Sensors in Analytical Techniques Module 5-1
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Chemistry for EEE stream BCHEE202
Module 5
Module 5
SENSORS IN ANALYTICAL TECHNIQUES
Electrochemical cells
A device which converts chemical energy into electrical energy or electrical energy into chemical
energy is called electrochemical cell. There are 2 types
1.
The one which converts chemical energy into electrical energy is called galvanic cell.
2.
The one which converts electrical energy into chemical energy is called electrolytic cell.
An electrochemical cell consists of two electrodes. When the electrodes are connected by a metallic
wire, an electric current flows as a result of a spontaneous redox reaction. Each electrode usually
consists of a metal in contact with a solution of its own ions. Since a cell is a combination of two
electrodes, each electrode is referred to as a single electrode or a half cell
.
Single electrode potential(E)
When a metal is in contact with a solution of its own ions, the potential developed at the interface
between the metal and the solution is called single electrode potential.
Standard electrode potential(E
0
)
The potential developed when the electrode is in contact with a solution of unit concentration at 298K.
If the electrode contains gas then its pressure is one atmosphere. It is denoted by E
◦
.
Types of electrodes
Some of the important types of electrodes are grouped into the following types.
i)
Metal
–
Metal ion electrode:
where a metal is in contact with its ionic solution.
Eg: Cu
+2
|Cu, Zn
+2
|Zn, Ag
+
|Ag
ii)
Metal- metal salt ion electrode (metal- insoluble salt electrode):
where a metal is in contact with its
insoluble salt which is in contact with a solution containing the anion of the salt.
Eg; Calomel electrode.
iii)
Gas electrode:
In this an inert metal such as platinum is in contact with gas molecules and also
with the ionic solution of the same gas molecule.
Eg: Standard Hydrogen electrode (SHE).
iv)
Amalgam electrode:
where metal-amalgam is in contact with a solution containing its own metal
ions.
Eg: Lead- amalgam electrode (Pb- Hg|Pb
+2
)
v)
Oxidation- reduction electrode:
An oxidation-reduction electrode is the one in which the electrode
potential arises from the presence of oxidized and reduced forms of the same metal substance in
solution. The potential developed is picked up by an inert metal like platinum.
Eg: Pt |Fe
+2
, Fe
+3
vi)
Ion selective electrode (Membrane electrode):
This electrode is more selective towards a particular
type of ion, in a mixture of ions and used to measure concentration of that particular ion.
Chemistry for EEE stream BCHEE202
Module 5
Eg; Glass electrode.
Ion Selective Electrodes
Ion selective electrodes are the electrodes that can interact more selectively with a particular ion in a
mixture of ions neglecting the other ions.
The
potential developed by an ion-selective electrode
depends only on the concentration of species or ions of interest.
For e.g., Glass electrode is H
+
ion
selective.
The electrodes consist of a membrane which is capable of exchanging the specific ions with the
solution with which it is in contact. Ion selective electrodes are also called membrane electrodes.
Construction & working of Glass Electrode (Ion selective electrode)
A glass electrode consists of a long glass tube with a thin-walled bulb at one end. The Glass is made
of silicate glass containing SiO
2
,
Na
2
O, CaO of high electrical conductance & low melting point.
The
bulb contains 0.1M HCl
&
Silver
-
silver chloride electrode (Internal reference electrode) is immersed
into the solution & connected by a platinum wire for external contact.
The electrode is represented as
Ag / AgCl
(s)
/ 0.1M HCl / glass
.
The membrane undergoes an ion exchange reaction. The Na
+
ions on the glass are exchanged for H
+
ions.
H
+
(soln)
+
Na
+
Gl
-
(membrane)
Na
+
(soln)
+
H
+
Gl
-
(membrane)
If a thin-walled glass bulb containing an acid is dipped in a solution(analyte), a potential is developed
across the glass membrane. The potential difference E
b
at the interface is referred to boundary
potential, given by,
E
b
= E
1
– E
2
E
2
= Potential of inner membrane
,
E
1
= Potential of outer membrane
Chemistry for EEE stream BCHEE202
Module 5
E
b
is related to the difference in the hydrogen ion concentration of the two solutions by the relation.
