Analytical Method for Turbidity Measurement
Standard Methods 2130 A and B
June 2003
Draft

-------
Standard Methods 2130: Turbidity
Reprinted by Permission from the 20th Edition
2130 A. Introduction
1.	Sources and Significance
Clarity of water is important in producing products destined for human consumption and in many
manufacturing operations. Beverage producers, food processors, and potable water treatment plants drawing
from a surface water source commonly rely on fluid-particle separation processes such as sedimentation and
filtration to increase clarity and insure an acceptable product. The clarity of a natural body of water is an
important determinant of its condition and productivity.
Turbidity in water is caused by suspended and colloidal matter such as clay, silt, finely divided organic and
inorganic matter, and plankton and other microscopic organisms. Turbidity is an expression of the optical
property that causes light to be scattered and absorbed rather than transmitted with no change in direction or
flux level through the sample. Correlation of turbidity with the weight or particle number concentration of
suspended matter is difficult because the size, shape, and refractive index of the particles affect the light-
scattering properties of the suspension. When present in significant concentrations, particles consisting of
light-absorbing materials such as activated carbon cause a negative interference. In low concentrations these
particles tend to have a positive influence because they contribute to turbidity. The presence of dissolved,
color-causing substances that absorb light may cause a negative interference. Some commercial instruments
may have the capability of either correcting for a slight color interference or optically blanking out the color
effect.
2.	Selection of Method
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 are relatively unaffected by small differences in design parameters and
therefore are specified as the standard instrument for measurement of low turbidities. Instruments of
different 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 non-standard design, such as forward-
scattering devices, may be more sensitive 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.
Draft
1
June 2003

-------
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 the nephelometric method
preferable to visual methods. Report nephelometric measurement results as nephelometric turbidity units
(NTU).
3. Storage of Sample
Determine turbidity as soon as possible after the sample is taken. Gently agitate all samples before
examination to ensure a representative measurement. Sample preservation is not practical; begin analysis
promptly. Refrigerate or cool to 4°C, to minimize microbiological decomposition of solids, if storage is
required. For best results, measure turbidity immediately without altering the original sample conditions
such as temperature or pH.
2130 B. Nephelometric Method
1.	General Discussion
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.
2.	Apparatus
a. Laboratory or process nephelometer consisting of a light source for illuminating the sample
and one or more photoelectric detectors with a readout device to indicate intensity of light
scattered at 90° to the path of incident light. Use an instrument designed to minimize stray
light reaching the detector in the absence of turbidity and to be free from significant drift
after a short warmup period. The sensitivity of the instrument should permit detecting
turbidity differences of 0.02 NTU or less in the lowest range in waters having a turbidity of
less than 1 NTU. Several ranges may be necessary to obtain both adequate coverage and
sufficient sensitivity for low turbidities. Differences in instrument design will cause
differences in measured values for turbidity even though the same suspension is used for
calibration. To minimize such differences, observe the following design criteria:
Draft
2
June 2003

-------
1)	Light source - Tungsten-filament lamp operated at a color temperature between
2200 and 3000°K.
2)	Distance traversed by incident light and scattered light within the sample tube -
Total not to exceed 10 cm.
3)	Angle of light acceptance by 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 and 600 nm.
b. Sample cells: Use sample cells or tubes of clear, colorless glass or plastic. Keep cells
scrupulously clean, both inside and out, and discard if scratched or etched. Never handle
them where the instrument's light beam will strike them. Use tubes with sufficient extra
length, or with a protective case, so that they may be handled properly. Fill cells with
samples and standards that have been agitated thoroughly and allow sufficient time for
bubbles to escape.
Clean sample cells by thorough washing with laboratory soap inside and out followed by
multiple rinses with distilled or deionized water; let cells air-dry. Handle sample cells only
by the top to avoid dirt and fingerprints within the light path.
Cells may be coated on the outside with a thin layer of silicone oil to mask minor
imperfections and scratches that may contribute to stray light. Use silicone oil with the same
refractive index as glass. Avoid excess oil because it may attract dirt and contaminate the
sample compartment of the instrument. Using a soft, lint-free cloth, spread the oil uniformly
and wipe off excess. The cell should appear to be nearly dry with little or no visible oil.
Because small differences between sample cells significantly impact measurement, use
either matched pairs of cells or the same cell for both standardization and sample
measurement.
3. Reagents
a.	Dilution water. High-purity water will cause some light scattering, which is detected by
nephelometers as turbidity. To obtain low-turbidity water for dilutions, nominal value 0.02
NTU, pass laboratory reagent-grade water through a filter with pore size sufficiently small
to remove essentially all particles larger than 0.1 (im *thc usual membrane filter used for
bacteriological examinations is not satisfactory. Rinse collecting flask at least twice with
filtered water and discard the next 200 mL.
Some commercial bottled demineralized waters have a low turbidity. These may be used
when filtration is impractical or a good grade of water is not available to filter in the
laboratory. Check turbidity of bottled water to make sure it is lower than the level that can
be achieved in the laboratory.
b.	Stock primary standard formazin suspension:
1) Solution I - Dissolve 1.000 g hydrazine sulfate, (NH2)2*H2S04 in distilled water and
dilute to 100 mL in a volumetric flask. CAUTION: Hydrazine sulfate is a
carcinogen; avoid inhalation, ingestion, and skin contact. Formazin suspensions
can contain residual hydrazine sulfate.
Draft
3
June 2003

