited electrolyte ∝ amount of electric charge required to deposit charge
m ∝ Q
m ∝ I x t Q= It
( where Z is proportionality constant
17
PRINCIPLE
m = ZIt

18. 2. Faraday’s Second Law of Electrolysis
The mass of different substance evolved/deposited by the passage of same
quantity of current are proportional to chemical equivalence.
 Mass of analyte ∝ equivalent weight of substance
 Mass ∝ molar mass of substance deposited/valency number of ions
m ∝ M/n
 If m1 and m2 are masses of two different substances liberated by passage
of same amount of current and M1 and M2 are respective molar masses and
n1 and n2 are their respective valence numbers then
How to calculate Equivalent weight?
There are two methods
1. If analyte contains H+ or OH- ions then
Equivalent weight = molecular weight/ no of H+ or OH- ions
2. If analyte is a salt then
Equivalent weight = molecular weight/ valence number of ions.
m1 n1 / M1 = m2 n2 / M2
18

19. How much charge is required for deposition of 1 mole of any ion?
Q = nF
Where n is number of moles of electrons required for reaction.
To determine no of moles of analyte deposited (nA) we can modify above equation
as
Q = nF nA
What is Faraday?
 The quantity of electricity carried by one mole of electrons is called Faraday
 Represented by F and its value is 96485C
1.603 * 10-19 X 6.022 * 1023 = 96485C
19

20. TYPES OF COULOMETRY
1. Controlled Potential Coulometry
2. Controlled Current Coulometry
1. Controlled Potential Coulometry
 In this technique, the potential of the electrode is held constant for a long
time, minutes to hours and the resulting integrated charge is recorded.
 100% current efficiency is ensured by holding working electrode at constant
potential in such a way that analyte reacts
completely without oxidizing or reducing
interfering species simultaneously.
 As electrolysis progresses, analyte’s
concentration decreases, as does the current.
 Integrating area under curve gives total charge.
20
Fig9:Current Vs Time curve
https://youtu.be/0E59bk6DU3E

21. Selecting a Constant potential
 To see how an appropriate potential for working electrode is selected, let’s develop
a constant potential Coulometric method for Cu+2 based on its reduction to Cu
metal.
Cu2+
(aq) + 2e- ↔ Cu(s)
 From ladder diagram for an aqueous solution of Cu+2
it is shown that above reaction is more favorable at + 0.342V
Problem
 I decreases overtime. As a result rate of electrolysis
become slower and exhaustive electrolysis of analyte
may require long time
Minimizing Electrolysis Time
 From the equation of time and rate constant
t = - 9.21/k
 Where k is rate constant and is directly proportional to area of working
electrode and rate of stirring and inversely proportional to volume of solution.
21
Fig10:Ladder Diagram
https://youtu.be/0E59bk6DU3E

22. INSTRUMENTATION
 A 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.
 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.
22
Fig10: Example of a cylindrical
Pt-gauze electrode for
controlled-potential coulometry.
https://chem.libretexts.org/Course
s/Northeastern_University

23. 2. Controlled Current Coulometry
 The current is kept constant
 The quantity of charge required to attain the end point is calculated from the
magnitude of the current and the time of its passage. Q= I x t
 Controlled-current coulometry, also known as Amperostatic Coulometry.
 It has two advantages over controlled potential coulometry
a) Analysis time is shorter and is less than 10 min, as compared to approximately 30-
60 min for controlled potential coulometry.
b) No need to integrate
Problems
1. To maintain a constant current we must
allow potential to change until another
oxidation, reduction reactions occur at
working electrode
2. Method is needed to determine the end
point
23
Fig11:Current Vs Time curve
https://youtu.be/0E59bk6DU3E