Where C
1
is the concentration of H
+
ions of the acid solution inside the glass bulb and C
2
is the
concentration of the acid solution into which glass bulb is dipped.
E
b
=
RT
F
ln c
1
-
RT
F
ln c
2
+
If the concentration C
1
of the solution inside the glass bulb is constant then the first term on RHS of
above equation is constant. Therefore
Therefore, E
b
=
constant
RT
F
ln c
2
+
Substituting the value of R and F at 298K
E
b
=
L
0.0591
log c
2
+
Where L is a constant which depends on the pH of the solution taken in the bulb and in glass electrode
assembly. Since C
2
= [H
+
] of the solution.
The equation can be written as
i.e., E
b
=
L
0.0591
pH
E
b
=
L
0.0591 log [H
+
]
+
This equation is used to determine pH of solution.
The boundary potential established due to the above reaction is mainly responsible for glass electrode
potential E
G
. The potential of the glass electrode
E
G
is given by
E
G
= E
b
+ E
Ag|AgCl
From equation (1) E
b
= 0 when C
1
=C
2
. But in practice it has been observed that even when C
1
=C
2
a
small potential is developed. This is called asymmetric potential
(E
asy
). Hence E
G
can be written as
E
G
= E
b
+ E
Ag|AgCl
+ E
asy
E
G
= L
–
0.0591pH + E
Ag|AgCl
+ E
asy
or
E
G
= L
1
–
0.0591pH where L
1
is a constant equal to L + E
Ag|AgCl
+ E
asy
Determination of pH
To determine the pH of a given solution the glass electrode is dipped in a solution whose pH need to
be determined. It is combined with a saturated calomel electrode which is a reference electrode.
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Chemistry for EEE stream BCHEE202
Module 5
Hg
Hg
2
Cl
2
Cl
-
Solution of unknown pH
Glass
0.1M HCl
AgCl
Ag
The emf of the so formed cell is determined potentiometrically.
E
cell =
E
G
- E
cal
E
cell
= L
1
–
0.0591pH -E
cal
L
1
value is determined by dipping the glass electrode in a solution of known pH and measuring the
emf of the cell formed when combined with a calomel electrode.
Calomel electrode
The calomel electrode consists of mercury, mercurous chloride (Hg
2
Cl
2
) and a solution of KCl.
Mercury is placed at the bottom of a glass tube. Mercury is covered by a paste of mercurous chloride
(calomel) with mercury and potassium chloride solution. A solution of KCl is introduced above the
paste through the side tube. A platinum wire sealed into a glass tube is dipped into mercury and used
to provide the external electrical contact. The concentration of KCl solution used is either decinormal,
normal or saturated. Correspondingly the electrode is known as decinormal, normal or saturated
calomel electrode respectively.
The calomel electrode is represented as
Hg| Hg
2
Cl
2
| Cl
-
Chemistry for EEE stream BCHEE202
Module 5
Depending upon the nature of the other electrode of the cell, the calomel electrode can act as anode or
cathode.
When it acts as anode the electrode reaction is
2Hg
Hg
2
+2
+ 2e
-
Hg
2
+2
+2Cl
-
Hg
2
Cl
2
2Hg +2Cl
-
Hg
2
Cl
2
+2e
-
When it acts as cathode, the electrode reaction is
Hg
2
+2
+ 2e
-
2Hg
Hg
2
Cl
2
Hg
2
+2
+2Cl
-
Hg
2
Cl
2
+2e
-
2Hg + 2Cl
-
The net electrode reaction is
Hg
2
Cl
2
+2e
-
2Hg + 2Cl
-
Electrode potential, E = E
o
2.303RT
nF
[Cl
-
]
2
log
i.e.,
E = E
o
2.303RT
2F
[Cl
-
]
2 log
i.e.,
E = E
o
2.303RT
F
[Cl
-
]
log
i.e.,
E = E
o
0.0591
[Cl
-
]
at 298K
log
The electrode potential depends on the concentration of the chloride ions.
At 298K, the electrode potentials are as follows
0.1
N KCl electrode (0.334V)
1N KCl electrode (0.281V)
Saturated KCl electrode (0.2422V)
Uses
is used as a secondary reference electrode in the measurement of single electrode potential.
