15.

1 Introductory
Nephelometry and Turbidimetry:
• These are analytical techniques related to colorimetry.
• Both methods involve scattering of light by non-transparent particles suspended in a
solution.
• The main difference between them is how the scattered light is measured.

Turbidimetry:

• Definition: Measures the intensity of transmitted light (i.e., how much light passes through
the sample).
• Concept: When light passes through a suspension:
o Some light is absorbed, reflected, or refracted by the particles.
o The rest is transmitted (passes through).
• Application: The amount of transmitted light decreases as the concentration of
suspended particles increases.
• This principle is the basis of turbidimetric analysis.
• Diagram (Fig. 15.1) shows:
o Light Source → Lens → Sample → Detector.

Nephelometry:

• Definition: Measures the intensity of scattered light.

• Concept: Light hits the suspended particles and is scattered in different directions.
o Commonly, the scattered light is measured at a 90° angle to the incident beam.
• Application: The intensity of scattered light increases with an increase in particle
concentration.
• This is the basis of nephelometric analysis.
• Diagram (Fig. 15.2) shows:
o Light Source → Lens → Sample → Scattered Light measured at 90° using a detector.

15.2 Turbidimetry and Colorimetry

• Both methods measure the intensity of transmitted light, but:

o In colorimetry, the color or absorbance due to a colored solute is measured.
o In turbidimetry, the light intensity decreases due to scattering by suspended
particles, not absorption by solute.
o
Feature Turbidimetry Nephelometry

What is measured? Transmitted light Scattered light

Direction of Same direction as incident light Usually 90° to incident beam

measurement

Based on Loss of light intensity due to scattering Intensity of scattered light

Common use Suspensions, e.g., wastewater analysis Protein, antigen-antibody

studies

15.3 Nephelometry and Fluorimetry

• Nephelometry is much similar to fluorimetric method because both involve measurement

of scattered light.
• Key difference:
o In fluorimetry, scattering is elastic.
o In nephelometry, scattering is inelastic.

• ➤ This means:

o In fluorimetry: both incident and scattered light are of the same wavelength.
o In nephelometry: scattered light is of a longer wavelength than the incident light.

15.4 Choice Between Nephelometry and Turbidimetry

• Choice depends on: amount of light scattered by suspended particles present in solution.

• ➤ Turbidimetry is satisfactory for relatively high concentrations of suspended particles.

o Because scattering is extensive due to presence of many particles.

• ➤ Nephelometry is most suited when:

o Suspension is less dense.

o Decrease in power of incident beam is small.
o More accurate results can be obtained due to small amount of scattered light
measured against a black background.

• ⚠ Important Caution:

o In such a case (low scattering), turbidimetry should NOT be used, because:

▪ Comparison would be between two large quantities of nearly equal values,
reducing accuracy.
 15.5 Theory

(1) Reflection vs. Scattering

• Both reflection and scattering phenomena are very important in turbidimetry and
nephelometry.

• ➤ If light is allowed to pass through solution having suspended particles:

o Reflection occurs if:

▪ Dimensions of suspended particles are larger than the wavelength of
incident light.
o Scattering occurs if:
▪ Dimensions of suspended particles are of same order of magnitude or
smaller than wavelength.

• This distinction is critical for:

o Sensitivity of measurement.
o Instrument design in nephelometry and turbidimetry.

(i) Key Point on Particle Size and Wavelength:

• In nephelometric measurements:
o Suspended particles should be small with respect to wavelength.
• Why?
o Ensures scattering rather than reflection predominates.

• ➤ Smaller particles → more likely to scatter light symmetrically.

o Creates symmetrical pattern of secondary rays in space.

• Max intensity at 90° to the primary incident beam.

• Therefore, most instruments used in nephelometry:

o Involve measurements at 90°

Concept Key Competitive Point

Fluorimetry vs
Elastic vs Inelastic scattering; same vs longer wavelength
Nephelometry

Turbidimetry suitability Best for high concentration, many particles, extensive scattering

Best for low concentration, weak scattering, accurate background

Nephelometry suitability
measurement

Reflection: particle size > wavelength; Scattering: particle size ≤

Reflection vs Scattering
wavelength
Concept Key Competitive Point

Nephelometry
Done at 90° to incident light, particles must be smaller than wavelength
measurement

Continuation of 15.5 Theory

(i) Particle Size and Scattering Angle (continued)

• If particles are large:

o Only a small fraction of light is scattered at right angles (90°) to the primary
beam.
o A larger fraction of light gets deviated at angles ≠ 90°.
o So, in nephelometric measurements, angles < 90° (like 5° to 20° or even 45°) are
often used.
• In nephelometry, suspended particles should be:
o Neither too large nor too small.
o If too small → scattering efficiency falls off.
o For UV and visible region measurements, optimum particle size should be in the
range of 0.1 to 1 µm (micrometer).

