Instrumental Method of Analysis

Types of Instrumental Method of Analysis

Instrumental Method of Analysis can be classified into two types
1. Electrical method: It involves the measurement of current ,voltage or resistance in relation
to the concentration of a certain species in solution
Ex: Potentiometric method, Conductometric method etc

2. Optical methods: The optical methods are based on how the sample acts towards the
electromagnetic radiation.
Ex: colorimetric method, Flame photometric method.
Advantages:
 The method is much faster than the chemical methods
 Applicable at concentrations too small for determination by classical method
 Find wide applications in industries
 The analytical process can be automated
Disadvantages:
 The instruments are expensive
 The initial or continuous calibration is required using a sample of material of known
composition
 The concentration range is limited
 Specialized training is needed for the operation of certain sophisticated instruments.

Theory, Instrumentation and Applications of:

1. Colorimetry:
Theory: Colorimetric analysis is a measurement of absorption of light in visible region (400
-700nm). The basis of colorimeter is the intensity of colored solution is a measure of
concentration of solution.
When a monochromatic light radiation passing through a medium a part of light is reflected, a
part is absorbed and a part is transmitted.
IO = I r + I a + I t
For glass medium Ir = 0
So IO = Ia + It
Extent of absorption depends on concentration of solution and thickness of medium. Quantitative
analysis by colorimetric is based on beer –Lambert Law.
Beer’s law: The intensity of the transmitted light decreases exponentially as the concentration of
the absorbing substance increases arithmetically.
Lamberts law: The intensity of the transmitted light decreases exponentially as the thickness of
the absorption medium increases arithmetically.
Beer- Lamberts law: The amount of light absorbed is directly proportional to the concentration
(c) of the solution and directly proportional to the path length (l).
A = ε Cl

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Therefore absorbance of the colored solution changes with both concentration and thickness of
the solution, if thickness kept constant, absorbance of the solution increases with increase in the
concentration. On measuring the Absorbance of known and unknown concentrate solutions with
respect to blank. Concentration of unknown solution can be determined from the graph
Instrumentation:

The essential parts of colorimeter are

 Source – Light
 Filter- To filter undesired radiations, it allows radiations of a definite wave length range
to pass through it and reach the sample
 Sample cell - To take sample
 Photocell- To receive the transmitted light
 Recorder – to record the absorbance
Application: Colorimetric determination of copper in copper sulfate solution
1. Draw out 2, 4, 6, 8 and 10 ml of copper sulfate solution into a 50 ml volumetric flask.
Add 5ml of ammonia solution to each flask and also to test solution or unknown solution.
Dilute all the solutions with distilled water up to the mark and mix well. Measure the
absorbance of all the solutions at 620 nm against blank (5 ml of ammonia and water in a
standard flask).
2. Plot a graph of absorbance or optical density against volume of copper sulfate
(Calibration curve) and determine the volume and concentration of copper in the
unknown solution.

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 O
p
t
i
c
a
l

D
e
n
s Volume of Unknown
i solution
t
y

a c m3
V o l u m e o f c o p p e r s u l p h a t e (c m 3)

2. Flame photometry:
Theory: Emission of characteristic radiation by element and the correlation of the emission
intensity with the concentration of the element is the basis of flame photometry.
When a solution containing the sample element or the ion is aspirated into the flame, a series of
changes take place at the flame.
Evaporation Vaporization Dissociation
M+X- M+X- MX MX M(gas) + X (gas)
[Solution] Mist [Solid] [gas]
Absorption of
heat

Flame emission
(gas)M M*(gas)

1. First, the solvent gets evaporated leaving behind the salt in the flame
2. The salt then gets evaporated into vapors of the salt, which further undergo disassociation into
the constituent atoms
3. Metal atoms formed in the flame absorb heat energy from the flame and get electronically
excited into their higher energy level
4. Excited metal atoms fall back to their ground state by emitting the energy in the form of
radiations
5. The intensity of the light radiation emitted is proportional to the no. of atoms in the excited
state which in turn is proportional to the no. of atoms in the flame are the concentration of the
solution fed into the flame
6. Thus the concentration of the solution is related to the intensity of emitted radiation
7. By measuring the intensity of the emitted radiation by a flame photometer concentration can
be determined
Instrumentation:

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When pressurized air is passed in to the atomizer produces suction. The suction draws a sample
solution in to the atomizer. And the mixture is passed in to the mixing chamber where it is mixed
with the gas and it is passed into the flame, the emitted radiation is passed through lens, through
filter and finally through photometer. The output is read out by suitable readout device.