It is the most commonly used reference electrode for potential measurement.
Concentration cells
A concentration cell is an electrochemical cell in which both the anode and the cathode are made up of
the same element (metal or non metal) in contact with the solutions of the same electrolyte, but of
different concentrations.
Chemistry for EEE stream BCHEE202
Module 5
Eg: Concentration cell consisting of two half cells. Both the electrodes are made up of copper metal
but dipped in copper sulphate solutions of different concentrations. It is represented as
Cu|Cu
+2
(C
1
)||Cu
+2
(C
2
)|Cu
Where C
1
and C
2
are the molar concentrations of the Cu
+2
ions in the two half cells.
By convention left hand electrode is the anode and the right hand electrode is the cathode. The cell
reactions are
Cu
Cu
+2
(C
1
)
+
2e
-
(anode reaction)
Cu
+2
(C
2
)
+
2e
-
Cu
( cathode reaction)
Adding the two half cell reactions, the net cell reaction is
Cu
+2
(C
2
)
Cu
+2
(C
1
)
The cell reaction is a change in concentration as a result of which current flows. This takes place till
the concentrations in the two half cells become equal.
The potential of the cell is Ecell
= E
R
-E
L
Where E
◦
is the standard reduction potential of the species in the concentration cell. On simplifying
E
cell
is positive if C
2
>C
1
SENSORS
A Sensor is a device that detects and measures physical input like light, heat, moisture, pressure or any other
environmental phenomena. The output produced by a sensor is generally a signal that is transmitted
electronically over a network for reading or further processing.
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A chemical sensor is a device that is capable of giving
real time analytical information about the
test sample. It interacts with the analyte, detects it
and produces measurable signal that is proportional to
the analyte concentration or quantity.
Main components of sensor
a)
Receptor
b)
Transducer
c)
Electrical signal and display
Receptor:
The receptor is the component of the chemical sensor that comes into direct contact with the analyte.
It produces signal corresponding to interaction in the form of change in potential, conductivity,
current, heat, pH and colour etc.
Transducer:
It is used to convert a non-electrical signal into an electrical signal. These are used to convert the
signal created by the receptor-analyte interaction into a measurable value.
In most chemical sensor the
sensing element and transducer are packaged together in the same unit.
Electrical signal and display
The electronic system analyzes the signal given by the transducer, helps in signal amplification and
converts the signal from analog to digital form. These amplified signals are then displayed. Signal can
be displayed in various forms such as numeric value, graph, image etc.
Electrochemical sensors
:
Working principle:
1.
Electrochemical sensors convert the information associated with electrochemical reactions (the
reaction between an electrode and analyte) into measurable signal.
2.
Electrochemical sensors are made up of three essential components: the analyte, the sensing
element or receptor that responds to the analyte, and a transduction element that converts the
output of the sensing element into an electrical signal.
3. electrochemical sensors, the surface of the electrode which is in contact with analyte acts as
sensing element or receptor where electrochemical reactions takes place and produces non-
electrical signal.
Chemistry for EEE stream BCHEE202
Module 5
4. The electrode acts as the transduction element because it will convert non-electrical signal
produced by receptor into electrical signal.
5. Electrochemical sensors require a closed circuit. Current must flow to make a measurement.
6. The electrode surface will either oxidize or reduce the analyte producing non -electrical signals.
The following steps are involved in the working of an electrochemical sensor.
1)
Diffusion of the analyte towards the electrode.
2)
Adsorption onto the electrode surface
3)
Oxidation and reduction of the analyte by electrode to produce non
-
electrical signal which is
proportional to the concentration of the analyte.
4)
Desorption of products from the electrode surface
5)
Diffusion of the products away from the elctrode to the bulk of electrolyte.
Application of Electrochemical sensors
The oxygen sensor is used in detecting dissolved oxygen in water boilers.
They are used in security and defence applications like detection of toxic gas like CO, H
2
S,
NO, etc.
They are used in water analysis to measure toxic metal concentration in water.
They are used in diagnostic and health care application to diagnose the viral illness such as
Coronavirus, HIV, Malaria and Zika virus.