-------
2)	Solution II - Dissolve 10.00 g hexamethylenetetramine, (CH2 )6 N4, in distilled
water and dilute to 100 mL in a volumetric flask.
3)	In a flask, mix 5.0 mL Solution I and 5.0 mL Solution II. Let stand for 24 h at 25
±3°C. This results in a 4000-NTU suspension. Transfer stock suspension to an
amber glass or other UV-light-blocking bottle for storage. Make dilutions from this
stock suspension. The stock suspension is stable for up to 1 year when properly
stored.
c.	Dilute turbidity suspensions: Dilute 4000 NTU primary standard suspension with high-
quality dilution water. Prepare immediately before use and discard after use.
d.	Secondary standards: Secondary standards are standards that the manufacturer (or an
independent testing organization) has certified will give instrument calibration results
equivalent (within certain limits) to the results obtained when the instrument is calibrated
with the primary standard, i.e., user-prepared formazin. Various secondary standards are
available including: commercial stock suspensions of 4000 NTU formazin, commercial
suspensions of microspheres of styrene-divinylbenzene copolymer,* and items supplied by
instrument manufacturers, such as sealed sample cells filled with latex suspension or with
metal oxide particles in a polymer gel. The U.S. Environmental Protection Agency1
designates user-prepared formazin, commercial stock formazin suspensions, and
commercial styrene-divinylbenzene suspensions as "primary standards," and reserves the
term "secondary standard" for the sealed standards mentioned above.
Secondary standards made with suspensions of microspheres of styrene-divinylbenzene
copolymer typically are as stable as concentrated formazin and are much more stable than
diluted formazin. These suspensions can be instrument-specific; therefore, use only
suspensions formulated for the type of nephelometer being used. Secondary standards
provided by the instrument manufacturer (sometimes called "permanent" standards) may be
necessary to standardize some instruments before each reading and in other instruments
only as a calibration check to determine when calibration with the primary standard is
necessary.
All secondary standards, even so-called "permanent" standards, change with time. Replace
them when their age exceeds the shelf life. Deterioration can be detected by measuring the
turbidity of the standard after calibrating the instrument with a fresh formazin or
microsphere suspension. If there is any doubt about the integrity or turbidity value of any
secondary standard, check instrument calibration first with another secondary standard and
then, if necessary, with user-prepared formazin. Most secondary standards have been
carefully prepared by their manufacturer and should, if properly used, give good agreement
with formazin. Prepare formazin primary standard only as a last resort. Proper application of
secondary standards is specific for each make and model of nephelometer. Not all secondary
standards have to be discarded when comparison with a primary standard shows that their
turbidity value has changed. In some cases, the secondary standard should be simply
relabeled with the new turbidity value. Always follow the manufacturer's directions.
Draft
4
June 2003

