## 2010 INTRODUCTION
This part deals primarily with measurement of the physicalproperties of a sample, as distinguished from the concentrations of chemical or biological components. Many of thedeterminations included here, such as color, electrical conductivity, and turbidity, fit this category unequivocally. However, physical properties cannot be divorced entirely from chemical composition, and some of the techniques of this part measure aggregate properties resulting from the presence of anumber of constituents. Others, for example, calcium carbonate saturation, are related to, or depend on, chemical tests. Also included here are tests for appearance, odor, and taste, which have been classified traditionally among physical properties, although the point could be argued. Finally, Section 2710, Tests on Sludges, includes certain biochemical tests. However, for convenience they are grouped with the other tests used for sludge.

With these minor exceptions, the contents of this part have been kept reasonably faithful to its name. Most of the methods included are either inherently or at least traditionally physical, asdistinguished from the explicitly chemical, radiological, biological, or bacteriological methods of other parts.



## 2020 QUALITY ASSURANCE/QUALITY CONTROL
Part 2000 contains a variety of analytical methods, many of which are not amenable to standard quality-control techniques. General information on quality control is provided in Part 1000and specific quality-control techniques are outlined in the individual methods. The following general guidelines may be applied to many of the methods in this part:

Evaluate analyst performance for each method. Determine competence by analyses of samples containing known concentrations. Calibrate instruments and ensure that instrument measurements do not drift.

Assess the precision of analytical procedures by analyzing at least 10% of samples in duplicate. Analyze a minimum of one duplicate with each set of samples.

Determine bias of an analytical procedure in each sample batch by analysis of blanks, known additions with a frequency of at least 5% of samples, and, if possible, an externally provided standard.



## 2110 APPEARANCE
To record the general physical appearance of a sample, use any terms that briefly describe its visible characteristics. These terms may state the presence of color, turbidity, suspended solids, organisms and their immature forms, sediment, floating material, and similar particulate matter detectable by the unaided eye. Use numerical values when they are available, as for color, turbidity, and suspended solids.



## 2120 COLOR*

2120 A. Introduction
Color in surface and ground waters results primarily from the presence of natural organic matter, particularly aquatic humic matter. Humic matter consists of humic and fulvic acids; both cause a yellow-brown color. Humic acids give a more intense color, and the presence of iron intensifies the color through the formation of soluble ferric humates. Suspended particles, especially colloidal-size particles such as clays, algae, iron and manganese oxides, give waters an appearance of color; they should be removed before measurement. Industrial wastewaterscan contain lignins, tannins, dyes, and other organic and inorganic chemicals that cause color. Humic materials and the colorcaused by these materials are removed from potable water supplies for aesthetic reasons and for health reasons because they are precursors in the formation of disinfection by-products. * Approved by Standard Methods Committee, 2001. Joint Task Group: James K. Edzwald (chair), Penny J. Bristol, Brian A. Dempsey, Darren A. Lytle, David J. Pernitsky, Mike J. Sadar, Jeff Throckmorton.
2-1Color also is removed to make water suitable for industrial applications. Colored industrial wastewaters may require color removal before discharge into watercourses.
1. Definitions

The term “ color” is used here to mean true color, that is, the color of water from which turbidity has been removed. Colloidal and larger suspended particles scatter light interfering with the determination of true color measurements in Method B and in the spectrophotometric procedures of Methods C through F. The term “ apparent color” includes not only color due to substances in solution, but also that due to suspended matter. Apparent color is determined on the original sample without fi ltration. In some waters and wastewaters, apparent color is contributed principally by colloidal or suspended material.
2. Selection of Method

Methods B and C are applicable to measurement of color caused primarily by natural organic matter. The measurementsapply to all surface and ground waters; wastewaters, both domestic and industrial; and especially potable waters. While all methods (B through F) are suitable for true color measurements, for apparent color measurements use only Method B; in suchcases, determine both true color and apparent color. For comparison among laboratories, calibrate Method B with Method C. Methods D through F allow color measurement for any dissolved chemical that gives the appearance of color in the visible-light wavelength range. They are especially applicable to colored waters and wastewaters having color characteristics different from, but not excluding, platinum-cobalt standards.