Conductometric sensors:
Working Principle:
1
Conductometric sensor is considered a type of electrochemical sensor.
2
In conductometric sensor determination of the concentration of analyte is based on the
measurement of changes in the electrolytic conductance of solution.
3
Conductance of the solution is based on
Concentrations (number) of ions in the solutions
Mobility of each type of ions in solution. Mobility of ions depends on its size.
Smaller the size higher is the mobility and hence higher is the conductance.
4
An electrode used in conductometric sensor is called as conductivity cell.
Chemistry for EEE stream BCHEE202
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5
It is made of two platinum electrodes with area of cross section 1cm
2
and distance between
two electrodes is 1 cm apart. Volume of the solution between the two electrodes is 1 cm
3
.
6
Conductance of unit volume of the solution is called specific conductance.
7
There will be change in specific conductance of solution when there is change in number of
ions or type of ions.
8
This change is measured using conductivity cell.
9
The conductivity cell is dipped in the electrolytic solution taken in a beaker and it is connected
to a conductance measuring device called conductivity meter.
Application of conductometric sensors
It is used to estimate acids, bases and their mixtures in a sample.
Used to check the amount of ionic impurities in waste samples.
Measure the acidity or alkalinity of sea water and fresh water.
In environment monitoring, bio technology and agriculture related applications.
Thermometric Sensors:
Working Principle:
1
Thermometric sensor is based on the measurement of thermal changes during the interaction
between the analyte and receptors.
2
Thermal changes are converted to measurable change in the temperature or potential.
3
Thermometric sensors are useful in those processes which generate sufficient heat to produce a
measurable change of temperature.
4
Main components of thermometric sensor are a tubular catalytic reactor fitted with temperature
transducer.
5
Analyte (reactant) is fed in to the reactor.
6
The wall of the reactor is coated with catalyst or enzymes capable of catalysing the reaction,
liberating heat energy.
7
The heat liberated is measured by temperature transducer.
8
The change in temperature is converted to the output voltage by transducer which is amplified
and fed to the data storage and processing unit.
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9
In order to convert change in temperature into an electrical signal, two main kinds of
transducers namely resistive transducer and thermocouple are used.
Applications of thermometric sensors
:
Temperature control in chemical reactions: Thermometric sensors are widely used in chemical
reactions to control the temperature of the reaction mixture which is essential for obtaining the
desired product.
Used to monitor temperatures in industrial processes such as heating and cooling systems,
boilers, and furnaces.
Used to monitor the temperature of medical equipment such as incubators, blood warmers, and
dialysis machines.
Environmental monitoring: Thermometric
sensors are used to monitor the temperature of the
environment, such as air temperature, soil temperature and water temperature. This helps to
understand the effects of temperature on the environment and to monitor climate change.
Medical applications: Thermometric sensors are used in medical applications to measure body
temperature, surface temperature and core temperature. This information is critical for
diagnosing and treating various medical conditions.
Optical Sensors:
Working Principle:
1.
Optical sensor is used to convert the light rays into electronic signals by photoelectric effect.
Optical sensors help in measuring the incident li
ght’s intensity & changing it into a readable
form through an integrated instrument.
2.
The working of an optical sensor is based on the interaction of incident light with analyte
sample.
3.
In optical sensor, the incident radiation in the ultraviolet, visible, infrared domain is used for
analytical purpose.
4.
When light falls on a analyte, it can be absorbed, reflected, scatterd or transmitted.
5.
The amount of light absorbed or reflected depends on the structure of the molecule in the
analyte and the wavelength of the penetrating light.
Optical sensors are made up of following components:
1)
Light source: To pass light on material.
2)
Monochromator: To select a light of particular wavelength.
3)
Photocell: The light source emits light, which falls on the analyte to be measured. The
photodetector then detects the amount of light that is absorbed, reflected, or transmitted. This
information is then converted into an electrical signal.
4) Recorder:
electrical signal is recorded by the recorder that can be used to measure the properties
of the material or concentration of particular component in a material.
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Applications of optical sensors are vast and include the following:
1.
In Medical equipments - Optical sensors are used in medical equipment such as pulse
oximeters, which measure the amount of oxygen in a person's blood. And also measures the
pulse rate by detecting the changes in the amount of light transmitted through the finger as the
heart beats. The heart causes changes in the volume of blood in the vessels, due to which there
is change in the light that is transmitted.
