Module 2
ElectroAnalytical
Techniques
Dr. Bhavna Vyas
Reference Electrode
Definition: Reference electrode is an electrode which has stable and
reproducible electrode potential
Reference electrode:
• Reversible and obeys the Nernst equation
• Exhibits a constant potential with time
• Attains its original potential after the disturbance of small currents
Types of Reference electrode:
1) Primary Reference electrode - SHE
2) Secondary Reference electrode – Convenient to handle
Calomel electrode
Silver-Silver Chloride electrode
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Calomel Electrode
➢ Secondary Reference Electrode
➢ Most widely used
Construction:
Narrow Glass tube at the bottom (porous) of which liquid Hg,
above it is a paste of Hg-Hg2Cl2 and remaining portion of
glass tube is filled with 1 N/0.1 N/saturated KCl solution
A Platinum wire dipped in Hg layer for electrical contact inside glass
electrode
Side tube filled with gel act as salt bridge
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Calomel Electrode
Representation of Calomel Electrode: (Half Cell)
Pt Hg(l)
Hg2Cl2(s)
KCl(aq) (Sat.)
Reaction:
If the calomel electrode acts as cathode; (e- comes through Pt wire)
Hg2Cl2 + 2 e- ⇆ 2 Hg + 2 ClAnd if it acts as anode (Only pure Hg loose e-), then the reaction is reverse
Potential of Calomel Electrode: Depends on Potassium Chloride conc.
The reduction potential:
Ecalomel=E0-0.0591log10[Cl-]
Std. Electrode potential of Calomel for different conc. of Cl- ions:
0.334 V for 0.1 N,
0.281V for 1 N and
0.2422 V for Saturated conc.
Demerits:
• Involves handling of Hg and Hg2Cl2
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Bhavna M. Vyas
• Should
2Cl2 gets decomposed
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Indicator Electrode
• Responds rapidly to the changes in the concentration of
solution under study
• Used in conjunction with the reference electrode
E.g.: Glass electrode (pH electrode), Quinhydrone electrode
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Ion Selective Electrode (Membrane)
• Transducer or electroanalytical sensor that converts the activity of a specific
ion dissolved in a solution into an electrical potential
• Used in conjunction with the reference electrode
• Consists of a thin membrane (selective, not specific always) which is
separated by two solutions containing same type of ions of different
concentrations and indented ion can only be transported
• Difference in concentration of ions results in Boundary
potential, from which unknown concentration can be
determined by using the known concentration of ions
in the internal solution
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Ion Selective Electrodes
Advantages:
➢ Portable
➢ Do not affect the test solution
➢ Short response time
➢ Wide range of concentration
➢ Unaffected by colour/turbidity
➢ Non contaminating
➢ Non-destructive
Limitations:
➢ Fouled by other solutes
➢ Interference by other ions
➢ Fragile and limited shelf life
➢ Electrodes respond to complexed ion activity
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hence ligands must be absent/masked
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Ion Selective Electrodes
Types of Ion Selective Electrodes:
Based on membranes used Ion Selective Electrodes (ISE) can be
classified as:
➢ Glass Electrode ( glass membrane specifically for H+ Ions)
➢ Solid State Electrode (crystalline membrane for F- or C- ions)
➢ Enzyme Electrode (urease on polyacrylamide gel for 𝑁𝐻4+ ions )
➢ Gas Sensing Electrode
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Glass Electrode
➢ Indicator Electrode & a type of ISE
➢ Used for pH determination
Construction:
Long glass tube with thin walled pH
sensitive glass membrane at the
bottom filled with 0.1 M HCl solution
A Platinum wire or AgCl coated Ag wire
dipped in HCl solution for electrical
contact
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Glass Electrode
Construction:
Glass used in glass membrane is porous in structure and may be
of silicate type or calcinogenide glass. Silicate glass is generally used for
monovalent cations while calcinogenide for divalent cations detection
Silicate or Special glass composition: 72% SiO2, 22% Na2O, 6%CaO
Representation of Glass Electrode: (Half Cell)
Ag, AgCl
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HCl (0.1 M)
Glass membrane H+ (Test)
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Glass Electrode
Working of Glass Electrode:
➢ Glass bulb acts as a semipermeable for H + ions
➢ On immersing glass electrode in a solution, glass membrane gets hydrated
➢ Exchange of H+ ions with Na+ ions takes place on both sides of hydrated
glass membrane
➢ Results into Boundary Potential
➢ An equilibrium gets established on both sides:
𝐻 + + 𝑁𝑎+ 𝐺𝑙𝑎𝑠𝑠 − ⇌ 𝑁𝑎+ + 𝐻+ 𝐺𝑙𝑎𝑠𝑠 −
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Glass Electrode
Working of Glass Electrode:
➢ Boundary Potential develops across the glass membrane as a result of a
concentration difference of H+ ions on two sides of the membrane (H+ ions
can pass through the membrane being small in size while Cl - cannot being
bigger, thus only H+ ions concentration is measured)
pH of Solution:
➢ Glass electrode coupled with calomel electrode can be used to determine
pH of a solution.
