Nephelometry and turbidimetry

NEPHELOMETRY AND TURBIDIMETRY
 These are the techniques which are used to measures the scattered light
by non-transparent particles suspended in a solution.
 Both the two are differ only in the manner of measuring the scattered
radiation.
 Both are closely allied to colorimetry.
When light is passed through the a suspension, the part of the incident
radiant energy is dissipated by,
1. Absorption
2. Reflection
3. Refraction
4. Transmission
PRINCIPLE:
TURBIDIMETRY
 Measurement of the intensity of the transmitted light as a function of
the concentration of suspended particles forms the basis of
turbidimetric analysis.
 The amount of radiation transmitted by the particles is measured at
an angle of 1800 to the incident beam.
Forturbidimetric measurements the transmitted intensity I can be
determined from the equation.
It = I0 × 10 –k ‘lc
 This equation is known as the basic equation of turbidimetry.
 It is similar to Bouguer – Lambert – Beer equation.
Where,
I0 = incident intensity.
1
8
0
0

It = transmitted intensity.
C = concentration of absorbing particles in the solution
1 = thickness of absorbing layer of solution.
NEPHELOMETRY
 Measurement of the intensity of the scattered light as a function of
concentration of the dispersed phase forms the basis of
nephelometric analysis.
 The amount of radiation scattered by the particles is measured at
an angle (usually 900) to the incident beam.
TURBIDIMETRYAND COLORIMETRY:
 Turbidimetry is much similar to colorimetry
 Because both involve measurement of the intensity of light transmitted
through a medium.
 But they differ in the sense that the light intensity is decreased by
transmission in turbidimetry and by the absorptionin colorimetry.
NEPHELOMETRYAND FLUORIMETRY:
 Nephelometry is much similar to fluorimetry.
 Because both involve the measurement of scattered light.
 Difference: scattering is elastic in fluorimetry and inelastic in
Nephelometry.
 i.e., both incident and scattered light are of the same wavelength in
Nephelometry.
CHOICE BETWEEN NEPHELOMETRYAND TURBIDIMETRY:
 It depends upon the amount of light scattered by suspended particles
present in the solution.
 Turbidimetry is suitable for high concentrated suspensions.
 Nephelometry is suitable for low concentrated suspensions.

THEORY
REFLECTION Vs SCATTERING
REFLECTION:
 It takes place when “dimensions of suspended particles are larger than
the wavelength of the incident light”.
SCATTERING:
 It takes place when “dimensions of suspended particles are same order of
magnitudeor smaller than the wavelength of incident light”.
1) In nephelometric measurements, the suspended particles should be
small with respect to the wavelength used.
2) This is required so that scattering rather than reflection predominates.
3) At the same time, smaller particles undergo scattering to give rise a
symmetrical pattern of secondaryrays in spacehaving maximum
intensity at 900 to the primary incident beam.
4) Thus most of the instruments used in Nephelometry involve
measurements at 900.
5) If the particles are large, a small fraction of light gets deviated at
right angle to the primary beam whereas the larger fraction gets
deviated at angles other than 900.
6) In such cases, Nephelometry measurements are made at angles less
than 900 from the primary beam (50 to 200 or even 450).
7) In nephelometry, suspended particles should neither be too large
nor too small otherwise scattering efficiency falls off.
8) For measurements to be made in the ultravioletand visibleregions
of spectrum the optimum particle size should be in the range of about
0.1 to 1µm.
9) In turbidimetric measurements, particles larger than the wavelength
of light.

10) Because measurement depends o the total radiation removed from
the primary beam irrespective of the mechanism by which it is
remove or the angle through which it undergoes deviation.
11) But with larger particles another problem arises, i.e., the
relationship between absorbanceand concentration doe not remain
linear. Thus, in such cases measurement s cannot be very accurate.
FACTORS AFFECTING MEASUREMENTS:
The amount of radiation removed or deviated from the primary radiation
beam depends on the following factors:
 Ratio of concentration of solutions mixed
 Order of mixing of solutions
 Concentration of ions forming the precipitate
 Temperature
 Rate of mixing
 Presence of extraneous electrolytes
 Stability of dispersion
 Presence of non-electrolytes
 Presence of protective colloids
 Time required to attaining the maximum turbidity
 Wavelength of incident light
 Refractive index deference
 Particle geometry
INSTRUMENTATION:
The instruments used in nephelometry and turbidimetry are similar as used
in spectrophotometry.
We will describe here some special features:
Sources:
 Monochromatic radiation is used both in turbidimeters and nephelometer
to minimise absorption.
 Generally a mercury arc or a laser with special filter combinations for
isolating one of its emission lines is the most suitable source.
 The tungsten lamp (which is polychromatic source)is used when one has
to determine the concentration of a particular substance.

