Analytical Method for Turbidity Measurement

Method 180.1

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Analytical Method for Turbidity Measurement
Method 180.1

1. Scope and Application

1.1 This method is applicable to drinking water samples in the range of turbidity from 0 to 40
nephelometric units (NTU). Higher values may be obtained with dilution of the sample. A
method detection limit of 0.100 NTUs is recommended for this procedure.

NOTE: NTUs are considered comparable to the previously reported Formazin Turbidity Units (FTU).

1.2 This method covers the determination of turbidity in drinking, ground, surface, and saline waters,
domestic and industrial wastes.

2.	Summary of Method

2.1 The method is based upon 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. The
higher the intensity of the scattered light, the higher the turbidity. Readings, in NTUs, are made
in a nephelometer designed according to specifications outlined in "Apparatus." A standard
suspension of Formazin, prepared under closely defined conditions, is used to calibrate the
instrument.

2.1.1	Formazin polymer is used as the turbidity reference suspension for water because it is
more reproducible than other types of standards previously used for turbidity standards.

2.1.2	A commercially available polymer primary standard is also approved for use for the
National Interim Primary Drinking Water Regulations. This standard is identified as
AMCO-AEPA-1, available from Advanced Polymer Systems.

3.	Sample Handling and Preservation

3.1 Collect each sample in a soft or hard plastic, or soft or hard glass container. Immediately
refrigerate or ice the sample to 4°C and analyze within 48 hours.

4.	Conditions Affecting Turbidity Reading

4.1	The presence of floating debris and coarse sediments, which settle out rapidly, will give low
readings. Finely divided air bubbles will affect the results in a positive manner.

4.2	The presence of color, that is the color of water which is due to dissolved substances which
absorb light, will cause turbidities to be low, although this effect is generally not significant.

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4.3 Light absorbing materials such as activated carbon in significant concentrations can cause low
readings.

5.	Apparatus

5.1	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 designed so
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.

5.2	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,
the following design criteria should be observed:

5.2.1	Light source: Tungsten lamp operated at a color temperature between 2200-3000°K.

5.2.2	Distance traversed by incident light and scattered light within the sample tube: Total not
to exceed 10 cm.

5.2.3	Detector: Centered at 90° to the incident light path and not to exceed ± 30° from 90°.
The detector, and filter system if used, shall have a spectral peak response between 400
nm and 600 nm.

5.3	The sensitivity of the instrument should permit detection of a turbidity difference of 0.02 NTU or
less in waters having turbidities less than 1 unit. The instrument should measure from 0-40 units
turbidity. Several ranges may be necessary to obtain both adequate coverage and sufficient
sensitivity for low turbidities.

5.4	The sample tubes to be used with the available instrument must be of clear, colorless optical
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 that they may be
handled. Tubes should be checked, indexed and read at the orientation that produces the lowest
background blank value.

6.	Reagents

6.1	Reagent water, turbidity-free: Pass deionized distilled water through a 0.45|J, pore size membrane
filter, if such filtered water shows a lower turbidity than unfiltered distilled water.

6.2	Stock standard suspension (Formazin):

6.2.1 Dissolve 1.00 g hydrazine sulfate, (NH2)2.H2S04, (CASRN 10034-93-2) in reagent water
and dilute to 100 mL in a volumetric flask. CAUTION ~ carcinogen.

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6.2.2 Dissolve 10.00 g hexamethylenetetramine (CASRN 100-97-0) in reagent water and dilute
to 100 mL in a volumetric flask. In a 100 mL volumetric flask, mix 5.0 mL of each
solution (Sections 6.2.1 and 6.2.2). Allow to stand 24 hours at 25 ±3°C, then dilute to the
mark with reagent water.

6.3	Primary calibration standards: Mix and dilute 10.00 mL of stock standard suspension (Section
7.2) to 100 mL with reagent water. The turbidity of this suspension is defined as 40 NTU. For
other values, mix and dilute portions of this suspension as required.

6.3.1 A new stock standard suspension (Section 6.2) should be prepared each month. Primary
calibration standards (Section 6.3) should be prepared daily by dilution of the stock
standard suspension.

6.4	Formazin in commercially prepared primary concentrated stock standard suspension (SSS) may
be diluted and used as required. Dilute turbidity standards should be prepared daily.

6.5	AMCO-AEPA-1 Styrene Divinylbenzene polymer primary standards are available for specific
instruments and require no preparation or dilution prior to use.

6.6	Secondary standards may be acceptable as a daily calibration check, but must be monitored on a
routine basis for deterioration and replaced as required.

7. Procedure

7.1	Turbidimeter calibration: The manufacturer's operating instructions should be followed. Measure
standards on the turbidimeter covering the range of interest. If the instrument is already
calibrated in standard turbidity units, this procedure will check the accuracy of the calibration
scales. At least one standard should be run in each instrument range to be used. Some
instruments permit adjustments of sensitivity so that scale values will correspond to turbidities.
Solid standards, such as those made of lucite blocks, should never be used due to potential
calibration changes caused by surface scratches. If a pre-calibrated scale is not supplied, then
calibration curves should be prepared for each range of the instrument.

7.2	Turbidities less than 40 units: If possible, allow samples to come to room temperature before
analysis. Shake the sample to thoroughly disperse the solids. Wait until air bubbles disappear.
Then pour the sample into the turbidimeter tube. Read the turbidity directly from the instrument
scale or from the appropriate calibration curve.