-------
4. Procedure
a.	General measurement techniques: Proper measurement techniques are important in
minimizing the effects of instrument variables as well as stray light and air bubbles.
Regardless of the instrument used, the measurement will be more accurate, precise, and
repeatable if close attention is paid to proper measurement techniques.
Measure turbidity immediately to prevent temperature changes and particle flocculation and
sedimentation from changing sample characteristics. If flocculation is apparent, break up
aggregates by agitation. Avoid dilution whenever possible. Particles suspended in the
original sample may dissolve or otherwise change characteristics when the temperature
changes or when the sample is diluted.
Remove air or other entrained gases in the sample before measurement. Preferably degas
even if no bubbles are visible. Degas by applying a partial vacuum, adding a nonfoaming-
type surfactant, using an ultrasonic bath, or applying heat. In some cases, two or more of
these techniques may be combined for more effective bubble removal. For example, it may
be necessary to combine addition of a surfactant with use of an ultrasonic bath for some
severe conditions. Any of these techniques, if misapplied, can alter sample turbidity; use
with care. If degassing cannot be applied, bubble formation will be minimized if the
samples are maintained at the temperature and pressure of the water before sampling.
Do not remove air bubbles by letting sample stand for a period of time because during
standing, turbidity-causing particulates may settle and sample temperature may change.
Both of these conditions alter sample turbidity, resulting in a nonrepresentative
measurement.
Condensation may occur on the outside surface of a sample cell when a cold sample is being
measured in a warm, humid environment. This interferes with turbidity measurement.
Remove all moisture from the outside of the sample cell before placing the cell in the
instrument. If fogging recurs, let sample warm slightly by letting it stand at room
temperature or by partially immersing it in a warm water bath for a short time. Make sure
samples are again well mixed.
b.	Nephelometer calibration: Follow the manufacturer's operating instructions. Run at least
one standard in each instrument range to be used. Make certain the nephelometer gives
stable readings in all sensitivity ranges used. Follow techniques outlined in |s 2b and 4a for
care and handling of sample cells, degassing, and dealing with condensation.
c.	Measurement of turbidity. Gently agitate sample. Wait until air bubbles disappear and pour
sample into cell. When possible, pour well-mixed sample into cell and immerse it in an
ultrasonic bath for 1 to 2 s or apply vacuum degassing, causing complete bubble release.
Read turbidity directly from instrument display.
d.	Calibration of continuous turbidity monitors: Calibrate continuous turbidity monitors for
low turbidities by determining turbidity of the water flowing out of them, using a
laboratory-model nephelometer, or calibrate the instruments according to manufacturer's
instructions with formazin primary standard or appropriate secondary standard.
Draft
5
June 2003

-------
5. Interpretation of Results
Report turbidity readings as follows:
Turbidity Range	Report to the
NTU	Nearest NTU
0-1.0	0.05
1-10	0.1
10-40	1
40-100	5
100-400	10
400-1000	50
>1000	100
When comparing water treatment efficiencies, do not estimate turbidity more closely than specified above.
Uncertainties and discrepancies in turbidity measurements make it unlikely that results can be duplicated to
greater precision than specified.
6. Reference
1. U.S. Environmental Protection Agency. 1993. Methods for Determination of Inorganic
Substances in Environmental Samples.EPA-600/R/93/100 - Draft. Environmental Monitoring
Systems Lab., Cincinnati, Ohio.
7. Bibliography
Hach, C.C., R.D. Vanous[nm & J.M. [smHeer[nm. 1985. Understanding Turbidity Measurement. Hach Co.,
Technical Information Ser., Booklet 11, Loveland, Colo.
Katz, E.L. 1986. The stability of turbidity in raw water and its relationship to chlorine demand. J. Amer.
Waterworks Assoc.78:72.
McCoy, W.F. & B.H. Olson. 1986. Relationship among turbidity, particle counts and bacteriological quality
within water distribution lines. Water Res.20:1023.
Bucklin, K.E., G.A. McFeters & A. Amirtharajah. 1991. Penetration of coliform through municipal drinking
water filters Water Res. 25:1013.
Hernandez, E., R.A. Baker & P.C. Crandal. 1991. Model for evaluating turbidity in cloudy beverages. J.
Food Sci. 56:747.
Hart, V.S., C.E. Johnson & R.D. Letterman. 1992. An analysis of low-level turbidity measurements. J.
Amer. Waterworks Assoc.,[cf 1 ] 84(12):40.
LeChevallier, M.W. & W.D. Norton. 1992. Examining relationship between particle counts and Giardia,
Cryptosporidium, and turbidity. J. Amer. Waterworks Assoc. 84(12):54.
Draft
June 2003

-------