Enve 208 Experiment 2.2
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Transcript of Enve 208 Experiment 2.2
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1. PURPOSE:
The purposes of this experiment are to explain the use of light scattering properties of
suspended matter in finding out their concentration and to show the principles of
nephelometric methods.
2. PROCEDURE:
Turbidity measurements:
1. For turbidimeter calibration prepare different dilutions of your 4000NTU formazin
standard (Table 2).
Table 2:
NTU mL of stock suspension diluted to 100
mL with turbidity- free water4000 100
1000 25
200 5
20 0.5
0 0
2. The sample size for all turbidity measurements should be 25(+or1) mL. Variations in
sample volume can affect the accuracy of the determinations.
3. When through with the measurements, best performance will be gained from the
photomultiplier tube by
Removing the sample cell from the cell holder Closing the sample compartment door, and Leaving the range switch in the 4000 position.
4. From the stock kaolinite solution, we take 10, 30, 80 and 70 mL (unknown sample) and
dilute to 100 mL volume using turbidity free water. Before any measurement, or any
solution preparation, we mix samples thoroughly.
5. We calibrate the turbidimeter with standard turbidity solutions.
6. We measure the turbidity of samples of all the kaolinite suspensions at the appropriate
turbidity range of the instrument calibrated.
7. We measure the turbidity of unknown sample before and after addition of
phenolphthalein.
3. THEORY:
A turbidity in water is caused by finely- or coarse-dispersed particles and undissolved
materials. The particle size of the present substances ranges between 1 to 300 m. The
turbidity can be measured by a decrease in the intensity of the radiation passed through the
liquid or by the intensity of the stray light. (URL 1)
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In water, turbidity is caused by suspended matter, such as clay, silt, finely divided organic
and inorganic matter, soluble colored organic compounds, and plankton and other
microscopic organisms. Turbidity is an expression of the optical property that causes light
to be scattered rather than transmitted in straight lines through the sample. Correlation of
turbidity with the weight concentration of suspended matter is difficult because the size,shape, and refractive index of the particulates also affect the light-scattering properties of
the suspension.
Interferences:
1. The presence of floating debris and coarse sediments which settle out rapidly will give
low readings. Finely divided air bubbles can cause high readings.
2. The presence of true color, that is the color of water which is due to dissolved
substances that absorb light, will cause turbidities to be low, although this effect is
generally not significant with drinking waters. Also, this interference is minimized by the
use of a ratiometric optical design which compares the transmitted and scattered signals.
3. Light absorbing materials such as activated carbon in significant concentrations can
cause low readings. (URL 2)
The major impacts of turbidity;
The main impact is merely esthetic: nobody likes the look of dirty water. But also, it is essential to eliminate the turbidity of water in order to effectively
disinfect it for drinking purposes. This adds some extra cost to the treatment of
surface water supplies.
The suspended particles also help the attachment of heavy metals and many othertoxic organic compounds and pesticides. (URL 3)Turbidity units are a measure of the cloudiness of water. If measured by a nephelometric
(deflected light) instrumental procedure, turbidity units are expressed in nephelometric
turbidity units (NTU) or simply TU. Those turbidity units obtained by visual methods are
expressed in Jackson turbidity units (JTU), which are a measure of the cloudiness of water;
they are used to indicate the clarity of water. There is no real connection between NTUs
and JTUs. The Jackson turbidimeter is a visual method and the nephelometer is an
instrumental method based on deflected light. (URL 4)
Historically, the standard method for determination of turbidity has been based on the
Jackson candle turbidimeter; however, the lowest turbidity value that can be measured
directly on this device is 25 Jackson Turbidity Units (JTU). Because turbidities of water
treated by conventional fluid-particle separation processes usually fall within the range of 0
to 1 unit, indirect secondary methods were developed to estimate turbidity. Electronic
nephelometers are the preferred instruments for turbidity measurement.