➢ Electrodes are dipped into the solution of unknown pH
Representation of cell:
Pt. Hg, Hg2Cl2 Cl- aq. Test Solution Glass electrode
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Glass Electrode
Advantages of Glass Electrode:
➢ Portable and compact
➢ Attains equilibrium easily
➢ Stable electrode, used in strong oxidizing and reducing agents
➢ Accurate and quick results
➢ Detect H+ ions in the presence of other type of ions as well
Uses:
• Pure research,
• Control of industrial processes
• Analysis of foods and cosmetics
• Measurement of environmental indicators
• Microelectrode measurements such as cell membrane electrical potential
• Soil acidity.
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pH of Solution:
Pt. Hg, Hg2Cl2 Cl- aq.
Glass Electrode
Test Solution (Unknown H+) Glass electrode
EMF of cell (ECell):
ECell = ECalomel – EGlass
= 0.2422 – (𝐸𝐺0 + 0.0591pH)
pH=
0
𝐸𝐶𝑒𝑙𝑙 +𝐸𝐺
−0.2422
0.0591
where, 𝐸𝐺0 - potential of glass electrode when electrode
is in contact with known pH
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pH-Metry
Objective:
Electro-analytical technique: pH-metry
Learning Outcomes:
• Method of material analysis
• Selection appropriate electroanalytical technique
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Introduction
pH: Represent Hydrogen Ion Concentration
Definition ??
Defined as the negative logarithm of hydrogen ion concentration
pH = -log [H+]
where [H+] is the concentration of hydrogen ion in moles/litre
Applications of pH measurements:
Water treatment, Food Industry, Agriculture etc.
Laboratory/Industries – Required to Maintain pH of solution
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pH of some real samples
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Buffer Solution: Resists the pH change in solution
Acidic buffer
A solution that contains a weak acid and its salt
with a strong base.
For example, a mixture of acetic acid (CH3COOH) and
sodium acetate (CH3COONa)
Basic buffer
A solution that contains a weak base and its salt
with a strong acid.
For example, a mixture of ammonium hydroxide
(NH4OH) and ammonium chloride (NH4Cl)
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pH Metry
What is pH Metry ????
Its an electro-analytical technique in which pH of a solution is
measured using pH meter
Where it is used ??
It is an important application of potentiometry
for the determination of pH
To determine
equivalence point of© Bhavna
titration
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M. Vyas
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pH-metric Titration of SASB
Objective: To determine the unknown concentration of strong acid
using pH-metry.
Consider a SASB titration in which both, the acid (HCl) and base
(NaOH) dissociate completely.
Reaction:
H + +Cl - + Na+ +OH - ® Na+ +Cl - + H 2O
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pH-metric Titration of SASB
Glass Electrode
Procedure:
➢ First calibrate the pH meter
using buffer solutions of
pH meter
known pH 4, 7 and 9
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pH-metric Titration of SASB
Procedure:
➢ Take known quantity of HCl in a beaker (V1) with magnetic needle in it and
then keep it on the magnetic stirrer.
A picture containing indoor, table, sitting,
glass
➢ Immerse the indicator electrode (glass) and reference
Description automatically generated
electrode in beaker, start the magnetic stirrer
➢ Note down the pH of the pure acid solution
➢ Fill the burette with 0.1 N NaOH
➢ Add 0.5 ml of N/10 NaOH to the acid in the
beaker
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pH-metric Titration of SASB
Procedure:
➢ Continue addition of NaOH from the burette and note down the
corresponding pH after each addition. Near the equivalence point the
change in pH is much more rapid than in any other region.
➢ Stop the addition when pH value remains
nearly constant.