2.
Air quality monitoring: Optical sensors can be used to monitor air quality by measuring the
amount of light absorbed or scattered by pollutants in the air, such as dust, smoke, and smog.
The sensors can detect the presence of these pollutants by analyzing changes in the amount of
light transmitted through the air.
3.
Water quality monitoring: Optical sensors can be used to monitor water quality by measuring
the amount of pollutants. This is done by analyzing changes in the amount of light transmitted
through the water as light is absorbed by pollutants in the water
4.
Lighting control: Optical sensors can be used to automatically turn lights on and off based on
the presence of people in a room. By
detecting changes in light levels in a room in the
presence of people.
Analytical Techniques
Analytical chemistry
methods refer to techniques used for the detection, identification,
characterization, and quantification of chemical compounds.Qualitative identifies analytes, while
quantitative analysis determines the numerical amount or concentration. Several Instruments and
sensors are being used for analysis as these methods are faster, accurate and require a minimum
quantity for analysis. Instrumental methods such as conductometry and potentiometry are some
examples of these methods.
CONDUCTOMETRY
Principle/ Theory
Conductometric titrations are the titrations in which amount of the analyte present in the given
solution is measured by the change in conductance. The principle underlying in conductometry is the
measurement of replacement of ions of a particular conductivity by ions of different conductivity
during a chemical reaction. Conductivity electrodes or sensors are used to determine the end point in
acid base titrations. Neutralization point is determined by observing the change in specific
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conductance of the solution. Specific conductance(K) is the conductance of a solution present between
2 parallel electrodes of area 1cm
2
which are kept 1cm apart. Here R is the resistance.
K =
R
1
a
l
X
a
l
= cell constant
The specific conductance of an electrolytic solution at any temperature depends on ions present and
hence varies with ionic concentration. In conductometric titration, there is a sudden change in
conductance of solution at equivalence point. Hence equivalence point is determined by plotting a
graph of conductance versus volume. Therefore, concentration of the analyte in the solution can be
calculated.
Instrumentation
Conductometer consists of 2 platinum electrodes and a conductance measuring device. The 2
electrodes have unit area of cross section and are placed unit distance apart. The solution to be
estimated is taken in a beaker. The system responds readily to changes in concentration of analyte.
To, conductometer
Stirrer
Platinum electrode
1cm
2
1cm
Advantages
Mixture of acids can be titrated accurately.
Accurate results are obtained in dilute as well as more concentrated solutions.
The method can be employed for colored solutions also.
Application:
Estimation of Weak Acid
Titration of weak acid with a strong base:
(CH
3
COOH Vs. NaOH)
The conductance of weak acid, acetic acid is initially low, due to poor dissociation. When a strong
base is added, the salt is formed, gets ionized and conductance increases slowly. After the complete
neutralization, further addition of base NaOH introduces more and more Na
+
and OH
-
ions into the
solution, conductance again increases rapidly. When conductance of the solution is plotted against
volume of base added, two straight lines are obtained. The point of intersection corresponds to the
volume of base required to neutralize weak acid.
CH
3
COOH
+
NaOH
→ CH
3
COONa
+
H
2
O
The plot of conductance versus volume of strong base is as follows.
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volume of strong base
conductance(mhocm
-1
)
equivalence point
Procedure
Fill the microburrete with standard NaOH solution. Pipette out the given weak acid into clean 100 cm
3
beaker. Immerse the conductivity cell connected to conductivity meter in acid
(CH
3
COOH)
given.
Add 0.5 cm
3
of the NaOH solution from the burette. Stir the solution gently and measure the
conductance. Continue the measurement of conductance after each addition of 0.5 cm
3
of NaOH
solution. The titration should be continued till the slow increase in conductance changes to sudden
increases near equivalence point; and at least 5-6 readings should be taken after that. Plot a graph of
conductance on y- axis versus volume of NaOH on x -axis. The volume corresponding intersection of
two lines is the neutralization point of Acetic acid.