Most commercial turbidimeters designed for measuring low turbidities give comparatively
good indications of the intensity of light scattered in one particular direction,
predominantly at right angles to the incident light. Turbidimeters with scattered-light
detectors located at 90 to the incident beam are called nephelometers. Nephelometers arerelatively unaffected by small differences in design parameters and therefore are specified
as the standard instrument for measurement of low turbidities. Instruments of different
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make and model may vary in response. However, interinstrument variation may be
effectively negligible if good measurement techniques are used and the characteristics of
the particles in the measured suspensions are similar. Poor measurement technique can
have a greater effect on measurement error than small differences in instrument design.
Turbidimeters of nonstandard design, such as forward-scattering devices, may be moresensitive than nephelometers to the presence of larger particles. While it may not be
appropriate to compare their output with that of instruments of standard design, they still
may be useful for process monitoring.
An additional cause of discrepancies in turbidity analysis is the use of suspensions of
different types of particulate matter for instrument calibration. Like water samples,
prepared suspensions have different optical properties depending on the particle size
distributions, shapes, and refractive indices. A standard reference suspension having
reproducible light-scattering properties is specified for nephelometer calibration.
Its precision, sensitivity, and applicability over a wide turbidity range make thenephelometric method preferable to visual methods. Report nephelometric measurement
results as nephelometric turbidity units (NTU).
Nephelometric Method
a. Principle: This method is based on a comparison of the intensity of light scattered by the
sample under defined conditions with the intensity of light scattered by a standard
reference suspension under the same conditions. The higher the intensity of scattered light,
the higher the turbidity. Formazin polymer is used as the primary standard reference
suspension. The turbidity of a specified concentration of formazin suspension is defined as
4000 NTU.
b. Interference: Turbidity can be determined for any water sample that is free of debris and
rapidly settling coarse sediment. Dirty glassware and the presence of air bubbles give false
results. True color, i.e., water color due to dissolved substances that absorb light, causes
measured turbidities to be low. This effect usually is not significant in treated water.
4. DATA ANALYSIS AND CALCULATIONS:
Table 1. Data
Concentration of kaolinite
suspension(mg/L)Read-out Turbidity (NTU)
0 0.030
50 mg/L 22.5
150 mg/L 37.6
400 mg/L 149
500 mg/L 232
70 mL->(350mg/L) without
With indicator
132
138
500 mg/L sample*10 mL /100mL=50 mg/L 500 mg/L sample*30 mL /100mL=150 mg/L 500 mg/L sample*80 mL /100mL=400 mg/L 500 mg/L sample*100 mL /100mL=500 mg/L
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500 mg/L sample *70/100 mL=350 mg/L
1. Turbidity units (NTU) = A*(B+C)/C
Where, A=turbidity units found in diluted samplesB=volume of dilution water used in (mL)
C= sample volume in (mL) taken for dilution
For sample 10 mL: ((22.5-0.030)*(90mL+10 mL)) /10mL=224.7 NTU
For sample 30 mL: ((37.6-0.030)*(70mL+30 mL)) /30mL=125.23 NTU
For sample 80 mL: ((149-0.030)*(20mL+80 mL)) /80mL=186.21 NTU
For sample 70 mL: ((135-0.030)*(30mL+70 mL)) /70mL=192.8 NTU
For sample 100 mL: ((232-0.030)*(0mL+100 mL)) /100mL=231.97 NTU
2.
y=0.4192x
135=0.4192x x= 322 mg/L
Error = (350-322)/350*100= 8% error
5. DISCUSSION AND CONCLUSION:
1. A comparison of the dilute solution approximation and the condensed phase theory
yielded different predictions for the increase in turbidity as a function of an increase in the
size of the scattering units. Even greater differences are found when a change in theconcentration of the scattering units is considered. The dilute solution approximation
y = 0.4192x
R = 0.9672
0
50
100
150
200
250
0 200 400 600
T
u
rb
i
d
i
t
y
N
T
U
Concentration of kaolinite suspension mg/L
Concentration vs. NTU
Series1
Linear (Series1)
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would predict a proportional increase in turbidity with increasing concentration of the
scattering units. (URL 6)
Therefore we expect our graph will be linearly but our graph do not increase linearly
because we did not read-out suitable turbidity in nephelometre.