➢ Note down all corresponding pH for
different Volumes of Base added
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pH-metric Titration of SASB
Observations:
➢ Initial Volume of acid taken = V1 ml
pH
➢ Volume of strong base added
➢ pH of the solution of reaction mixture
Volume of Strong Base added
(ml)
Calculations: Plot a pH metric Titration curve
Plot of pH verses Volume of Strong Base added
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pH metric Titration Curve for SASB
pH
Volume of Strong Base added (ml)
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pH metric Titration Curve for SASB
pH
Volume of Strong Base added (ml)
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pH metric Titration Curve for SASB
pH
Volume of Strong Base added (ml)
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pH metric Titration Curve for SASB
pH
Volume of Strong Base added (ml)
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pH metric Titration Curve for SASB
At equivalence point, equal amounts of H+ and OH-
pH
combine to form water at pH=7
pH=7
Vb at Equivalence point
Vb
Volume of Strong Base added (ml)
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pH-metric Titration of SASB
Calculations: Using the following equivalence equation:
Strong Acid vs Strong Base
N1V1= N2V2
Normality of HCL is obtained and from that Strength of acid using
following:
𝟑𝟔. 𝟓 ∗ 𝑵𝒐𝒓𝒎𝒂𝒍𝒊𝒕𝒚 𝒐𝒇 𝑨𝒄𝒊𝒅 ∗ 𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒃𝒂𝒔𝒆 𝒂𝒕 𝑬𝒒. 𝒑𝒐𝒊𝒏𝒕
𝑺𝒕𝒓𝒆𝒏𝒈𝒕𝒉 𝒐𝒇 𝑨𝒄𝒊𝒅 𝒈Τ𝒍 =
𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝑨𝒄𝒊𝒅 𝒕𝒂𝒌𝒆𝒏
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Test yourself…. Tick mark the right answer:
1. pH is defined as:
a) Positive logarithm of Hydrogen ions
b) Negative logarithm of Hydrogen ions
c) Positive logarithm of Hydroxide ions
d) Negative logarithm of Hydroxide ions
2. In SASB pH-metric titration curve, value of pH shows inflection at
a) near equivalence point
a) at equivalence point
a) after equivalence point
a) at all points
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Test yourself…. Tick mark the right answer:
1. pH-metry is:
a) an electroanalytical technique
b) instrumental method of analysis
c) gravimetric technique
d) volumetric analysis using indicators
2. Indicator electrode used in pH meter for the determination of Hydrogen ions
a) Calomel electrode
b) Fluoride electrode
c) Glass electrode
d) Conductivity cell
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Few subjective questions
1.
Define pH. What do you mean be pH scale?
2.
What is pH-metry?
3.
Draw the titration curve for pH-metric titration of HCL and NaOH.
4.
Explain pH-metric titration curve of Strong Acid (HCL) and Strong Base (NaOH).
5.
Why pH of the solution is 7.0 at equivalence point of strong acid-strong base
titration?
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Conductometry
Conductometry is an electroanalytical technique which deals with the
measurement of electrical conductance of solutions containing
electrolytes (acids, bases and salts in water). These solutions conduct
electric current due to the movement of ions towards oppositely charged
electrodes. Solutions of electrolytes also obey Ohm’s Law like metallic
conductors (R=E/I).
Conductance: It is the reciprocal of resistance. It is the ease of the flow of
current through a solution of electrolyte
Unit: Ohm-1 or Mho or Seimen
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Conductometry
Conductivity is a parameter used to measure the ionic concentration and
activity of a solution.
The more salt, acid or alkali in a solution, the greater its conductivity
The unit of conductivity is S/m or S/cm.
The scale for aqueous solutions begins with pure water at a conductivity of
0.05 µS/cm (25 °C) and naturally occurring waters such as drinking water or
surface water have a conductivity in the range 100 - 1000 µS/cm.
Conductivity cells are specially used for
the measurements of the conductance
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Basic Definitions of Conductometry
Conductance: It is the reciprocal of resistance. Unit: Ohm-1 or Mho or Siemen
Specific Conductance: Specific conductance of a solution can be defined as the
conductance of solution between two parallel electrodes which have cross sectional area 1
Unit: Mho-cm-1
cm2 and which are kept 1 cm apart.
Equivalent Conductance: It is defined as the conductance of a solution containing one gram
equivalent of an electrolyte at any particular concentration, when placed between two
Unit: Mho-cm2-gm-equiv.-1
electrodes 1 cm apart.
Molar Conductance: It is defined as the conductance of a solution containing one mole of an
electrolyte at any particular concentration, when placed between two electrodes 1 cm apart.