Application:
Estimation of sodium content in the given sample of water
1. Transfer 2, 4, 6, 8, and 10 cm3 of standard sodium solution into different 50 cm3
volumetric flasks, make up all the solutions using distilled water
2. Make up the volumetric flask containing unknown concentration up to the mark using
distilled water.
3. Adjust the air supply from the compressor 10 lbs/square inch using pressure regulating
knob
4. Place the sodium filter (589 nm) in position
5. Dip the capillary tube in a cell containing distilled water, adjust the flame photometer to
zero by zero control knob
6. Now feed the 100 ppm sodium ion solution and adjust the reading to 100, repeat the
process to confirm the accuracy of the calibration
7. Feed the various sodium ion solutions through the flame one by one including the
unknown solution and note down the flame photometer readings.
8. plot a graph of flame photometer reading against concentration to get calibration curve
9. Using calibration curve concentration of unknown solution can be determined.

3. Potentiometry:
Theory: The electrode potential and metal ion concentration is related by Nernst equation
0 . 0591
E=E 0 + . log [ M n+ ]
n

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So the electrode potential depends on the concentration of metal ions.

 A Potentiometric titration may be defined as a titration in which the end point is detected
by measuring the change in potential of a suitable electrode during the titration.
 The electrode which responds to the change in concentration of the ion in the solution is
called the indicator electrode.
 The indicator electrode is combined with the reference electrode to form the cell and the
emf of the so formed cell is measured during the titration. The emf of the cell changes
gradually till the end point and changes rapidly at very close to the end point and again the
change is gradual after the end point.
 When emf is plotted as ordinate and the volume of titrant added as abscissa, the point of
inflection of the curve corresponds to the equivalence point or end point of the titration
Instrumentation:
 Instrument used- potentiometer or pH meter
 Electrodes: Indicator electrode eg – platinum (whose electrode potential depends on the
concentration of ions in a solution)
Reference electrode eg- calomel for potentiometer, glass electrode for pH meter
 The solution to be titrated is taken in the beaker and titrant is taken in the micro burette.
 Immerse the electrodes in the beaker along with stirrer
 The electrodes are connected to the potentiometer which gives the emf values

Emission intensity

Volume of Na ion solution

Application:
Oxidation Reduction reactions (FAS v/s K2Cr2O7): Estimation of amount FAS.
1. Burette out 25 cm3 of FAS solution into a 100 ml beaker. Add 1 test tube full of dilute sulfuric
acid.
2. Dip the platinum and calomel electrode assembly into the solution & connect to a
potentiometer and measure the potential.
3. Add 0.5cm3 of K2Cr2O7 from a burette, stir the solution well and measure the potential after
each addition.
4. Continue the process till the potential shows a tendency to increase rapidly and carry out the
experiment for five more readings.

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 5. Plot a graph of ∆E/∆V against volume of titrant and find out equivalence point.
6. Using formula, calculate the normality and amount of FAS in the given solution.

N FAS = ( NXV )K 2 Cr 2 O 7
V FAS

Amount = NFAS x 392

4. Conductometry:
ΔE/ΔV
∆E/∆v
Volume of K2Cr2O7
Theory: Ohm’s law states that the current I (amperes) flowing through a conductor is directly
proportional to the applied potential, E(volts) and inversely proportional to the resistance
R(ohms) of the conductor.
I= E/R
The reciprocal of resistance is called conductance. The resistance of a homogeneous material of
uniform cross section with an area of a sq. cm and length l cm is given by,
R= ρ l /a. Where ρ is called specific resistance.
k=1/ ρ is called specific conductance.
k=1/R 1/a when l =1cm and a=1cm2 k=1/R
Specific conductance is the conductance of a solution placed between two electrodes of 1 cm2
area and kept 1cm apart. Unit is Ohm-1 cm-1 or Sm-1
In conductometric titrations, there is a sudden change in the conductance of the solution near the
neutralization point. However the change is not sharp and hence the neutralization point is
determined graphically by plotting conductivity against titer values. The principle involved in
conductometric titrations is the replacement of ions of a particular conductivity by ions of
different conductivity during titration
Instrumentation: It consists of two platinum electrodes each of unit area of cross section placed
unit distance apart. The electrodes are dipped in the electrolytic solution taken in a beaker. It is
connected to a conductance measuring device. The titrant is added from a burette and the
solution is stirred. The conductance is measured after the addition of the titrant at intervals of 0.5
ml.