2. Turbidity is not color related, but relates rather to the loss of transparency due to the
effect of suspended particulate, colloidal material, or both. A lack of turbidity results in
clarity or clearness because it is, in part, the effect of these various suspended materials on
light passing through a liquid. (URL 7)
3. Excessive turbidity, or cloudiness, in drinking water is aesthetically unappealing, and
may also represent a health concern. Turbidity can provide food and shelter for pathogens.
If not removed, turbidity can promote regrowth of pathogens in the distribution system,
leading to waterborne disease outbreaks, which have caused significant cases of
gastroenteritis throughout the United States and the world. Although turbidity is not a
direct indicator of health risk, numerous studies show a strong relationship betweenremoval of turbidity and removal of protozoa. The particles of turbidity provide shelter
for microbes by reducing their exposure to attack by disinfectants. Microbial attachment to
particulate material or inert substances in water systems has been documented by several
investigators and has been considered to aid in microbe survival. Fortunately, traditional
water treatment processes have the ability to effectively remove turbidity when operated
properly. (URL 8)
Also, simply stated, turbidity is the measure of relative clarity of a liquid. Clarity is
important when producing drinking water for human consumption and in many
manufacturing uses. Once considered as a mostly aesthetic characteristic of drinking water,
significant evidence exists that controlling turbidity is a competent safeguard against
pathogens in drinking water. (URL 9)
4. The Disadvantages;
The presence of floating debris and coarse sediments will give high readings. Air bubbles will cause high results. Colored samples will cause low results.
Also, the sample tubes to be used with the available instrument must be clear, colorless
glass. They should be kept scrupulously clean, both inside and out, and discarded when
they become scratched or etched. They must not be handled at all where the light strikes
them, but should be provided with sufficient extra length, or with a protective case, so thatthey may be handled. Differences in physical design of turbidimeters will cause
differences in measured values for turbidity even though the same suspension is used for
calibration. To minimize such differences, design criteria should be observed.
The Advantages;
The turbidimeter shall consist of a nephelometer with light source for illuminating the
sample and one or more photo-electric detectors with a readout device to indicate the
intensity of light scattered at right angles to the path of the incident light. The turbidimeter
should be so designed that little stray light reaches the detector in the absence of turbidity
and should be free from significant drift after a short warm-up period. (URL 8)
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6. REFERENCES:
1. URL 1. Determination of Turbidity in Waste Water, retrieved 11 December 2012 from,
http://analitica.inycom.es/es-es/notas-de-aplicacion/Documents/UV_ALL_23_01_e.pdf
2. URL 2. Determination of Turbidity by Nephelometry, Thermo Scientific Orion Method
AQ4500, retrieved 11 December 2012 from,
http://www.thermo.com/eThermo/CMA/PDFs/Various/File_51982.pdf
3. URL 3.
http://www.lenntech.com/turbidity.htm#How%20do%20we%20measure%20turbidity?#ixz
z2Elu5a03w
4. URL 4. http://www.owp.csus.edu/glossary/turbidity-units-tu.php
5. URL 5. Standard Methods for the Examination of Water and Wastewater, retrieved 11
December 2012 from, http://www.umass.edu/tei/mwwp/acrobat/sm2130turbidity.PDF
6. URL 6. Effect of Change in Concentration upon Lens Turbidity as Predicted by Random
Fluctuation Theory, retrieved from 11 December 2012 from,
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1329010/pdf/biophysj00223-0035.pdf
7. URL 7. What is Turbidity and how is it measured? Retrieved 11 December 2012 from,
http://www.optek.com/Turbidity.aspTurbidity Overview
8. URL 8.Standard Operating Procedure for GLNPO Turbidity: Nephelometric Method.,
retrieved 12 December 2012from, http://www.epa.gov/glnpo/lmmb/methods/turbid.pdf
9.URL 9. http://www.epa.gov/ogwdw/mdbp/pdf/turbidity/chap_07.pdf