Unit: Mho-cm2 -gmole-1
𝑺𝒑𝒆𝒄𝒊𝒇𝒊𝒄 𝑪𝒐𝒏𝒅𝒖𝒄𝒕𝒂𝒏𝒄𝒆
Cell Constant= 𝑶𝒃𝒔𝒆𝒓𝒗𝒆𝒅 𝑪𝒐𝒏𝒅𝒖𝒄𝒕𝒂𝒏𝒄𝒆 or ratio of distance between the two electrodes to the area
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of electrodes (l/a)
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Unit: cm-1 or m-1
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Basic Definitions of Conductometry
Factors influencing Conductance:
➢ Number of free ions: More the no of free ions, greater is the conductance
➢ Charge on the free ions: Greater the charge, greater is the conductance
➢ Mobility of ions: Greater the mobility, greater is the conductance
Effect of Temperature:
➢ Conductance increases with increase in temperature due to increase in
velocity of ions, decrease in viscosity of the medium and decrease in the
interaction between the ions
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Basic Definitions of Conductometry
Effect of dilution on Conductivity:
➢ Equivalent or Molar Conductance increases
Equivalent Conductance
with increase in dilution.
Strong Electrolyte
➢ Increased Dissociation of the electrolyte on
dilution while for strong electrolyte increase
Weak Electrolytes
is due to the increased mobility of ions.
➢ Specific conductance of the solution
Root of Concentration C
Dilution
decreases on dilution (due to decrease in
the concentration of ions per unit volume)
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Conductivity Cell
➢ Conductivity Cells are of various shapes and sizes
➢ The simplest kind of measuring cell used consists of two similar electrodes.
➢ They are made up of pyrex glass (or some other resistant glass) which is fitted
with a pair of Platinum electrodes. These electrodes are platinum plates fused
into the glass tubes. The tubes are firmly supported by an ebonite cover.
Copper Wire
Ebonite Cover (Epoxy Resin)
Glass Cell
Mercury Contact
Platinum Electrodes
Electrolytic Solution
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Conductivity Cell
➢ The cover along with the platinum electrodes is fitted into the pyrex glass vessel so
that the distance between the electrodes may not change.
➢ An alternating voltage applied to one of the conductivity electrodes causes the ions
in the solution to migrate towards the electrodes.
➢ The more ions in the solution, the greater the current which flows between the
conductivity electrodes.
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Conductivity Cell
Conductivity cell measurements: Conductivity is measured by using a
conductivity cell to make a measurement of the electrical resistance.
➢ The conductivity meter measures the current produced by the conductivity
cell and uses Ohm's law to calculate first the conductance of the solution
and then - by taking the cell data into account - the conductivity.
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Conductometric Titrations
➢ It is a standard technique used in conductometry which has notable
application in analytical chemistry.
➢ In this method we determine the point where reaction is completed
(equivalence point) with the help of a conductometer that measures the
changes in conductance of solution produced by the ions in the
solution.
➢ At equivalence point we measure the volume of base used to neutralize
the acid ions completely in the solution. Putting these values in formula
we can get the strength of acid
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Conductometric Titrations
➢The principle of conductometric titration is based on the fact that
during the titration, one of the ions is replaced by the other and
invariably these two ions differ in the ionic conductivity with the result
that conductivity of the solution varies during the course of titration.
➢ The equivalence point may be located graphically by plotting the
change in conductance as a function of the volume of titrant added.
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Strong Acid with a Strong Base Conductometric Titration
Consider titration of HCl against NaOH wherein Strong base is filled in burette
and acid is taken in a beaker
Chemical reaction:
HCl + NaOH → NaCl + H2O
H+ + Cl- + Na+ + OH- → Na+ +Cl- + H2O
1. Initially, before titration the conductance is high which is due to
HCl → H+ + Cl- and
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Bhavna M.
mobility
of H+ is 350 and that of Cl-©ion
isVyas73.
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Strong Acid with a Strong Base Conductometric Titration Curve
Conductance
Conductometric
Titration of Strong
Acid vs. a Strong
Base
End point
Volume of Titrant (ml)
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Strong Acid with a Strong Base Conductometric Titration
2. Before Equivalence Point: Upon addition of NaOH, the H ion reacts with
OH ion to form the very weakly ionized water molecule. This means that the H+
ion is removed from the medium and replaced by Na+ ion which has a mobility
of 43; thus, a continuous abrupt decrease in conductance occurs during the
titration till the end point.