3. Bibliography

B LACK , A.P. & R.F. C HRISTMAN . 1963. Characteristics of colored surface waters. J. Amer. Water Works Assoc. 55:753. C HRISTMAN , R.F. & M. G HASSEMI . 1966. Chemical nature of organic color in water. J. Amer. Water Works Assoc. 58:723. T HURMAN , E.M. 1985. Organic Geochemistry of Natural Waters. MartinusNijhoff/Dr. W. Junk Publishers, Dordrecht, Netherlands. S AWYER , C.N., P.O. M C C ARTY & G.F. PARKIN . 1994. Chemistry for Environmental Engineering, 4th ed. McGraw-Hill, Inc., New York, N.Y.



## 2120 B. Visual Comparison Method

1. General Discussion

a. Principle: Color is determined by visual comparison of thesample with known concentrations of colored solutions. Comparison also may be made with special, properly calibrated glass color disks. The platinum-cobalt method of measuring color is the standard method, the unit of color being that produced by 1 mg platinum/L in the form of the chloroplatinate ion. The ratio of cobalt to platinum given (¶ 3 b ) matches the color of natural waters.

b. Application: The platinum-cobalt method is applicable tonatural waters, potable waters, and to wastewaters, both domestic and industrial.

c. Interference: Even a slight turbidity causes the apparent colorto be noticeably higher than the true color; therefore remove turbidity by the filtration procedure described in Method C.

The color value of water is extremely pH-dependent and invariably increases as the pH of the water is raised. Whenreporting a color value, specify the pH at which color is determined. For research purposes, or when color values are to be compared among laboratories, determine the color response of a given water over a wide range of pH values. 1

d. Field method: Because the platinum-cobalt standard method is not convenient for fi eld use, compare water color with that of glass disks held at the end of metallic tubes containing glass comparator tubes fi lled with sample and colorless distilled water. Match sample color with the color of the tube of clear water plus the calibrated colored glass when viewed by looking toward a white surface. Calibrate each disk to correspond with the colors on the platinum-cobalt scale. The glass disks give results in substantial agreement with those obtained by the platinum-cobalt method and their use is recognized as a standard fi eld procedure. e. Nonstandard laboratory methods: Using glass disks or liquids other than water as standards for laboratory work is permissible only if these have been individually calibrated against platinum-cobalt standards. Waters of highly unusual color, such as those that may occur by mixture with certain industrial wastes, may have hues so far removed from those of the platinum-cobalt standards that comparison by the standard method is dif fi cult or impossible. For such waters, use the methods in Sections 2120C through F. However, results so obtained are not directly comparable to those obtained with platinum-cobalt standards.
2. Apparatus

a. Nessler tubes , matched, 50-mL, tall form. b. pH meter, for determining sample pH (see Section 4500-H  ). c. Filter and filter assembly (for true color measurements): Use a 0.45-  m-pore-diam cellulose membrane fi lter of 22 or 47 mm diam. Glass fi ber fi lters also can be used. Rinse fi lters before use and monitor fi lter blanks. Smaller-pore fi lters of 0.2 or 0.22 

m or even ultrafi ltration may be needed to remove colloidal particles for certain samples such as Mn or Fe oxides or other colloids. Use a glass, TFE, or stainless steel assembly to hold the selected fi lters.
3. Reagents

a. Organic-free water: Type I reagent water (see Section 1080) or equivalent water. Use for all standard preparation and other procedures.

b. Potassium chloroplatinate , K2PtCl 6 , analytical grade. c. Cobaltous chloride , CoCl 2  6H 2 O, analytical grade. 2-2 PHYSICAL & AGGREGATE PROPERTIES (2000)d. Hydrochloric acid, HCl, analytical grade. e. Sodium hydroxide , NaOH, analytical grade.