 It has been observed that even in such a case blue spectral region gives
the best results.
Cells:
 We can use cylindrical cells having flat faces where entering and existing
beams are passed.
 This is to minimize reflections and multiple scatterings from the cell
walls.
 Generally a cell with a rectangular cross section is preferred.
 The octagonal faces of cell allow measurements to be made at 00, 450, 900
or 1350 to primary beam.
 Generally, walls through which light beam are not to pass are coated a
dull black to absorb unwanted radiation.
Detectors:
 In turbidimeters, phototubes are used as detectors.
 The photo-multiplier tubes are used as detectors in case of nephelometers
because intensity of scattered radiation is generally very small.
 Generally the detector is fixed at 90° to primary but for maximum
sensitivity the detector angle should vary which is generally close to the
primary beam.
 There are some nephelometers where detectoris mounted on a circular
disc which allows measurement at many angles, i.e. at 0° and from 35° to
135°.
Filters:
1) Turbidimeter (Blue filter of 530nm)
2) Nephelometer (visible Filter)
TURBIDIMETER

NEPHELOMETER
APPLICATIONS:
1. Inorganic Analysis:
 The main uses of nephelometry and turbidimetry are the
determination of sulphate as BaSO4, chloride as AgCl, fluoride
as CaF2, cyanide as AgCN, calcium as oxalate, carbonate as
BaCO3 and Zinc as ferrocyanide.
 Out of these sulphur determination is of particular importance
and serves for the routine determination of total sulphur in
coke, coal, oils, rubbers, plastics and other organic substances.
 Determination of carbondioxide where it involves the bubbling
of the gas through an alkaline solution of a barium salt and then
analyzing for the barium carbonate suspension with
nephelometry or turbidimetry.
 These are more precise and sensitive than colorimetric
methods.
 Also in the basic of the water treatment plants, in sewage works
and in power and steam generating plants.
2. Organic Analysis:
 In food and beverages, turbidimeter is used for analysis of turbidity
in sugar products, and clarity of citrus juices.
3. BiochemicalAnalysis:
 Turbidimetry is used to measure the amount of growth of a test
bacterium in a liquid nutrient medium also used to find out the
amount of amino acids, vitamins and antibiotics.

 Nephelometry has been used for the determination of protein and
the determination of yeast, glycogen and of beta and gamma
globulin in blood serum and plasma.
4. Air and Water Pollution:
 In air, dust and smoke are monitored whereas in water, turbidity is
monitored.
5. Turbidimetric Titrations:
 It is analogous to photometric titrations.
 In these titrations, the absorbanceis to be plotted against the
volume of titrant added.
 With the increase in the volume of titrant, the concentration of
precipitate increases and hence the absorbanceincreases.
 When all the substancegets precipitated, the absorbancebecome
constant.
 The change in the slope indicates the end-point.
6. Phase Titrations:
 Turbidimetry can be used for titrating a mixture of two liquids by a
third which is miscible with one but not with the other.
7. Determination of MolecularWeights of Higher Polymers:
 The measurement of the intensity of light scattered by polymer
solutions constitutes an important method for determining the
molecular weights of macromolecules.
 The turbidity of a solution of macromolecule is related to its
molecular weight by the following relation.
Where,
H = constant for given polymer
C = concentration of the solution in grams per ml
Turbidity = fraction of incident light scattered per cm length of the
solution.
8. Atmospheric Pollution :
 Smokesand fogs are visible largely due to light scattering effects,
 Thus instruments for measuring these effects are very useful in
monitoring atmosphericpollution.
Limit HC 1
C 0 ——— = —
Turbidity M

Nephelometry and turbidimetry

Nephelometry and turbidimetry

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