3. After Equivalence Point: Beyond the end point there is excess Na+ and OHions with 43 and 198 mobility due to continuous addition of NaOH So there is
continuous increase in conductance and the curve will have a V shape.
4. The equivalence point (end point) is the intersection of this two lines which is
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a minimum
of the curve.
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Strong Acid with a Strong Base Conductometric Titration
Calculations: Using the following equivalence equation:
Strong Acid vs Strong Base
N1V1= N2V2
Normality of HCL is obtained and from that Strength of acid using
following:
𝑺𝒕𝒓𝒆𝒏𝒈𝒕𝒉 𝒐𝒇 𝑨𝒄𝒊𝒅 𝒈Τ𝒍
𝟑𝟔. 𝟓 ∗ 𝑵𝒐𝒓𝒎𝒂𝒍𝒊𝒕𝒚 𝒐𝒇 𝑨𝒄𝒊𝒅 ∗ 𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒃𝒂𝒔𝒆 𝒂𝒕 𝑬𝒒. 𝒑𝒐𝒊𝒏𝒕
=
𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝑨𝒄𝒊𝒅 𝒕𝒂𝒌𝒆𝒏
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Weak Acid with a Strong Base Conductometric Titration
Consider titration of CH3COOH against NaOH wherein Strong base is filled in
burette and weak acid is taken in a beaker
Chemical reaction:
CH3COOH + NaOH → CH3COONa + H2O
H+ + CH3COO- + Na+ + OH- → CH3COO- + Na+ + H2O
1. Initially, before titration the conductance is low which is due to weak acid.
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Weak Acid with a Strong Base Conductometric Titration Curve
Conductance
Conductometric
Titration of Weak
Acid vs. a Strong
Base
End point
Volume of Titrant (ml)
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Weak Acid with a Strong Base Conductometric Titration
2. Before Equivalence Point: First conductance decreases on addition of strong base
owing to the formation of the salt which suppresses the ionization of weak acid. Mostly
this decrease is not observed unless one adds very small volumes of NaOH. As more of
NaOH is added, highly ionized salt, sodium acetate is formed thereby increasing the
conductance. Conductance increase occurs during the titration till the end point.
3. After Equivalence Point: Beyond the end point there is excess Na+ and OH- ions
with 43 and 198 mobility due to continuous addition of NaOH, So there is continuous
increase in conductance.
4. Equivalence point (end point) is the intersection of this two lines.
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Strong Acid with a Weak Base Conductometric Titration
Consider titration of HCl against NH4OH wherein weak base is filled in burette
and acid is taken in a beaker
Chemical reaction:
HCl + NH4OH → NH4Cl + H2O
H+ + Cl- + NH4+ + OH- → NH4+ +Cl- + H2O
1. Initially, before titration the conductance is high which is due to HCl → H+ + Cland high mobility of H+.
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Strong Acid with a Weak Base Conductometric Titration Curve
Conductance
Conductometric
Titration of Strong
Acid vs. a Weak
Base
End point
Volume of Titrant (ml)
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Strong Acid with a Weak Base Conductometric Titration
2. Before Equivalence Point: Upon addition of NH4OH, the conductance first
decreases due to the replacement of H+ ions. It keeps on decreasing upto the
equivalence point.
3. After Equivalence Point: Beyond the end point i.e. after the whole of acid is
neutralised, the conductance remains almost constant because the weakly ionised
excess base do not produce appreciable change in conductance.
4. The equivalence point (end point) is the intersection of this two lines.
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Weak Acid with a Weak Base Conductometric Titration
Consider titration of CH 3COOH and NH4OH, wherein weak base is filled in burette
and weak acid is taken in a beaker
Chemical reaction:
CH3COOH + NH4OH → CH3COONH4 + H2O
H+ + CH3COO- + NH4+ + OH- → CH3COO- + NH4+ + H2O
1. Initially, before titration the conductance is low which is due to weak acid.
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Weak Acid with a Weak Base Conductometric Titration Curve
Conductance
Conductometric
Titration of
Weak
End point Acid vs.
a Weak Base
Volume of Titrant (ml)
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Weak Acid with a Weak Base Conductometric Titration
2. Before Equivalence Point: First conductance decreases on addition of weak base
due to the replacement of H+ ions. But it soon begins to increase due to the salt
formation. Conductance increase occurs during the titration till the end point.