4. Preparation of Standards

Dissolve 1.246 g potassium chloroplatinate and 1.00 g crystallized cobaltous chloride in water with 100 mL conc HCl and dilute to 1000 mL. This stock solution has a color of 500 color units (CU). Platinum-cobalt standards of 500 CU are available commercially, and are suitable for use as the primary standard.

Prepare standards having CU of 5, 10, 15, 20, 25, 30, 40, 50, and 100 by diluting 1.0, 2.0, 3.0, 4.0, 6.0, 8.0, 10.0, and 20.0 mL stock color standard with water in 100-mL volumetric fl asks. Transfer to nessler tubes for use as standards. Protect standards against evaporation and contamination when not in use. Keep in the dark when not in use, and keep only for 1 month.

5. Procedure

a. Sample collection: Collect samples in acid-washed amber glass bottles or plastic bottles covered to keep out light. Rinse bottles once with sample before fi lling bottle with sample. Preferably take a sample of at least 100 mL. Analyze sample within 24 h of collection. Keep samples cold until analysis, and warm them up to room temperature before measurement.

b. Sample preparation: Check sample pH. If outside the range of 4 to 10, preferably adjust sample to pH 7 and note the adjustment.

If true color is to be measured, wash membrane fi lter and fi lter assembly by passing at least 50 mL water through fi lter. Filter about 25 mL sample and discard fi ltrate. Filter a further portion of about 50 mL through the same fi lter and retain for analysis.

c. Sample measurement: Observe sample color by fi lling amatched nessler tube to the 50-mL mark with sample and comparing it with standards. Look vertically downward through tubes toward a white or specular surface placed at such an angle that light is re fl ected upward through the columns of liquid. If turbidity is present and has not been removed, report as “ apparent color. ” If the color exceeds 100 units, dilute sample in known proportions until the color is within the range of the standards.

6. Calculation

a. Calculate color units (CU) by the following equation: Color A  50 B

where: A estimated color of a diluted sample and B mL sample taken for dilution. b. The correct units for true color are CU. One CU is equivalent to one Hazen unit and to one Pt-Co unit. If samples are not fi ltered, report data as Apparent CU. Report color results in whole numbers and record as follows:

CU Record to Nearest
1

–50 1 51

– 100 5 101

–250 10 251

–500 20

c. Report sample pH.

7. Quality Control

a. Replicate measurements: Use at least two portions of fi ltered sample.

b. Duplicate analyses: Analyze every tenth sample in duplicate (i.e., duplicating the entire procedure) to assess method precision.

c. Pre-programmed spectrophotometers: For spectrophotometers with pre-programmed calibration curves, verify calibrationcurve regularly with the platinum-cobalt standards prepared under 2120C.4, and adjust pre-programmed curves as needed.

8. Reference

1. B LACK , A.P. & R.F. C HRISTMAN . 1963. Characteristics of colored surface waters. J. Amer. Water Works Assoc. 55:753.
9. Bibliography

C HRISTMAN , R.F. & M. G HASSEMI . 1966. Chemical nature of organic color in water. J. Amer. Water Works Assoc. 58:723. S AWYER , C.N., P.L. M C C ARTY & G.F. PARKIN . 1994. Color. In Chemistry for Environmental Engineering, 4th ed., Chap. 14. McGraw Hill, New York, N.Y.



## 2120 C. Spectrophotometric - Single-Wavelength Method (PROPOSED)

1. General Discussion

a. Principle: Color is determined spectrophotometrically at a wavelength between 450 and 465 nm, with platinum-cobalt solutions as standards. 1

– 3 True color of real samples and plati

num-cobalt standards follows Beer ’ s Law. b. Application: The spectrophotometric platinum-cobalt method is applicable to natural waters, potable waters, and wastewaters, both domestic and industrial.

c. Interference: The primary interference is from the presence of colloidal and suspended particles that absorb or scatter light at the wavelength of the spectrophotometric method. While inMethod 2120B color measurements can be made without removal of particulate matter as long as they are reported as COLOR (2120)/Spectrophotometric - Single-Wavelength Method 2-3“ Apparent CU ” , Method C requires removal of particulate matter before color determination.