(ii) Turbidimetric Measurements — Particle Size

• In turbidimetric measurements:
o Particles larger than the wavelength of light do not pose much problem.
o Why? Because the measurement is based on total radiation removed from the
primary beam:
▪ This includes absorption, reflection, and scattering — it does not depend
on the exact mechanism.

• ⚠ However, with larger particles, a problem arises:

o The relationship between absorbance and concentration becomes non-linear.

o So, measurements may not be very accurate.

(2) Factors Affecting Measurements

The amount of radiation removed or deviated from the primary radiation beam depends on:

(a) Concentration

• In turbidimetry, one measures the transmittance of a primary beam of radiation.

• Definition of transmittance follows (likely on the next page).

Quick Competitive Recap Table:

Topic Key Point

If particles are large → scattering angle ≠ 90° → measure at <

Scattering angle in nephelometry
90°

Optimal particle size for

0.1 to 1 µm, otherwise efficiency falls
nephelometry

Large particles don’t affect detection mechanism, but cause

Turbidimetry: particle size impact
non-linearity

Nephelometry particle size Must be comparable to or smaller than wavelength

Measurement mechanism Based on total radiation removed, not type

(turbidimetry) (absorption/scattering)

Continuation of 15.5 Theory

(2) Factors Affecting Measurements (continued)

(a) Concentration in Turbidimetry

• The transmittance TTT of a primary beam is defined as:

where:
• I0 = intensity of incident light after passing through a comparison cell (containing only
solvent).
• I = intensity of light after passing through the sample solution.
Transmittance TTT is related to the concentration ccc of suspended material using a form of Beer’s
Law:

Important Notes on Equation (2):

• Equation (2) is valid only for small particles under Rayleigh’s scattering.
• Applicable to dilute suspensions (where multiple scattering is unlikely).
• If the suspension is too dilute, then:
o I≈I0, and
o Accurate measurement is not possible due to minimal attenuation.

• ⚠ Equation (2) also shows departures in real cases, similar to how Beer's Law deviates
under non-ideal conditions.

Working Curve in Turbidimetry:

• A calibration or working curve is prepared by plotting:

S vs known concentrations
• Then, unknown concentrations can be determined by comparing their S values.

Nephelometry: Relation Between Intensity and Concentration

• In nephelometry, there's no simple theoretical equation to relate scattered intensity to

concentration.
• This is due to:
o Properties of the suspension
o Scattering angle
o Geometry of the instrument
• The best practical equation is empirical:
(b) Particle Geometry
• In both turbidimetric and nephelometric analysis, one of the most critical factors is:
o Control of particle size and shape.
• Ideal condition:
o All samples and calibration solutions must possess the same distribution of small,
medium, and large particles.
• Implication:
o Samples and standards must be prepared under identical conditions.

• ⚠ This is not a simple task: Conditions include:

o Concentration of reactants
o Temperature
o Agitation
o pH
o Presence of non-reactants
o Order of mixing
o Time allowed for particle growth
• If not maintained:
o Different particle sizes may form, leading to major errors in both turbidimetry
and nephelometry.
(c) Wavelength of Incident Light
• In turbidimetry, wavelength of incident light is a very important factor.
 • General rule:
o Select a wavelength at which the sample solution does not absorb light strongly.

• Practices:

o If sample is colorless → Use incident light of same color.

o If sample is dark/colored → Use red or infrared light (longer wavelength), where
absorption is minimum.
• In nephelometry, absorption is less of a problem.
o So, white light is generally used for convenience.
(d) Refractive Index Difference
• Best results are obtained when:
o There is a significant refractive index difference between:
▪ The particle and
▪ The surrounding medium.

• ➤ Sometimes, solvents are changed to increase this difference, improving measurement

accuracy.

Factor Description & Competitive Highlight

Particle Size & shape must be controlled. Samples and standards must be made
Geometry under identical conditions.

Wavelength of Turbidimetry → choose wavelength not absorbed.

Light

1. Basic Concept of Light Scattering

• When radiant energy strikes particles suspended in a medium with a different refractive
index, the light is scattered at angles ≠ 180°.
• This is called light scattering.
2. Difference Between Turbidimetry and Nephelometry

Technique What It Measures Measurement Direction

Turbidimetry Decrease in transmitted (non-scattered) light Along the incident beam (180°)

Nephelometry Intensity of scattered light At an angle to the incident beam

3. Nephelometry vs Fluorimetry

• Both measure at an angle to incoming light.