3. After Equivalence Point: After the whole of acid is neutralised, the conductance
remains almost constant because the weakly ionised excess base do not produce
appreciable change in conductance.
4. The equivalence point (end point) is the intersection of this two lines
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Advantages of Conductometric Titrations
➢ Employed to very dilute solutions (upto the order of 10-4 M solutions).
➢ Very accurate end points (with an error of ±0.5%).
➢ Very useful for coloured solutions (colour change of indicator is not clear)
➢ Useful for WAWB titration, which otherwise do not give sharp end point
➢ Keen observation is not necessary near the end point as it is obtained graphically
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Spectroscopic Techniques
UV-Visible Spectroscopy
Spectroscopy
• Spectroscopy is branch of science deals with the study of interaction of
electromagnetic radiation with matter
• Spectroscopic methods of analysis are based on the measurement of
electromagnetic radiation absorbed or emitted by the sample
• Physical and chemical properties of the sample substance are directly
related to the structure of the sample and sample substance absorb
radiations based on its bonding and structure
• Structure elucidation can be done based on wavelength absorbed by
sample substance
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Spectroscopy
Electromagnetic Radiations:
• Consists of discrete packets of energy which are called photons of light.
• Energy of photon of EMR is given by:
E = hc/ λ
• EMR travels at 3 x 108 m/sec. The speed of the wave is given by:
Velocity = Wavelength x Frequency
•
These waves made of two components electrical and magnetic. They
oscillates in space perpendicular to each other and perpendicular to the
direction of propagation.
• Wavelength of radiation is inversely proportional to its energy.
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Spectroscopy
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Fundamental Laws of Spectroscopy
Lambert’s law : When a beam of light is allowed to pass through a
transparent solution, the rate of decrease in intensity of light is directly
proportional to thickness of solution.
Beer’s law: When a beam of light is allowed to pass through a transparent
solution, the rate of decrease in intensity of light is directly proportional to
concentration of solution when thickness of solution is kept constant.
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Fundamental Laws of Spectroscopy
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Spectroscopy
Spectroscopic methods of analysis are based on the measurement of
electromagnetic radiations absorbed or emitted by the sample solution.
Most common photometric methods are:
1. Colorimetric: (400 to 800 nm)
2. UV spectroscopy (180 to 400 nm)
3. Visible spectroscopy (400 to 800 nm)
4. UV visible spectroscopy (180 to 800nm)
5. IR (0.76 to 15 micrometer)
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Range of UV-Visible Spectroscopy
▪
Range of wavelengths belong to UV and Visible region.
UV region: 10nm to 400nm
Visible region: 400nm to 800nm
▪
Subdivision of Ultraviolet region based on wavelengths:
Far UV (below 200nm) & Near UV(from 200 – 400nm)
▪
UV-Visible wavelengths are used in Quantitative as well as Qualitative analysis
in which absorption laws are used.
▪
In Quantitative analysis graph of absorbance against concentrations is usually
plotted.
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Spectroscopy
Interaction of electromagnetic radiation with matter
UV visible Radiation: bring excitation of electrons in the bond to
higher energy anti bonding level
Absorption of IR: Cause vibration in certain covalent bonds
∆ E rotational < ∆ E vibrational < ∆ E electronic
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Absorption of UV radiation leading to
Electronic transitions in organic molecules
• Absorption of UV-visible radiation by a molecule brings changes in the electronic
energy of molecule because of transition of valence electrons from lower energy
to higher energy. Types of electrons present in organic molecule:
1. Sigma (σ) electrons: Electrons forming single bonds like C-H bond present in
saturated hydrocarbon
2. pi (π) electrons: Electrons forming double bonds like C=C bond present in
unsaturated hydrocarbon
3. Non bonded (ɳ) electrons: Non-bonded or lone pair of electrons and not
involved in bonding between atoms in
molecules. Lone pair of electrons
present on Oxygen, Nitrogen etc.
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Absorption of UV radiation leading to
Electronic transitions in organic molecules
When energy is absorbed in UV Visible region, electronic transitions occur:
➢ Possible Electronic Transitions:
1) σ→ σ* transition
2) π→ π* transition
3) ɳ→ σ*transition
4) ɳ→π*transition
➢ Forbidden Transitions:
1) σ→π*transition
2) 2) π →σ* transition
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Electronic Energy Levels and Transitions
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Electronic transitions in organic molecules
➢ Possible Electronic Transitions:
1. σ→ σ* transition: (Less than 150 nm)
Such transitions occur only in the compounds containing sigma electrons
which are involved in single bonds and no lone pair of electrons on any atom in the
molecule from sigma bonding orbital to the corresponding antibonding orbital σ* .
