180.1-1

METHOD 180.1

DETERMINATION OF TURBIDITY BY NEPHELOMETRY

Edited by James W. O'Dell

Inorganic Chemistry Branch

Chemistry Research Division

Revision 2.0

August 1993

ENVIRONMENTAL MONITORING SYSTEMS LABORATORY

OFFICE OF RESEARCH AND DEVELOPMENT

U.S. ENVIRONMENTAL PROTECTION AGENCY

CINCINNATI, OHIO 45268
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METHOD 180.1

DETERMINATION OF TURBIDITY BY NEPHELOMETRY

1.0 SCOPE AND APPLICATION

1.1 This method covers the determination of turbidity in drinking, ground, surface,

and saline waters, domestic and industrial wastes.

1.2 The applicable range is 0-40 nephelometric turbidity units (NTU). Higher

values may be obtained with dilution of the sample.

2.0 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 scattered light, the

higher the turbidity. Readings, in NTU's, are made in a nephelometer

designed according to specifications given in Sections 6.1 and 6.2. A primary

standard suspension is used to calibrate the instrument. A secondary standard

suspension is used as a daily calibration check and is monitored periodically

for deterioration using one of the primary standards.

2.1.1 Formazin polymer is used as a primary turbidity suspension for water

because it is more reproducible than other types of standards

previously used for turbidity analysis.

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.

3.0 DEFINITIONS

3.1 Calibration Blank (CB) -- A volume of reagent water fortified with the same

matrix as the calibration standards, but without the analytes, internal

standards, or surrogates analytes.

3.2 Instrument Performance Check Solution (IPC) -- A solution of one or more

method analytes, surrogates, internal standards, or other test substances used

to evaluate the performance of the instrument system with respect to a defined

set of criteria.

3.3 Laboratory Reagent Blank (LRB) -- An aliquot of reagent water or other blank

matrices that are treated exactly as a sample including exposure to all

glassware, equipment, solvents, reagents, internal standards, and surrogates

that are used with other samples. The LRB is used to determine if method
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analytes or other interferences are present in the laboratory environment, the

reagents, or the apparatus.

3.4 Linear Calibration Range (LCR) -- The concentration range over which the

instrument response is linear.

3.5 Material Safety Data Sheet (MSDS) -- Written information provided by

vendors concerning a chemical's toxicity, health hazards, physical properties,

fire, and reactivity data including storage, spill, and handling precautions.

3.6 Primary Calibration Standard (PCAL) -- A suspension prepared from the

primary dilution stock standard suspension. The PCAL suspensions are used

to calibrate the instrument response with respect to analyte concentration.

3.7 Quality Control Sample (QCS) -- A solution of the method analyte of known

concentrations that is used to fortify an aliquot of LRB matrix. The QCS is

obtained from a source external to the laboratory, and is used to check

laboratory performance.

3.8 Secondary Calibration Standards (SCAL) -- Commercially prepared, stabilized

sealed liquid or gel turbidity standards calibrated against properly prepared

and diluted formazin or styrene divinylbenzene polymers.

3.9 Stock Standard Suspension (SSS) -- A concentrated suspension containing the

analyte prepared in the laboratory using assayed reference materials or

purchased from a reputable commercial source. Stock standard suspension is

used to prepare calibration suspensions and other needed suspensions.

4.0 INTERFERENCES

4.1 The presence of floating debris and coarse sediments which settle out rapidly

will give low readings. Finely divided air bubbles can cause high readings.

4.2 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.

4.3 Light absorbing materials such as activated carbon in significant concentrations

can cause low readings.

5.0 SAFETY

5.1 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.

5.2 Each laboratory is responsible for maintaining a current awareness file of

OSHA regulations regarding the safe handling of the chemicals specified in
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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.

5.3 Hydrazine Sulfate (Section 7.2.1) 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. 

6.0 EQUIPMENT AND SUPPLIES

6.1 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.

6.2 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:

6.2.1 Light source: Tungsten lamp operated at a color temperature between

2200-3000°K.

6.2.2 Distance traversed by incident light and scattered light within the

sample tube: Total not to exceed 10 cm.

6.2.3 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.

6.3 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.

6.4 The sample tubes to be used with the available instrument must be of clear,

colorless glass or plastic. They should be kept scrupulously clean, both inside

and out, and discarded when they become scratched or etched. A light

coating of silicon oil may be used to mask minor imperfections in glass tubes. 

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.

6.5 Balance -- Analytical, capable of accurately weighing to the nearest 0.0001 g.
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6.6 Glassware -- Class A volumetric flasks and pipets as required.

7.0 REAGENTS AND STANDARDS

7.1 Reagent water, turbidity-free: Pass deionized distilled water through a 0.45μ

pore size membrane filter, if such filtered water shows a lower turbidity than

unfiltered distilled water.

7.2 Stock standard suspension (Formazin):

7.2.1 Dissolve 1.00 g hydrazine sulfate, (NH ) .H SO , (CASRN 10034-93-2) in 22 2 4

reagent water and dilute to 100 mL in a volumetric flask. CAUTION--

carcinogen.

7.2.2 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 7.2.1 and 7.2.2). 

Allow to stand 24 hours at 25 ±3°C, then dilute to the mark with

reagent water.

7.3 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.

7.3.1 A new stock standard suspension (Section 7.2) should be prepared each

month. Primary calibration standards (Section 7.3) should be prepared

daily by dilution of the stock standard suspension.

7.4 Formazin in commercially prepared primary concentrated stock standard

suspension (SSS) may be diluted and used as required. Dilute turbidity

standards should be prepared daily. 