• However, mechanisms are entirely different.
4. Turbidimetry vs Absorption Spectrophotometry

Aspect Turbidimetry Absorption Spectrophotometry

Sample Type Suspension True solution

Beam Interaction Light loss due to scattering Light loss due to absorption

5. Energy Absorption by Particles

• Particles absorb radiant energy and return to lower energy by emitting it.
• If wavelength > particle size, light passes unchanged.
• If wavelength ≈ particle size, scattering increases.

Important Line:

“As the wavelength of impinging radiation approaches the particle size, the scattered fraction of
light becomes greater.”

7. Factors Affecting Light Scattering

1. Number of particles (i.e., concentration)

2. Size and shape of the particles
3. Wavelength of light
o Blue light is scattered more than red light (shorter λ = more scattering)

8. Instrumentation Comparison

• Nephelometry and Turbidimetry use modified spectrophotometers.

• Nephelometer: Measures scattered light at angles.
• Turbidimeter: Measures transmitted light loss (similar to absorption instruments).
9. Sample Type Comparison

Instrument Sample Type

Absorption Spectrophotometer True solution

Turbidimeter / Nephelometer Suspension

10. Improving Sensitivity of Turbidimetry

• Use a large end-on detector close to sample.

• Collect scattered light up to 90° angle.
• Creates a hybrid system between turbidimeter and nephelometer.

11. Fluorimeter as Nephelometer

• Convert fluorimeter to nephelometer by:

o Removing emission filter (for filter-based systems)
o Setting emission monochromator = excitation monochromator (in grating/prism
systems)

1. Sources (of Radiation)

• White light can be used in nephelometers, but monochromatic radiation is preferred.

• Monochromatic radiation is also used in turbidimeters to minimize absorption.
• High-intensity monochromatic sources (e.g., mercury arc, laser) improve Rayleigh
scattering.
• Blue spectral region gives the best results; polychromatic sources like tungsten lamps may
also be use

2. Detectors

• Photomultiplier tubes are preferred in nephelometers due to the very small intensity of
scattered radiation.
• Detector is generally fixed at 90° to the primary beam.
• For greater versatility and sensitivity, detector angle can be varied (usually close to primary
beam).
• Some detectors are mounted on circular discs, allowing readings at multiple angles (e.g.,
0°, 30° to 135°).
• The disc is graduated in degrees and viewed from outside.

• In turbidimeters, ordinary detectors like phototubes may be used.

3. Cells (Sample Holders)

• Though cylindrical cells can be used, flat-faced cells are better to reduce reflections and
scattering.
• Rectangular cross-section cells are preferred.
• Semioctagonal cells and octagonal cells (referenced in Figs. 154, 155) allow multi-angle
measurements (e.g., 0°, 45°, 90°, 135°).
• Interior cell walls should be dull black to absorb unwanted radiation.
• A blackened curved screen is often placed opposite to trap unscattered light.
• Light traps or blackening helps minimize stray radiation in experimental setups.

4. Turbidimeters

• For turbidity measurements, ordinary colorimeters or spectrophotometers may be adapted.

• Simple instruments like the Parr turbidimeter or the Duboscq colorimeter are also

Du Pont Model 430 Turbidimeter – Principle and Working

1. Unique Feature

• It is a double-beam instrument.
• Measures transmitted and scattered light based on the polarisation of light.

2. Working Principle

• The scattering of light by suspended particles in solution causes a change in the plane of
polarisation.
• Light from a lamp passes through a primary polarizer, producing plane-polarised light.
• This polarised light then passes through the sample.

3. Beam Splitting and Detection

• After passing through the sample:

o Light beam is split by a half-silvered mirror.
o Two beams go to:
▪ Photocell A (through the straight path).
▪ Photocell B (through a crossed polarizer — detects scattered/deflected
light).
4. Interpretation of Detector Response

Sample Condition Photocell A Photocell B

No suspended particles Maximum response Minimum/zero response

With suspended particles Decreases (↓) Increases (↑)

• The ratio of signal B to signal A = Measure of turbidity.

5. Advantages

• Double-beam setup reduces absorption interference.

• Works as an on-line monitor for flowing streams.
• Useful for diluted and industrial samples.
• Insensitive to:
o Color of particles
o Lamp fluctuations

6. Limitations

• Cannot be used with samples containing optically active substances.