Conductometer

Pt Electrode
Stirrer

Applications:
i) Strong acid V/s strong base:

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 ( NXV )NaOH
N=
V HCL
HCl (H+ + Cl-) + NaOH (Na+ + OH-) NaCl (Na+ + Cl-) + H2O
 During the titration highly mobile H+ ions are replaced by less mobile Na+ ions. Therefore,
there is a decrease in the conductance of the solution. This continues till the neutralization
point.
 After the end point conductance increases due to free OH – ions.
 Intersection of two straight lines is the end point of the reaction.
 Using formula, normality and amount can be calculated.

Amount =N x Eq wt of HCl

ii) Weak acid V/s strong base:

CH3COOH + NaOH CH3COONa + H2O
 During the titration formation of highly ionizable sodium acetate salt is formed.
 Therefore, there is an increase in the conductance of the solution. This continues till the
neutralization point.
 After the end point conductance increases due to
free OH – ions.
 Intersection of two straight lines is the end point
of the reaction.
 Using formula normality and amount can be
calculated.

( NXV )NaOH
N=
V

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 Amount =N x Eq wt of CH3COOH

iii) Mixture of strong acid (HCl) and weak acid

(CH3COOH) V/s strong base

HCl (H+ + Cl-) + NaOH (Na+ + OH-) NaCl (Na+ + Cl-) + H2O
 In the first line, the conductance decreases due to the replacement of highly mobile H + ions
of the strong acid (HCl) by less mobile Na+ ions of the base. This continues till the first
neutralization point.
CH3COOH + NaOH CH3COONa + H2O
 In the second line, due to the formation of highly ionizable sodium acetate salt, there is an
increase in the conductance of the solution. This continues till the second neutralization
point.
 In the third line conductance increases due to free OH –ions.
 Using formula normality and amount of each acid can be calculated.

( NXV )NaOH
N=
V

Amount =N x Eq wt of acid

5. Atomic Absorption Spectroscopy

Theory: It is a technique for measuring the
concentration of various elements in the sample
through their absorption of light.
Atomic absorption spectroscopy is based on the principle that when a beam of electromagnetic
radiation is passed through a substance, the radiation may either be absorbed or transmitted
depending upon the wavelength of the radiation.

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When a monochromatic radiation of frequency v, is incident on a molecule, the molecule in the
gaseous state absorbs a photon of energy, it undergoes a transition from lower energy level to
higher energy level.
∆E = hv
The greater the concentration of metal atoms in solution, then more photons is absorbed in the
flame from the wavelength source. This follows Beer – Lambert law.
A = ε Cl
Instrumentation:

Atomic absorption spectrometer includes the following components

 Nebulizer: It creates a fine spray of the sample.
 Atomizer: To dry the sample and produce atoms.
 Fuel and oxidant: To burn the sample by heat. Commonly used fuels include propane,
acetylene etc., and oxidants are mostly air or oxygen.
 Hallow cathode lamp: To produce light of the desired wavelength. It is coated with a metal
of analyte to be analyzed.
 Detector: To detect the absorption intensity.

Applications:
 It is used for quantitative analysis of metal elements in samples like soil, plant material.
 To determine metal elements in food industry
 To estimate lead in petroleum products
 To determine metal concentrations in ground water
 To determine heavy metals like in environmental samples.

Determination of Nickel:
1. A stock solution of nickel is prepared by dissolving nickel with HNO3 and H2SO4 and make
up the volume to the mark by distilled water
2. Prepare 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 ppm of nickel solution using stock solution in 50 ml
volumetric flask
3. Prepare the blank solution and unknown nickel solution.

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4. Spray the prepared nickel solutions and unknown in a series in the flame and the absorbance
is recorded with respect to the blank.
5. Plot a graph of absorbance against concentration
6. Using calibration curve concentration of unknown solution can be determined

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