24. Solution to Problems
1. Maintaining Current Efficiency ( Solution to 1st Problem)
a) Let us consider Coulometric analysis of Fe2+ based on oxidation to Fe3+ at Pt
electrode in 1M sulphuric acid.
Fe2+
(aq) ↔ Fe3+
(aq) + e-
 From ladder diagram it is shown that in beginning
potential of working electrode is kept nearly constant
at a level near its initial value.
 As concentration of Fe2+ decreases, the electrode
potential shifts toward more positive value until
oxidation of water begins.
2H2O(l) ↔ O2(g) + 4e- + 4H+
(aq)
b) We can use mediators e.g. Ce3+ to maintain 100% current efficiency.
2. End Point Determination ( Solution to 2nd Problem)
 By using redox indicators e,g. ferrion provides a useful visual end point as
changing a colour from red to blue when electrolysis of Fe2+ is complete.
24
Fig12:Ladder Diagram
https://youtu.be/0E59bk6DU3E

25. INSTRUMENTATION
 Controlled-current coulometry normally is carried out using a two-electrode
galvanostat, consisting of a working electrode and a counter electrode.
 The working electrode often a simple Pt electrode is also called the generator
electrode since it is where the mediator reacts to generate the species that reacts
with the analyte.
 Alternatively, we can generate the oxidizing agent or the reducing agent
externally, and allow it to flow into the analytical solution.
 A solution containing the mediator flows into a small-volume electrochemical
cell, with the products exiting through separate tubes. Depending upon the
analyte, the oxidizing agent or the reducing reagent is selectively delivered to
the analytical solution. For example, we can generate Ce4+ using an aqueous
solution of Ce3+, directing the Ce4+ that forms at the anode into our sample.
 There are two other crucial needs for controlled-current coulometry: an accurate
clock for measuring the electrolysis time, te, and a switch for starting and
stopping the electrolysis.
25

26. CONTD….
 There are two other crucial needs for controlled-current
coulometry: an accurate clock for measuring the electrolysis
time, te, and a switch for starting and stopping the electrolysis.
26
Fig13: Controlled-Current coulometry.
https://chem.libretexts.org/Courses/North
eastern_University

27. APPLICATIONS OF COULOMETRY
1) Inorganic Analysis
 Determination of several metal ions e.g. Ca2+
 To determine the purity of inorganic compound.
2) Analysis of Radioactive Materials
 Used for the determination of uranium and plutonium
 Extensively used in nuclear energy field.
3) Microanalysis
 Useful for accurate determination of small amount of analyte (0.01-1 mg)
4) Electrolytic Determination of Organic Compounds
 Controlled potential coulometry can be used in electrolytic determination of
organic compounds.
27

28. REFERENCES
 Material Provided by DR. Muhammad Waseem Mumtaz
 Https://www.gamry.com/Framework%20Help/HTML5 ( Retrieved on January 2, 2022
at 11.00pm)
 https://youtu.be/0E59bk6DU3E ( Retrieved on January 2, 2022 at 11.16pm)
 http://web.iyte.edu.tr/~serifeyalcin/lectures/chem306/cn_4.pdf ( Retrieved on January 2,
2022 at 12.00am)
 http://web.iyte.edu.tr/~serifeyalcin/lectures/chem306/cn_4.pdf ( Retrieved on January 3,
2022 at 08.15pm)
 https://slideplayer.com/slide/14679336/ ( Retrieved on January 3, 2022 at 09.00pm)
 https://www.aplustopper.com/electrolysis-icse-solutions-class-10-chemistry ( Retrieved
on January 4, 2022 at 4.15pm)
 https://dipslab.com/ohms-law-statement-formula-limitations-applications ( Retrieved on
January 5, 2022 at 1.00am)
 https://slidetodoc.com/electrochemical-polarization-materials-engineering-dr-lubna-
ghalib-electrochemical/ ( Retrieved on January 5, 2022 at 1.26am)
 https://youtu.be/ALE7uU1qWdU ( Retrieved on January 5, 2022 at 1.45 am)
28

29. ANY QUESTION?
29

30. 30