Diagram Labels (Fig. 15.6)

1. Lamp → Light source

2. Lens → Focuses beam
3. Primary Polarizer → Polarizes light
4. Sample → Contains suspended particles
5. Half-Silvered Mirror → Splits beam
6. Crossed Polarizer → For scattered beam to Photocell B
7. Photocell A & B → Detect transmitted and scattered beams

5. Nephelometers

General Information

• Ordinary fluorimeters are commonly used for nephelometric measurements.

• In some cases, even spectrophotometers can be used as nephelometers.

Instrument Description (Fig. 15.7)

• Mercury lamp → Source of light.

• Achromatic lens → Focuses the beam.
 • Monochromatic filter → Isolates specific wavelength.
• Semioctagonal cell → Holds the sample.
• Collimating tube → Aligns the light beam.
• Photomultiplier tube → Sensitive detector.
• Demountable polariser & Opal glass diffuser → Polarisation and depolarisation.
• Graduated disc with deflector → Allows rotation of detector.
• Light trap → Absorbs undetected light.

Working Principle

• Multiplier phototube used as receiver is mounted on a turntable and can be positioned at

any angle from 0° to 180°.
• Standard nephelometric measurements are typically taken at 45° or 90° to the primary
beam.
• Undeviated beam is passed into a light trap to prevent interference.

Additional Modification (Suggested by Debye)

• Detector and turntable are placed below the cell in a closed compartment.
• Scattered light is deflected by a small right-angle prism onto the photomultiplier tube.
• A floor shutter allows controlled detection.

Complete Applications of Turbidimetry and Nephelometry

(As per your image – no keywords missed )

General Scope

• Can be used on gaseous, liquid, or even transparent solid samples.

• Samples may vary greatly in proportions.

(1) Inorganic Analysis

• Useful when precipitates are difficult to filter due to:

o Small size
o Gelatinous nature
• Gravimetric methods not possible here.
• Turbidimetry/nephelometry help by converting precipitate into ideal suspension under
rigidly controlled conditions.
• Scattering of light depends on:
 o Size and number of particles
o Concentration

Important line preserved:

“This is done because the scattering of light depends on the size and number of the particles involved
as well as their concentration.”

(2) Quantitative Analysis

• Calibration curves are prepared from known metal concentrations.

• Unknown samples’ suspensions are compared with the curves.

Key line:

“The results of suspensions of unknown concentrations are obtained from these calibration curves.”

(3) Specific Analyte Determination (must be memorized as list)

Used for determination of:

Analyte Measured As

Sulphate BaSO₄

Chloride AgCl

Fluoride CaF₂
Cyanide AgCN

Oxalate Ca oxalate

Preserved phrase:

“The important uses... are the determination of sulphate as BaSO₄, chloride as AgCl, fluoride as CaF₂,
cyanide as AgCN, calcium as oxalate.”

(4) Industrial Application

• Used in routine determination of total sulphur in:

o Coke
o Coal
o Oils
o Rubbish
o Plastics
o Other organic materials

Specific lines included:

“...total sulphur in coke, coal, oils, rubbish, plastics and other organic materials.”

(5) Example Procedure: Barium Sulphate

 • Add dilute BaCl₂ to sulphate-containing solution.
• Mix → get BaSO₄ suspension.
• Turbidity measured → compared to standard calibration curve.

Preserved description:

“...mixed with BaCl₂ to get a suspension of BaSO₄... excess of solid BaCl₂ to get a suspension...
turbidity... obtained from the calibration curve.”

(6) Example: Carbonate Determination via Gas

• Gas (CO₂) bubbled through alkaline Ba(OH)₂.

• Forms BaCO₃ suspension.
• Measured by nephelometry or turbidimetry.

Exact keyword lines:

“Another important application... is the determination of carbonate content... through an alkaline

solution of barium hydroxide and then analyzing for the barium carbonate suspension.”

Conclusion:
Yes, every keyword and concept related to applications of nephelometry and turbidimetry has
been covered without skipping:

• Technique-specific uses

• Specific ion examples

• Industrial samples

• Suspension conditions

• Quantitative analysis and calibration

• Real-world examples (BaSO₄ and BaCO₃ methods)

Explanation of Introductory Content Above Table 15.1

1. Sensitivity of Nephelometric and Turbidimetric Methods

• These methods are more precise and sensitive than colorimetric methods.

2. Example – Phosphorus Estimation

• Phosphorus can be estimated at concentration of:

o 1 part in more than 300 million parts of water.
• Detected as a precipitate with:
o Strychnine-molybdate reagent.
 3. Example – Ammonia Estimation

• Ammonia (1 part in 160 million parts) can be detected by:

o Adding Nessler’s reagent.