POTENTIOMETRY
Principle
In potentiometric titrations, concentrations of the solutions can be calculated by measuring the emf
between two electrodes dipped in the analyte solution.
The potential of an electrode is given by Nernst equation,
E = E
o
+ 0.0591 log
n
M
n+
i.e. The potential of an electrode depends on concentration of ion to which it is reversible. This
method can be used in the determination of end points of acid-base titrations, red-ox titrations etc.
When a known volume of analyte is titrated with a standard solution, neutralization or red-ox reaction
takes place. During the course of titration, concentration of product will be continuously altered. If a
metal electrode, reversible with respect to corresponding ions is placed in the solution, the potential
will vary throughout the titration, which can be determined using a suitable device. Initially, the
change in potential is very small. At the equivalence point, when the amount of titrant added is
equivalent to the amount of analyte present, there will be a sharp rise in potential. Beyond the
equivalence point, there will be no significant change in potential. By plotting a graph of change in
potential against the volume of titrant added, the equivalence point can be determined.
Instrumentation
Potentiometer consists of a reference electrode(calomel electrode), indicator electrode, and a potential
measuring device. The indicator electrode responds to the changes in concentration of analyte. A
known volume of analyte is taken in the beaker and its potential is determined. The titrant is added in
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Module 5
increments of 0.5 ml and emf is measured each time. At equivalence point, emf increases rapidly. A
few readings are taken beyond the end point. Thus, the changes in potential at different volumes of
titrant are recorded.
To, potentiometer
Indicator electrode
Reference electrode
Stirrer
Advantages:
Potentiometric titrations can be carried out in colored solutions where indicators cannot be
used.
By potentiometry, it is possible to determine the end point in titrations of redox, precipitation
and acid-base titrations.
Application:
Estimation of Iron (Fe) using acidified Potassium dichromate solution
Potentiometry can be used to determine the end point of a redox titrations by measuring the change in
potential during titration using a platinum-calomel electrode combination.
The titrations involving Mohr's salt solution and Potassium dichromate solution can be done by
Potentiometry.
The platinum electrode is used as an indicator electrode and coupled with a calomel electrode
(reference electrode) and dipped in a solution of known volume of analyte (Mohr's salt
solution) and connected to potentiometer and emf is measured.
The titrant (Potassium dichromate) of known concentration is added in increments of 0.5ml
from burette to the analyte solution and emf is measured each time after stirring the solution.
FAS reacts with K
2
Cr
2
O
7
under acidic conditions, the redox electrode potential is set up at
indicator electrode. Pt/Fe
2+
,Fe
3+
.
The electrode potential
of the redox electrode is given by Nernst equation
The electrode potential depends upon concentration of Fe
2+
and Fe
3+
,
As the titration proceeds Fe
3+
increases and Fe
2+
decreases and redox electrode potential
increases and emf of the cell goes on increasing and steep rise is seen at equivalence point.
Chemistry for EEE stream BCHEE202
Module 5
At equivalence point all the Fe
2+
is converted to Fe
3+
ions and redox potential of Pt/Fe
2+
,Fe
3+
stops and further addition of dichromate new redox potential comes into existence Pt/Cr
6+
,Cr
3+
because the solution contains Cr
3+
ions produced due to reduction of dichromate
by Fe
2+
ions.
This redox electrode Pt/Cr
6+
,Cr
3+
has higher potential than Pt/Fe
2+
,Fe
3+
.
Plot a graph of ΔE/ ΔV versus volume of
K
2
Cr
2
O
7
gives the equivalent point.
COLORIMETRY :
Principle and instrumentation of colorimetric sensors and its application in the estimation of
copper sulphate:
“Colorimerty
is an analytical technique used for the determination of concentration of coloured
species
in a solution”.
It is used for solution which are themselves coloured or which gives a colour when mixed with
suitable reagent. The intensity of the colour of the solution is the measure of the concentration of the
substance.
The intensity is determined with the help of colorimeter.