Such transitions requires very large amount of energy, the absorption band occurs
in the far ultraviolet region.
e.g. Saturated Hydrocarbons like Methane, Ethane etc.
Methane (CH4) containing C-H bonds undergo this transition, shows absorbance
maxima at 125 nm
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Electronic transitions in organic molecules
➢ Possible Electronic Transitions:
2. π→ π* transition: (200 nm-700 nm)
Such transitions occur only in the compounds containing pi electrons which are
involved in multiple bonds in the molecule. These electrons get excited from pi
bonding orbital to the corresponding antibonding orbital π* .
e.g. Unsaturated Hydrocarbons like alkenes, alkynes, nitriles, aromatic compounds
etc. show these transitions.
Generally alkenes absorb in the region of 180-200 nm while compounds containing
2 or more conjugated bonds shows absorbance maxima above 200 nm
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Electronic transitions in organic molecules
➢ Possible Electronic Transitions:
3. ɳ→ σ* transition: (150 - 250 nm)
Such transitions occur in the saturated compounds containing atoms with
lone pair of electrons (non-bonding electrons) like O, N, S and halogens. These
electrons get excited from non-bonding orbital to the antibonding orbital σ*.
e.g. Various functional groups show absorbance maxima at shorter wavelength
in the range of 150 - 250 nm
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Electronic transitions in organic molecules
➢ Possible Electronic Transitions:
4. ɳ→π* transition: (200 nm-700 nm)
Such transitions occur in the compounds containing double bonds involving
hetero atoms with lone pair of electrons (non-bonding electrons) like O, N, S and
halogens. These electrons get excited from non-bonding orbital to the antibonding
orbital π*.
These transition requires minimum energy and show absorption at longer
wavelength between 200 to 700 nm
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Electronic Transitions in Organic Molecules
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UV-Visible Spectra
➢ UV-Visible spectrophotometer on analysis of an analyte solution gives
data in the form of UV-Visible Spectra
➢ The UV-Vis spectra is a graph:
Absorbance verses wavelength () or
% Transmittance verses Wavelength ()
➢ It gives information about max
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UV-Visible Spectroscopy
max : It is defined as a particular wavelength at which maximum
absorbance is observed. It is a charactristic property of an analyte and
depends on strucutre of that analyte
 : It is called as molar extinction coefficient or molar absorptivity
It is the extent of absorption for a solution specific concentration
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UV-Visible Spectroscopy
The UV-Vis spectra for isoprene is as follows:
Source: https://www.chemistry.msu.edu
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Content: UV-visible spectroscopy
Terms Involved in UV Visible Spectroscopy:
Definition and suitable examples to illustrate the following terms:
➢ Chromophore:
➢ Auxochrome:
➢ Bathochromic shift:
➢ Hypsochromic shift:
➢ Hyperchromic shift:
➢ Hypochromic shift:
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Terms involved in UV-visible spectroscopy
CHOMOPHORE: Any isolated covalently bonded unsaturated group responsible for
electronic absorption or shows characteristic absorption in the UV Visible region.
E.g.: -C=C-, -C=O, NO2
Consequently, chromophore is any substance (groups) which absorbs radiation at
particular wave length which may or may not impart colour to the compound.
AUXOCHROME: Any saturated group with non bonded electrons, which does not
itself act as a chromophore but whose presence brings about a shift of the
absorption band towards the red end of the spectrum (longer wavelength)
Auxochrome is a colour enhancing group. Auxochrome when attached to a
chromophore, alters both the wavelength and the intensity of the absorption
E.g. –NH2, -OH, -OR, -NHR, -SH etc.
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Chromophore + Auxochrome = Newer Chromophore
80
Terms involved in UV-visible spectroscopy
BATHOCHROMIC SHIFT or RED SHIFT:
The shift of maximum absorption to a longer wavelength due to
substitution or solvent effect.
Causes:
1. An auxochrome
2. Change of solvent
E.g.: Absorption Maximum of Benzene λmax= 255 nm (εmax= 203)
While for Pheonl λmax= 270 nm and Aniline λmax= 280 nm (εmax= 1430)
In alkaline medium, p-nitrophenol shows red shift with λmax=265 nm from
λmax=255 nm. Because negatively charged Oxygen delocalizes electron more
effectively than the unshared n electron pair present on –OH groups.