Light absorbance of organic matter depends on pH; however, the variation in absorbance is small for the pH range of most waters.Because color measurements are made for aesthetic reasons, preferably do not adjust sample pH as long as it is between 4 and 10. If pH is adjusted, adjust to 7, and note. Further, pH can affect the solubility of substances, which can then interfere with the color measurement if particulate matter is formed.

d. Method detection level: The minimum detectable color depends on the cell path length. Choose a cell size that provides an absorbance within the range that results in good accuracy and linearity of response. This range depends on the quality of the spectrophotometer. If a 50-mm cell is used in the wavelength range of 450 to 465 nm, then an absorbance of 0.005 yields aminimum detectable color of 1 CU. With newer spectrophotometers, a method detection level of 2 CU can be obtained with a path length of 25 mm. Dilute samples with high color to fall within the range of the standard curve. Absorbance readings should fall within the range of 0.005 to 0.8.
2. Apparatus

a. Spectrophotometer: Choose a wavelength between 450 and 465 nm. Use matched glass cells providing a light path of at least 25 mm. Cells with path lengths of 40, 50, or 100 mm may be used. Beer ’ s Law allows fl exibility in selecting the cell path length.

b. Filter and fi lter assembly: See 2120B.2 c
. 3. Reagents

See Section 2120B.3.
4. Preparation of Standards Prepare stock color solution of 500 CU according to 2120B.4. Prepare standards having CU of 5, 10, 15, 20, 30, 40, 50, and 100 by diluting 1.0, 2.0, 3.0, 4.0, 6.0, 8.0, 10.0, and 20.0 mL stock color standard with water in 100-mL volumetric fl asks. Protect standards against evaporation and contamination when not in use. Keep in the dark when not in use, and keep for only 1 month. 5. Spectrophotometric Standard Curve Let spectrophotometer warm up according to manufacturer ’ s instructions. Choose a wavelength between 450 and 465 nm to develop the standard curve; a good choice is 456 nm. The absorbance of Pt-Co has a broad maximum absorbance within this wavelength range. Use matched spectrophotometer cells. Fill one cell with water to zero the instrument. Read absorbance for each color standard, and prepare a standard curve of CU versus absorbance.

Pre-programmed color curves are available with some spectrophotometers. The curves can be veri fi ed by use of the standards prepared in 2120C.4.

6. Procedure

a. Sample collection: See 2120B.5 a
. b. Sample preparation: See 2120B.5 b . Always fi lter sample. c. Spectrophotometric measurement: Let spectrophotometer warm up according to manufacturer ’ s instructions. Set wavelength at same setting used to develop the standard curve; be sure that the cell path length is the same as that used for the standard curve. Fill one spectrophotometer cell with water and zero the instrument. Rinse the other cell with sample and then re fi ll. Placecell in spectrophotometer and read absorbance. Repeat for remaining samples. Determine sample color using absorbance readings and standard curve relating absorbance and CU. For spectrophotometers with pre-programmed calibration curves for color, zero instrument and take sample measurements according to manufacturer ’ s instructions.

7. Quality Control

See 2120B.7.

8. References

1. C ROWTHER , J. & J. EVANS . 1981. Estimating color in Hazen units by spectrophotometry. J. Amer. Water Works Assoc. 73:265. 2. B ENNETT , L. & M. D RIKAS . 1993. The evaluation of color in natural waters. Water Res. 27:1209. 3. H ONGVE , D. & G. Å KESSON . 1996. Spectrophotometric determination of water colour in Hazen units. Water Res. 30:2771.