Theory:
In colorimetric analysis light from a suitable source is passed through a filter to produce
monochromatic light. The monochromatic light is passed through the solution to be tested where a
part of the light is absorbed by the solution. The extent of absorption depends on the concentration of
the solution and on the path length. These are stated in the form of “
Beer’s –
Beer’s law:
It states that “when monochromatic light passes through a transparent light adsorbing
medium, the amount of light absorbed is proportional to concentration of the solution”
A
∝
C
Lambert’s law:
It states that “when monochromatic light passes through a transparent light
adsorbing medium of amount of light absorbed is proportional to thickness of the medium”
A
∝
t
Thus combining both Beer Lamberts law we get ,
A
∝
C t
Thus at constant thickness, absorbance is directly proportional to concentraion
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Chemistry for EEE stream BCHEE202
Module 5
Instrumentation:
Instrument used to measure absorbance of a solution is called photo electric colorimeter. Photo
electric colorimeter used for measuring absorbance of a coloured solution.
The photo electric colorimeter consist of the following parts
1)
Tungsten lamp as the source ( 400-760nm)
2)
Filter
3)
Sample cell ( to keep sample)
4)
recorder
Colorimeter use light from a tungsten lamp. The optical filter selects the light of definite wavelength
at which the light absorbing substance absorb light to the maximum extent.
Photo cell collect the
transmitted light and converts it into an electrical signal. The output from the photocell is proportional
to the intensity of light falling on the photocell. The output is then amplified and measured on
ammeter directly as absorbance.
Series of standard solution is prepared. First a blank solution is kept in the sample cell and placed in
the path of the light beam. Its absorbance is adjusted to zero on the meter. Next the analyte solution is
placed in the path of the light beam and quantity of light absorbed is measured as its absorbance.
Application of colorimetry:
Colorimetry is a versatile method of determining the concentration of metals & non metals present in
small quantities in ores, soil samples & alloys. The sample is dissolved in a suitable acid and known
amount of solution is treated with a reagent that produce characteristic colour. The absorbance of the
solution is measured at an optimum wavelength (
λ
max
) using photo electric colorimeter. This is
illustrated for the analysis of copper.
Eg: Colorimetric estimation of copper:
Principle:
when a solution containing cupric ions is treated with ammonia, a soluble complex with a
characteristic deep blue colour is formed.
Cu
+2
+ 4 NH
3
[Cu ( NH
3
)
4
]
+2
The absorbance of this solution is measur
e at 620nm (λ
max
) since the complex shows maximum
absorbance at this wavelength.
Chemistry for EEE stream BCHEE202
Module 5
Series of standard solution is measured at 620nm after treating ammonia solution. A plot of
absorbance against concentration gives a calibration curve. The absorbance of test solution is
measured in similar ways. Its concentration is determined from the graph.
Procedure:
Prepare 0.1 M copper sulphate solution (Std solution). 2, 4,6, 8 and 10 ml of the given
copper sulphate solution is transferred into separate 50 ml standard flasks. Each is treated with 5ml of
ammonia and made up to the mark with distilled water. To the given
test solution taken in a 50ml
standard flask add 5ml of ammonia and made up to the mark with distilled water. Absorbance of each
standard solution and test solution is measured using colorimeter at 620 nm., each time by adjusting
zero absorbance for blank solution.
Draw a calibration curve by plotting the absorbance against
concentration of copper as shown in the fig. using the calibration curve determine the concentration of
test solution.
x
x
x
x
x
Absorbance
Concentration
Questions
1.
Write a note on type of electrodes.
2.
What are ion selective electrodes? Discuss construction, working and application of glass
electrode. Explain the determination of pH using glass electrode.
3.
Explain the construction and working of calomel electrode.
4.
Explain the theory and instrumentation of Potentiometry
5.
Explain the theory and instrumentation of Conductometry
6.
Explain the application of Conductometry in estimation of weak acid.
7.
Explain the application of Potentiometry in estimation of Fe.
8.
Write a note on concentration cells.
9.
Discuss the working principle of electrochemical sensors and write any four applications of
electrochemical sensors.
10.
Discuss the working principle of conductometric sensors and write any four applications of
conductometric sensors.
11.
Discuss the working principle of thermometric sensors and write any four applications of
thermometric sensors.
12.
Discuss the working principle of optical sensors and write any four applications.
Dr.Lavanya D Kateel
Assistant Professor
Department of Science & Humanities
CEC
Chemistry for EEE stream BCHEE202
Module 5
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