4. Environmental and Industrial Applications

• These methods are used in:

o Water treatment plants
o Sewage works
o Power plants
o Steam-generating plants

Uses include:

• Estimation of elements in water

• Determination of clarity
• Control of treatment processes

5. Table Introduction

• The text introduces Table 15.1 with:

o "Some turbidimetric and nephelometric methods are given in Table 15.1."

Summary of Keywords Captured:

• More sensitive than colorimetric

• Strychnine-molybdate for phosphorus
• Nessler’s reagent for ammonia
• Used in: water, sewage, power plants
• Purpose: clarity check, element estimation, process control
15.8 Applications of Turbidimetry and Nephelometry (Image 1 - Bottom Section)
1. General Scope:
o Applicable to gaseous, liquid, and transparent solid samples.
o Used for samples in greatly varying proportions.
2. 1. Inorganic Analysis:
o Useful when precipitates are small or gelatinous.
o Avoids gravimetric methods in tough filtrations.
o Measures scattering of light depending on:
 ▪ Size
▪ Number
▪ Concentration of particles
3. Quantitative Analysis:
o Calibration curves used from known metal concentrations.
o Results from suspensions of unknown concentrations.
1. Examples:
o Sulphate (BaSO₄)
o Carbonate (BaCO₃)
o Chloride (AgCl)
o Fluoride (CaF₂)
o Cyanide (AgCN)
o Others: oxalate, arsenite, arsenate
2. Determination of Sulphate:
o Solution of sulphate + BaCl₂ → Suspension of BaSO₄.
o Measured using turbidimetry or nephelometry.
3. Determination of Carbonate:
o Bubbling CO₂ gas through alkaline Ba(OH)₂.
o Forms BaCO₃ suspension → Measured for turbidity.
Table 15.1 – Turbidimetric (T) and Nephelometric (N) Methods

🔬 Element ⚗️ 🌫️ Suspension 🧪 Reagent 🚫 Interferences

Method Formed Used

Ag (Silver) T, N AgCl (Silver chloride) NaCl –

As (Arsenic) T As (elemental arsenic) KH₂PO₂ Se, Te

Au (Gold) T Au (elemental gold) SnCl₂ Ag, Hg, Pd, Pb, Ru,

Se, Te

Ca (Calcium) T CaC₂O₄ (Calcium H₂C₂O₄ Mg, Na, SO₄²⁻

oxalate)

Cl⁻ T, N AgCl (Silver chloride) AgNO₃ Br⁻, I⁻

(Chloride)

Se T Se (elemental SnCl₂ Te
(Selenium) selenium)
Te T Te (elemental NaH₂PO₂ Se
(Tellurium) tellurium)
Applications Continued
2. Organic Analysis:
o Turbidity in food and beverages, sugar solutions, fruit juices.
o Detects clarity and alcohol dilution in beverages.
3. Biochemical Analysis:
o Measures bacterial growth in nutrient media.
o Monitors amino acids, vitamins, glycogen, beta/gamma globulins in
blood/serum/plasma.
4. Air and Water Pollution:
o Used for continuous monitoring.
o Air: dust/smoke
o Water: turbidity levels

Special Techniques

5. Turbidimetric Titrations:
o Like photometric titrations.
o Absorbance vs. volume plotted → end point = sudden slope change.
o Example: Curve 1 (ideal), 2 & 3 (errors due to mixing, particle size).
o Accuracy: ±5%

➤ Used for:

o Fluoride with Ca²⁺

o Bromide with Ag⁺
o Sulfate with Ba²⁺
6. Phase Titrations
• Concept: Turbidimetry (measurement of cloudiness due to suspended particles) is used to
determine the endpoint in titrations involving immiscible liquids (e.g., oil and water).
• Application: When titrating a mixture of two liquids where only one forms a precipitate,
turbidity increases at the interface as the precipitate forms.
• Diagram (Fig. 15.9): It shows a volume vs. turbidity graph, indicating the "end point"
where turbidity sharply changes.
• Example: Water-pyridine mixture titrated with chloroform.
8. Atmospheric Pollution – Smoke and Fog Analysis
• Principle: Smoke and fog scatter light, so turbidimetry/nephelometry can measure air
pollution.
• Instrument: Nephelometer (Fig. 15.11) measures light scattering by suspended particles in
air.
• How it works:
o A lamp emits light through air.
o Particles scatter the light.
o A photocell (placed at right angles to avoid direct light) detects scattered light.
o The signal is used to quantify the particle concentration.
• Use:
o Portable and can be mounted on cars or planes.
o Draws air via a pump, passes through a trap (to remove moisture), then measures
turbidity.