## 2120 D. Spectrophotometric - Multi-Wavelength Method

1. General Discussion

a. Principle: The color of a filtered sample is expressed in terms that describe the sensation realized when viewing the sample. The hue (red, green, yellow, etc.) is designated by the term “ dominant wavelength, ” the degree of brightness by “luminance, ” and the saturation (pale, pastel, etc.) by “purity. ” These values are best determined from the light transmission characteristics of the filtered sample by means of a spectrophotometer. b. Application: This method is applicable to potable and surface waters and to wastewaters, both domestic and industrial.

d. Interference: The primary interference is from the presence of colloidal and suspended particles that absorb or scatter light. 2-4 PHYSICAL & AGGREGATE PROPERTIES (2000)2. Apparatus a. Spectrophotometer, having absorption cells of a minimum of 10 mm, a narrow (10-nm or less) spectral band, and an effective operating range from 400 to 700 nm.

b. Filter: See 2120B.2 c
.
3. Procedure

a. Sample preparation: Bring two 50-mL samples to room temperature. Use one sample at the original pH; adjust pH of the other to 7.0 by using sulfuric acid (H 2 SO 4 ) and sodium hydrox

ide (NaOH) of such concentrations that the resulting volume change does not exceed 3%. A standard pH is necessary because of the variation of color with pH. Remove particulate matter from samples before color determination (see 2120B.5 b ).

b. Determination of light transmission characteristics: Thoroughly clean 1-cm absorption cells. Rinse twice with fi ltered sample, and fi ll cell with fi ltered sample.

Determine transmittance values (in percent) at each visiblewavelength value presented in Table 2120:I, using the 10 ordinates marked with an asterisk for fairly accurate work and all 30 ordinates for increased accuracy. Set instrument to read 100%transmittance on the distilled water blank and make all determinations with a narrow spectral band.
4. Calculation

a. Tabulate transmittance values corresponding to wavelengths shown in Columns X, Y , and Z in Table 2120:I. Total each transmittance column and multiply totals by the appropriate factors (for 10 or 30 ordinates) shown at the bottom of the table, to obtain tristimulus values X, Y , and Z. The tristimulus value Y is percent luminance.

b. Calculate the trichromatic coef fi cients x and y from the tristimulus values X, Y , and Z by the following equations:

x X X Y Z

y Y X Y Z Locate point (x, y) on one of the chromaticity diagrams inFigure 2120:1 and determine the dominant wavelength (in nanometers) and the purity (in percent) directly from the diagram.

Determine hue from the dominant-wavelength value, according to the ranges in Table 2120:II.
5. Expression of Results

Express color characteristics (at pH 7.0 and at the original pH) in terms of dominant wavelength (nanometers, to the nearest unit), hue (e.g., blue, blue-green, etc.), luminance (percent, to the nearest tenth), and purity (percent, to the nearest unit). Report T ABLE 2120:I. S ELECTED O RDINATES FOR S PECTROPHOTOMETRIC C OLOR D ETERMINATIONS *

Ordinate

No. XYZ Wavelength
nm 1 424.4 465.9 414.1 2 435.5* 489.5* 422.2* 3 443.9 500.4 426.3 4 452.1 508.7 429.4 5* 461.2* 515.2* 432.0* 6 474.0 520.6 434.3 7 531.2 525.4 436.5 8* 544.3* 529.8* 438.6* 9 552.4 533.9 440.6 10 558.7 537.7 442.5 11* 564.1* 541.4* 444.41 12 568.9 544.9 446.3 13 573.2 548.4 448.2 14* 577.4* 551.8* 450.1 15 581.3 555.1 452.1 16 585.0 558.5 454.0 17* 588.7* 561.9* 455.9* 18 592.4 565.3 457.9 19 596.0 568.9 459.9 20* 599.6* 572.5* 462.0* 21 603.3 576.4 464.1 22 607.0 580.4 466.3 23* 610.9* 584.8* 468.7* 24 615.0 589.6 471.4 25 619.4 594.8 474.3 26* 624.2* 600.8* 477.7* 27 629.8 607.7 481.8 28 636.6 616.1 487.2 29* 645.9* 627.3* 495.2* 30 663.0 647.4 511.2 Factors when 30 Ordinates Used 0.032 69 0.033 33 0.039 38 Factors when 10 Ordinates Used 0.098 06 0.100 00 0.118 14* Insert in each column the transmittance value (%) corresponding to the wavelength shown. Where limited accuracy is suf fi cient, use only the ordinates marked with an asterisk.

