34
3.1.2 Turbidity
It is a significant parameter in determining the
opaqueness of water that affects photosynthetic
processes and fish life. It mainly depends on particulate matter present in the water.
Suspension of particles in water interfering
with passage of light is called ‘turbidity’. The
suspended particles may be clay, silt, finely
divided organic and inorganic matter, plankton
and other microscopic organisms. Turbid water
should be unfit for drinking and is undesirable for
food products, beverages, etc. Turbidity is said to
be a measure of performance of water treatment
plants.
The following methods are generally used for
determining the turbidity of a water sample:
(a) By Jackson’s Candle turbiditimeter (JCT)
(b) By electrical/electronic turbiditimeters commonly called Nephelometers
The JCT is generally used for high-turbid
waters.
3.1.2.1 Jackson’s Candle Turbiditimeter
Principle
It is based on the transmittance of light from a
flame of a ‘standard candle’ (made of bee’s wax
and spermaceti designed to burn at 114–126
grains per hour) through the sample column of
certain path length, such that the flame becomes
indistinguishable against background illumination. Turbidity is inversely proportional to the
path length. In brief, it is based on comparison of
the intensity of light scattered by the sample and
a standard reference under comparable conditions. The higher the intensity of scattered light,
the higher is the turbidity.
The lowest turbidity which could be measured
with a JCT is 25 units (JTU). As such, indirect
secondary methods are required for measuring
turbidities in the range of 0–5 units. However, the
results obtained with different types of secondary
instruments do not match with one another
because of fundamental differences in the optical
systems; even though, the instruments are all precalibrated against JCT.
This method is not in much in use today
mainly because of difficulties in getting the ‘standard candle’.
3.1.2.2 Nephelometric Method
Principle
Formazin polymer is the turbidity standard preference suspension for water. It is easy to prepare
and is more reproducible in its light-scattering
properties. A given concentration of formazin
suspension having 40 NTU has approximately a
turbidity of 40 JTU. Therefore, turbidity based
on formazin will approximate units derived from
JCT, but will not be identical.
Materials
Turbiditimeter: It consists of a light source for
illumination of the sample and one or more photoelectric detectors with readout device to indicate
the intensity of light scattered at 90° to incident
light. The sensitivity of the instrument permits
detection of turbidity difference of 0.02 NTU or
less in water having turbidity <1.0 NTU. The
instrument measures generally from 0 to 40 NTU.
Various ranges are necessary to obtain both adequate coverage and sensitivity. However, meters
having wider ranges are also available today.
Sample Tubes: Clear colourless glass, scrupulously cleaned both inside and outside without
scratch. The tubes be sufficiently long, so that
they are not required to be touched where light
strikes.
Reagents
(a) Turbidity-Free Water: It is obtained by passing distilled water (DW) through membrane
filter having pore size <100 μm. If filtration
does not reduce turbidity, DW itself may be
used.
(b) Stock Turbidity Suspension:
(i) Solution I: It is prepared by dissolving
1.0 g of hydrazine sulphate in DW and
diluted to 100 ml in a volumetric flask.
(ii) Solution II: It is prepared by dissolving
10.0 g of hexamethylenetetramine in DW
and diluted to 100 ml in volumetric flask.
3 Lentic Water (Physico-chemical Characteristics of Water)
3.1.2 Turbidity
It is a significant parameter in determining the
opaqueness of water that affects photosynthetic
processes and fish life. It mainly depends on particulate matter present in the water.
Suspension of particles in water interfering
with passage of light is called ‘turbidity’. The
suspended particles may be clay, silt, finely
divided organic and inorganic matter, plankton
and other microscopic organisms. Turbid water
should be unfit for drinking and is undesirable for
food products, beverages, etc. Turbidity is said to
be a measure of performance of water treatment
plants.
The following methods are generally used for
determining the turbidity of a water sample:
(a) By Jackson’s Candle turbiditimeter (JCT)
(b) By electrical/electronic turbiditimeters commonly called Nephelometers
The JCT is generally used for high-turbid
waters.
3.1.2.1 Jackson’s Candle Turbiditimeter
Principle
It is based on the transmittance of light from a
flame of a ‘standard candle’ (made of bee’s wax
and spermaceti designed to burn at 114–126
grains per hour) through the sample column of
certain path length, such that the flame becomes
indistinguishable against background illumination. Turbidity is inversely proportional to the
path length. In brief, it is based on comparison of
the intensity of light scattered by the sample and
a standard reference under comparable conditions. The higher the intensity of scattered light,
the higher is the turbidity.
The lowest turbidity which could be measured
with a JCT is 25 units (JTU). As such, indirect
secondary methods are required for measuring
turbidities in the range of 0–5 units. However, the
results obtained with different types of secondary
instruments do not match with one another
because of fundamental differences in the optical
systems; even though, the instruments are all precalibrated against JCT.
This method is not in much in use today
mainly because of difficulties in getting the ‘standard candle’.
3.1.2.2 Nephelometric Method
Principle
Formazin polymer is the turbidity standard preference suspension for water. It is easy to prepare
and is more reproducible in its light-scattering
properties. A given concentration of formazin
suspension having 40 NTU has approximately a
turbidity of 40 JTU. Therefore, turbidity based
on formazin will approximate units derived from
JCT, but will not be identical.
Materials
Turbiditimeter: It consists of a light source for
illumination of the sample and one or more photoelectric detectors with readout device to indicate
the intensity of light scattered at 90° to incident
light. The sensitivity of the instrument permits
detection of turbidity difference of 0.02 NTU or
less in water having turbidity <1.0 NTU. The
instrument measures generally from 0 to 40 NTU.
Various ranges are necessary to obtain both adequate coverage and sensitivity. However, meters
having wider ranges are also available today.
Sample Tubes: Clear colourless glass, scrupulously cleaned both inside and outside without
scratch. The tubes be sufficiently long, so that
they are not required to be touched where light
strikes.
Reagents
(a) Turbidity-Free Water: It is obtained by passing distilled water (DW) through membrane
filter having pore size <100 μm. If filtration
does not reduce turbidity, DW itself may be
used.
(b) Stock Turbidity Suspension:
(i) Solution I: It is prepared by dissolving
1.0 g of hydrazine sulphate in DW and
diluted to 100 ml in a volumetric flask.
(ii) Solution II: It is prepared by dissolving
10.0 g of hexamethylenetetramine in DW
and diluted to 100 ml in volumetric flask.
3 Lentic Water (Physico-chemical Characteristics of Water)