7.3	Turbidities exceeding 40 units: Dilute the sample with one of more volumes of turbidity-free
water until the turbidity falls below 40 units. The turbidity of the original sample is then
computed from the turbidity of the diluted sample and the dilution factor. For example, if 5
volumes of turbidity-free water were added to 1 volume of sample, and the diluted sample
showed a turbidity of 30 units, then the turbidity of the original sample was 180 units.

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8. Calculations

8.1	Nephelometric turbidity units (NTU)

= A x (B + C) where: A = NTU found in diluted sample

C	B = volume of dilution water, in mL

C = sample volume taken for dilution, in mL

8.2	Report results as follows:

NTU

Record to Nearest

o

1

o
o

0.05

1 - 10

0.1

10-40

1

40-100

5

100-400

10

400- 1000

50

> 1000

100

9.	Precision and Accuracy

9.1	In a single laboratory (EMSL-Cincinnati), using surface water samples at levels of 26, 41, 75, and
180 NTU, the standard deviations were ± 0.60, ± 0.94, ± 1.2, and ± 4.7 units, respectively.

9.2	The interlaboratory precision and accuracy data in Table 1 were developed using a reagent water
matrix. Values are in NTU.

10.	Safety

10.1	The toxicity or carcinogenicity of each reagent used in this method has not been fully established.
Each chemical should be regarded as a potential health hazard and exposure should be as low as
reasonably achievable.

10.2	Each laboratory is responsible for maintaining a current awareness file of OSHA regulations
regarding the safe handling of the chemicals specified in this method. A reference file of Material
Safety Data Sheets (MSDS) should be made available to all personnel involved in the chemical
analysis. The preparation of a formal safety plan is also advisable.

10.3	Hydrazine Sulfate (Section 6.2.1) is a carcinogen. It is highly toxic and may be fatal if inhaled,
swallowed, or absorbed through the skin. Formazin can contain residual hydrazine sulfate.

Proper protection should be employed.

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11.	Quality Assurance

11.1 Each laboratory using this method in regulated environmental monitoring is required to operate a
formal quality assurance/control program. The minimum initial requirements of this program
consist of the demonstration of the laboratory's capability with this method. On a continuing
basis, the laboratory should check its performance (accuracy and precision) by analyzing reagent
blanks and check standards, fortified blanks, and/or fortified samples, preferably at a minimum
frequency of 10% of the total samples analyzed by the method. The laboratory should maintain
the performance records that define the quality of the data generated with the method.

12.	Pollution Prevention

12.1	Pollution prevention encompasses any technique that reduces or eliminates the quantity or
toxicity of waste at the point of generation. Numerous opportunities for pollution prevention
exist in laboratory operation. The EPA has established a preferred hierarchy of environmental
management techniques that places pollution prevention as the management option of first
choice. Whenever feasible, laboratory personnel should use pollution prevention techniques to
address their waste generation. When wastes cannot be feasibly reduced at the source, the
Agency recommends recycling as the next best option.

12.2	The quantity of chemicals purchased should be based on expected usage during its shelf life and
disposal cost of unused material. Actual reagent preparation volumes should reflect anticipated
usage and reagent stability.

12.3	For information about pollution prevention that may be applicable to laboratories and research
institutions, consult "Less is Better: Laboratory Chemical Management for Waste Reduction,"
available from the American Chemical Society's Department of Government Regulations and
Science Policy, 1155 16th Street N.W., Washington D.C. 20036, (202)872-4477.

13.	Waste Management

13.1 The U.S. Environmental Protection Agency requires that laboratory waste management practices
be conducted consistent with all applicable rules and regulations. Excess reagents, samples and
method process wastes should be characterized and disposed of in an acceptable manner. The
Agency urges laboratories to protect the air, water and land by minimizing and controlling all
releases from hoods, and bench operations, complying with the letter and spirit of any waste
discharge permit and regulations, and by complying with all solid and hazardous waste
regulations, particularly the hazardous waste identification rules and land disposal restrictions.
For further information on waste management consult the "Waste Management Manual for
Laboratory Personnel," available from the American Chemical Society at the address listed in
Section 12.3.

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Bibliography

1.	Annual Book of ASTM Standards, Volume 11.01 Water (1), Standard D1889-88A, p. 359, (1993).

2.	Standard Methods for the Examination of Water and Wastewater, 18th Edition, pp. 2-9, Method
2130B, (1992).

3.	Standard Methods for the Certification of Laboratories Analyzing Drinking Water: Criteria and
Procedures, Quality Assurance, EPA/570/9-90/008, April, 1990.

14.0 Tables, Diagrams, Flowcharts and Validation Data

TABLE 1. INTERLABORATORY PRECISION AND ACCURACY DATA

Number of
Values
Reported

True
Value
(T)

Mean
IX)

Residual
for X

Standard
Deviation
(S)

Residual
for S

373

0.450

0.4864

0.0027

0.1071

-0.0078

374

0.600

0.6026

-0.0244

0.1048

-0.0211

289

0.65

0.6931

0.0183

0.1301

0.0005

482

0.910

0.9244

0.0013

0.2512

0.1024

484

0.910

0.9919

0.0688

0.1486

-0.0002

489

1.00

0.9405

-0.0686

0.1318

-0.0236

640

1.36

1.3456

-0.0074

0.1894

0.0075

487

3.40

3.2616

-0.0401

0.3219

-0.0103

288

4.8

4.5684

-0.0706

0.3776

-0.0577

714

5.60

5.6984

0.2952

0.4411

-0.0531

641

5.95

5.6026

-0.1350

0.4122

-0.1078

REGRESSIONS: X = 0.955T + 0.54, S = 0.074T + 0.082

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