T ABLE 2120:II. C OLOR H UES FOR D OMINANT WAVELENGTH RANGES Wavelength Range
nm Hue 400

–465 Violet 465

–482 Blue 482

–497 Blue-green 497

–530 Green 530

–575 Greenish yellow 575

–580 Yellow 580

–587 Yellowish orange 587

–598 Orange 598

– 620 Orange-red 620

–700 Red 400

–530c* Blue-purple 530c

–700* Red-purple *See Figure 2120:1 for signi fi cance of “ c ”

. COLOR (2120)/Spectrophotometric - Multi-Wavelength Method 2-5Figure 2120:1. Chromaticity diagrams. 2-6 PHYSICAL & AGGREGATE PROPERTIES (2000)type of instrument (i.e., spectrophotometer), number of selected ordinates (10 or 30), and the spectral band width (nanometers) used. 6. Bibliography

H ARDY , A.C. 1936. Handbook of Colorimetry. Technology Press, Boston, Mass.



## 2120 E. Tristimulus Spectrophotometric Method

1. General Discussion

a. Principle: Tristimulus values are a set of three numbers obtained from a spectrophotometer or colorimeter that, whencombined in various ways, describe how the human eye perceives a given color. Calculations using the three tristimulus values typically are used to de fi ne the color of a specimen for color matching speci fi cations or for routine control purposes.

The percentage of tristimulus light transmitted by the solution is determined and the transmittance values are then converted to trichromatic coef fi cients and color characteristic values. b. Application: This method is applicable to potable and surface waters and to domestic and industrial wastewaters.

c. Interference: See Section 2120C.1 c
. 2. Apparatus

a. Spectrophotometer with narrow spectral band width (10 nm or less), an effective operating range of 380 to 780 nm, and a tungsten lamp light source. Instrument must be able to obtain transmittance values at a multitude of wavelengths and calculate tristimulus values X, Y , and Z 1 to produce a fi nal color result. * Calibrate calculation algorithm software against platinum-cobalt standard reference.

b. Spectrophotometer cells, 1 cm. c. Filtering apparatus and fi lter: See 2120 B.2 c
. 3. Procedure a. Sample collection: See 2120B.5 a
. b. Sample preparation: Filter turbid samples according to 2120B.5 b

.

c. Spectrophotometric measurement: Let spectrophotometer warm up in accordance with manufacturer ’ s directions. Set instrument to pre-programmed calibration curve for tristimulus color. Zero instrument and perform measurements according to manufacturer ’ s method directions. Express results as prescribed in 2120D.5.

Alternatively, obtain tristimulus values for color by a published computation method. 1

4. Quality Control

See Section 2120B.7.
5. Reference

1. AMERICAN S OCIETY FOR T ESTING AND M ATERIALS . 1995. Standard practice for computing the colors of objects by using the CIE system. E308-95, ASTM Standards on Color and Appearance Measurement, 5th ed. American Soc. Testing & Materials, West Conshohocken, Pa. 6. Bibliography

H ACH C OMPANY . 1999. Hach DR/4000 Spectrophotometer Procedures Manual, 9th ed. Hach Co., Loveland, Colo.



## 2120 F. ADMI Weighted-Ordinate Spectrophotometric Method

1. General Discussion

a. Principle: Three properties describe color: hue, chroma, and value. Hue is the “ color ” (blue or red, for example); chroma is the intensity, brightness, or dullness of color; and value is the amount, or lightness or darkness, of color. In accordance with Adams-Nickerson chromatic value formula, 1 this method calculates single-number color difference values, i.e., uniform color differences. For example, if two colors, A and B, are judged visually to differ from colorless to the same degree, their ADMI color values will be the same. 2 Values are independent of chroma and hue. Spectrophotometrically,transmittance is measured at multiple wavelengths and converted to a set of abstract numbers, which then are converted to a single number that indicates color value. This number isexpressed on a scale used by the American Dye Manufacturers Institute (ADMI). 2

b. Applications: This method is applicable to colored waters and wastewaters having color characteristics signi fi cantly different from, but not excluding, platinum-cobalt standards.