7.5 AMCO-AEPA-1 Styrene Divinylbenzene polymer primary standards are

available for specific instruments and require no preparation or dilution prior

to use.

7.6 Secondary standards may be acceptable as a daily calibration check, but must

be monitored on a routine basis for deterioration and replaced as required.

8.0 SAMPLE COLLECTION, PRESERVATION AND STORAGE

8.1 Samples should be collected in plastic or glass bottles. All bottles must be

thoroughly cleaned and rinsed with turbidity free water. Volume collected

should be sufficient to insure a representative sample, allow for replicate

analysis (if required), and minimize waste disposal.

8.2 No chemical preservation is required. Cool sample to 4°C.
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8.3 Samples should be analyzed as soon as possible after collection. If storage is

required, samples maintained at 4°C may be held for up to 48 hours. 

9.0 QUALITY CONTROL

9.1 Each laboratory using this method is required to operate a formal quality

control (QC) program. The minimum requirements of this program consist of

an initial demonstration of laboratory capability and analysis of laboratory

reagent blanks and other solutions as a continuing check on performance. The

laboratory is required to maintain performance records that define the quality

of data generated.

 9.2 INITIAL DEMONSTRATION OF PERFORMANCE.

9.2.1 The initial demonstration of performance is used to characterize

instrument performance (determination of LCRs and analysis of QCS).

9.2.2 Linear Calibration Range (LCR) -- The LCR must be determined

initially and verified every six months or whenever a significant change

in instrument response is observed or expected. The initial

demonstration of linearity must use sufficient standards to insure that

the resulting curve is linear. The verification of linearity must use a

minimum of a blank and three standards. If any verification data

exceeds the initial values by ±10%, linearity must be reestablished. If

any portion of the range is shown to be nonlinear, sufficient standards

must be used to clearly define the nonlinear portion.

9.2.3 Quality Control Sample (QCS) -- When beginning the use of this

method, on a quarterly basis or as required to meet data-quality needs,

verify the calibration standards and acceptable instrument performance

with the preparation and analysis of a QCS. If the determined

concentrations are not within ±10% of the stated values, performance of

the determinative step of the method is unacceptable. The source of

the problem must be identified and corrected before continuing with

on-going analyses.

9.3 ASSESSING LABORATORY PERFORMANCE 

9.3.1 Laboratory Reagent Blank (LRB) -- The laboratory must analyze at least

one LRB with each batch of samples. Data produced are used to assess

contamination from the laboratory environment. 

9.3.2 Instrument Performance Check Solution (IPC) -- For all determinations,

the laboratory must analyze the IPC (a mid-range check standard) and

a calibration blank immediately following daily calibration, after every

tenth sample (or more frequently, if required) and at the end of the

sample run. Analysis of the IPC solution and calibration blank

immediately following calibration must verify that the instrument is
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within ±10% of calibration. Subsequent analyses of the IPC solution

must verify the calibration is still within ±10%. If the calibration cannot

be verified within the specified limits, reanalyze the IPC solution. If the

second analysis of the IPC solution confirms calibration to be outside

the limits, sample analysis must be discontinued, the cause determined

and/or in the case of drift the instrument recalibrated. All samples

following the last acceptable IPC solution must be reanalyzed. The

analysis data of the calibration blank and IPC solution must be kept on

file with the sample analyses data. NOTE: Secondary calibration

standards (SS) may also be used as the IPC.

9.3.3 Where additional reference materials such as Performance Evaluation

samples are available, they should be analyzed to provide additional

performance data. The analysis of reference samples is a valuable tool

for demonstrating the ability to perform the method acceptably. 

10.0 CALIBRATION AND STANDARDIZATION

10.1 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, calibration curves should be

prepared for each range of the instrument.

11.0 PROCEDURE

11.1 Turbidities less than 40 units: If possible, allow samples to come to room

temperature before analysis. Mix 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.

11.2 Turbidities exceeding 40 units: Dilute the sample with one or 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.

11.2.1 Some turbidimeters are equipped with several separate scales. The

higher scales are to be used only as indicators of required dilution

volumes to reduce readings to less than 40 NTU.
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Note: Comparative work performed in the Environmental Monitoring

Systems Laboratory - Cincinnati (EMSL-Cincinnati) indicates a

progressive error on sample turbidities in excess of 40 units.

12.0 DATA ANALYSIS AND CALCULATIONS

12.1 Multiply sample readings by appropriate dilution to obtain final reading.

12.2 Report results as follows:

 NTU Record to Nearest:

0.0 - 1.0 0.05

 1 - 10 0.1

 10 - 40 1

 40 - 100 5

 100 - 400 10

 400 - 1000 50

 >1000 100

13.0 METHOD PERFORMANCE

13.1 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.

13.2 The interlaboratory precision and accuracy data in Table 1 were developed

using a reagent water matrix. Values are in NTU.

14.0 POLLUTION PREVENTION

14.1 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.

14.2 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.

14.3 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
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Chemical Society's Department of Government Regulations and Science Policy,

1155 16th Street N.W., Washington D.C. 20036, (202)872-4477. 

15.0 WASTE MANAGEMENT

15.1 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 14.3.

16.0 REFERENCES

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).
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17.0 TABLES, DIAGRAMS, FLOWCHARTS AND VALIDATION DATA

TABLE 1. INTERLABORATORY PRECISION AND ACCURACY DATA

Number of True Standard

Values Value Mean Residual Deviation Residual

Reported (T) (X) for X (S) 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