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Terms involved in UV-visible spectroscopy
HYPSOCHROMIC SHIFT or BLUE SHIFT: The shift of maximum absorption to
a shorter wavelength.
Causes:
1) Change of solvent towards higher polarity or
2) Removal of conjugation
E.g.: Absorption Maximum of Aniline (conjugation of pair electrons of nitrogen
with benzene ring) λmax= 280 nm while Aniline shows λmax= 203 nm as in acidic
medium as it will form –NH+3, due to removal of lone pair of electrons or due to
removal of –conjugation.
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Terms involved in UV-visible spectroscopy
HYPERCHROMIC EFFECT:
An increase in maximum absorption intensity
Cause: Introduction of auxochrome usually increases the intensity of
absorption
E.g. : Pyridine λmax= 257 nm and εmax= 2750 while pyridine with –CH3 group
shows increase in intensity of absorption that is 2-methyl pyridine λmax= 262
nm and εmax= 3560
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Terms involved in UV-visible spectroscopy
HYPOCHROMIC EFFECT:
A decrease in maximum absorption intensity.
Cause: Due to introduction of any group to the compounds which is going to alter
the molecular pattern of the compound or distort the original geometry results in a
hypochromic shifts
E.g.: Biphenyl (or naphthalene) has an absorption λmax= 250 nm and εmax= 19000
whereas 2-methyl biphenyl has an absorption of λmax= 237 nm and εmax= 10250
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Effect of substituents on the position and
intensity of an absorption band
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Source of Image: epgp.inflibnet.ac.in
85
Instrumentation: Single Beam UV Visible Spectrophotometer
Block Diagram of Single Beam UV Visible Spectrophotometer:
Source
Monochromator
Sample
Cell
Detector
Amplifier/
Recorder
Components of Single Beam UV Visible Spectrophotometer:
1) Light Source: Provides wide range of UV Visible radiation
and sufficient intensity over required wavelength.
Eg: Deuterium/Hydrogen Discharge Lamp (190-290 nm);
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Tungsten filament (370-780
86
Instrumentation: Single Beam UV Visible Spectrophotometer
Sample holder (Cuvette)
Source of radiation
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Instrumentation: Single Beam UV Visible Spectrophotometer
Source
Monochromator
Sample
Cell
Detector
Amplifier
/Recorde
r
2) Sample Holder: All UV Visible spectra are recorded for the
solution phase. Hence, Cells/Cuvettes made up of Glass or
Quartz are used. But Quartz Cells only are transparent in full
range of 200-780 nm.
3) Monochromator: Converts polychromatic radiation into
monochromatic radiation using Prism/Filters/Gratings. Quartz
prism is used for UV region
4) Detector: Transducer which converts transmitted radiation
into electric signal/current. Commonly used are
16-11-2024 Photomultiplier tube(PMT),
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M. Vyas
Phototube
etc.
88
Instrumentation: Single Beam UV Visible Spectrophotometer
Source
Monochromator
Sample
Cell
Detector
Amplifier
/Recorde
r
5) Amplifier: Amplify the signal received from detector
6) Recorder: Record output of the UV Visible spectrum data
and Display it either in the graphical manner as absorption
spectra or in the form of digital values of the Absorbance or
transmittance verses wavelength
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Applications of UV-Visible Spectroscopy
➢Structural Information: max is the characteristic property of a
molecule due to presence of chromophore along with auxochrome. By
knowing this value structural information can be extracted.
➢Qualitative analysis: UV-Visible spectra of an unknown sample can
be matched with spectra of known compound to analyse sample
qualitatively.
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Applications of UV-Visible Spectroscopy
• Quantitative analysis: By use of UV-Visible Spectroscopy, unknown
concentration of analyte can be evaluated. Here same analyte solution
is
prepared
with
different
concentration
and
its
absorption
is
determined. A graph is plotted with absorption verses concentration,
which gives a straight line.
• Now absorption of solution containing unknown concentration is
evaluated and by use of graph its corresponding concentration can be
determined.
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Applications of UV-Visible Spectroscopy
➢Chemical Kinetics: According to Beer’s Law absorbance is proportional
to concentration. Hence during the chemical reaction absorption
intensity of reactant decreases, while that of Product increases which will
help to study chemical kinetics.
➢Detection of Impurity: Any change in the spectra of a specific
compound will indicate the presence of impurity
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