c. Interferences: See Section 2120C.1. c
. 2. Apparatus

a. Spectrophotometer, with narrow spectral band width (10 nm or less), an effective operating range of 400 nm to 700 nm and a tungsten lamp light source. Instrument must be able to obtain transmittance values at a multitude of wavelengths and calculate * Hach DR/4000 Spectrophotometer, Program No. 1666, or equivalent. COLOR (2120)/ADMI Weighted-Ordinate Spectrophotometric Method 2-7tristimulus (ordinate) values, preferably by using the weightedordinate method.* Calibrate calculation algorithm software against platinum-cobalt standard reference.

b. Spectrophotometer cells, 1 cm. c. Filtering apparatus and fi lter: See 2120B.2 c
.
3. Procedure

a. Sample collection: See 2120B.5 a
. b. Sample preparation: Prepare two 100-mL sample portions, one at the original pH and one at pH 7.0. Filter turbid samples according to 2120B.5 b

.

c. Spectrophotometric measurement: Let spectrophotometer warm up in accordance with the manufacturer ’ s instruction. Set instrument to pre-programmed calibration curve for ADMIWeighted Ordinate Method. Zero instrument and take measurements of original and pH-adjusted samples according to manufacturer ’ s directions. Express results as prescribed in 2120C.6 c for both original and pH-adjusted samples.

Alternatively, obtain ADMI weighted-ordinate values for color by a published computation method. 2
4. Quality Control

See Section 2120B.7. 5. References

1. M CLAREN , K. 1970. The Adams-Nickerson colour-difference formula. J. Soc. Dyers Colorists 86:354. 2. ALLEN , W., W.B. PRESCOTT , R.E. D ERBY , C.E. GARLAND , J.M. PERET & M. S ALTZMAN . 1973. Determination of color of water and wastewater by means of ADMI color values. Proc. 28th Ind. Waste Conf. , Purdue Univ., Eng. Ext. Ser. No. 142:661. 6. Bibliography

H ACH C OMPANY . 1999. Hach DR/4000 Spectrophotometer Procedures Manual, 9th ed. Hach Co., Loveland, Colo.



## 2130 TURBIDITY*

2130 A. Introduction

1. Sources and Signi fi cance

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 fl uid-particle separation processes such as sedimentation and fi ltration 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, fi nely divided organic and inorganicmatter, 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 fl ux level through the sample. Correlation of turbidity with the weight or particle number concentration of suspended matter is diffi cult because the size, shape, and refractive index of the particles affect the light-scattering properties of the suspension. When present in signi fi cantconcentrations, particles consisting of light-absorbing materials such as activated carbon cause a negative interference. In low concentrations these particles tend to have a positive in fl uence 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 atright angles to the incident light. Turbidimeters with scatteredlight detectors located at 90 ° to the incident beam are called nephelometers. Nephelometers are relatively unaffected by small differences in design parameters and therefore are speci fi ed 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 * Hach DR/4000 Spectrophotometer, Program No. 1660, or equivalent.

* Approved by Standard Methods Committee, 2001. Joint Task Group: 20th Edition

—Raymond D. Letterman (chair), John A. Arrington, Alvin Lieberman, Kemon J. Papacosta, Theodore S. Tanaka, Brannon H. Wilder. † Nephelometers that instrument manufacturers claim meet the design speci fi cations of this method may not give the same reading for a given suspension, even when each instrument has been calibrated using the manufacturer ’ s manual. This differential performance is especially important when measurements are made for regulatory purposes. Consult regulatory authorities when selecting a nephelometer to be used for making measurements that will be reported for regulatory purposes. 2-8 PHYSICAL & AGGREGATE PROPERTIES (2000)