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 wastewaters

can contain lignins, tannins, dyes, and other organic and inorganic chemicals that cause color. Humic materials and the color

caused 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.

Color 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 filtration. 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 measurements

apply 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 such

cases, 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

BLACK, A.P. & R.F. CHRISTMAN. 1963. Characteristics of colored surface

waters. J. Amer. Water Works Assoc. 55:753.

CHRISTMAN, R.F. & M. GHASSEMI. 1966. Chemical nature of organic

color in water. J. Amer. Water Works Assoc. 58:723.

THURMAN, E.M. 1985. Organic Geochemistry of Natural Waters. MartinusNijhoff/Dr. W. Junk Publishers, Dordrecht, Netherlands.

SAWYER, C.N., P.O. MCCARTY & 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

the sample 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 (2120B.4)

matches the color of natural waters.

b. Application: The platinum-cobalt method is applicable to

natural waters, potable waters, and to wastewaters, both domestic and industrial.

c. Interference: Even a slight turbidity causes the apparent

color to 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. When

reporting 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 field use, compare water color

with that of glass disks held at the end of metallic tubes

containing glass comparator tubes filled with sample and

colorless distilled water. Match sample color with the color of

the tube of clear water plus the calibrated colored glass when

* Approved by Standard Methods Committee, 2001. Editorial revisions, 2011.

Joint Task Group: 21st Edition—James K. Edzwald (chair), Penny J. Bristol,

Brian A. Dempsey, Darren A. Lytle, David J. Pernitsky, Mike J. Sadar, Jeff

Throckmorton.

1
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 field 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 difficult 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 filter of 22 or 47

mm diam. Glass fiber filters also can be used. Rinse filters before

use and monitor filter blanks. Smaller-pore filters of 0.2 or 0.22

m or even ultrafiltration 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 filters.

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, K2PtCl6, analytical grade.

c. Cobaltous chloride, CoCl2  6H2O, analytical grade.

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 flasks.

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 filling 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 filter and filter

assembly by passing at least 50 mL water through filter. Filter

about 25 mL sample and discard filtrate. Filter a further portion

of about 50 mL through the same filter and retain for analysis.

c. Sample measurement: Observe sample color by filling a

matched 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 reflected 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

filtered, 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

The QC practices considered to be an integral part of each

method are summarized in Tables 2020:I and II.

a. Replicate measurements: Use at least two portions of filtered 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 calibration

curve regularly with the platinum-cobalt standards prepared under 2120C.4, and adjust pre-programmed curves as needed.

8. Reference

1. BLACK, A.P. & R.F. CHRISTMAN. 1963. Characteristics of colored

surface waters. J. Amer. Water Works Assoc. 55:753.

COLOR (2120)/Visual Comparison Method

2
9. Bibliography

CHRISTMAN, R.F. & M. GHASSEMI. 1966. Chemical nature of organic

color in water. J. Amer. Water Works Assoc. 58:723.

SAWYER, C.N., P.L. MCCARTY & 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 platinum-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 in Section 2120B

color measurements can be made without removal of particulate matter

as long as they are reported as “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 a minimum 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 flexibility in selecting the cell path

length.

b. Filter and filter assembly: See 2120B.2c.

3. Reagents

See 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 flasks.

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 verified by use of the standards prepared in 2120C.4.

6. Procedure

a. Sample collection: See 2120B.5a.

b. Sample preparation: See 2120B.5b. Always filter 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 refill. Place

cell 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. CROWTHER, J. & J. EVANS. 1981. Estimating color in Hazen units by

spectrophotometry. J. Amer. Water Works Assoc. 73:265.

2. BENNETT, L. & M. DRIKAS. 1993. The evaluation of color in natural

waters. Water Res. 27:1209.

3. HONGVE, D. & G. ÅKESSON. 1996. Spectrophotometric determination

of water colour in Hazen units. Water Res. 30:2771.

COLOR (2120)/Spectrophotometric—Single-Wavelength Method

3
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.

c. Interference: The primary interference is from the presence of colloidal and suspended particles that absorb or

scatter light.

d. Quality control (QC): The QC practices considered to be an

integral part of each method are summarized in Tables 2020:I

and II.

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.2c.

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 (H2SO4) and sodium hydroxide (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.5b).

b. Determination of light transmission characteristics: Thoroughly clean 1-cm absorption cells. Rinse twice with filtered

sample, and fill cell with filtered sample.

Determine transmittance values (in percent) at each visible

wavelength 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 coefficients 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 in

Figure 2120:1 and determine the dominant wavelength (in nanometers) and the purity (in percent) directly from the diagram.

TABLE 2120:I. SELECTED ORDINATES FOR SPECTROPHOTOMETRIC COLOR

DETERMINATIONS*

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 sufficient, use only the ordinates marked

with an asterisk.

COLOR (2120)/Spectrophotometric—Multi-Wavelength Method

4
Figure 2120:1. Chromaticity diagrams.

COLOR (2120)/Spectrophotometric—Multi-Wavelength Method

5

0.350 

::.., 

...._ 

0 

Q) 0.325 ::I 

~ 

0.300 

0.275 0.300 0.325 0.350 0.375 

Value of x 

0.80 

0.70 

0.60 

0.50 

::.., 

...._ 

0 

Q) 

::I 0.40 

~ 

0.30 

0.20 

0.10 

0 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 

Value of x 
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

type of instrument (i.e., spectrophotometer), number of selected

ordinates (10 or 30), and the spectral band width (nanometers)

used.

6. Bibliography

HARDY, 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, when

combined in various ways, describe how the human eye perceives a given color. Calculations using the three tristimulus

values typically are used to define the color of a specimen for

color matching specifications 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 coefficients and color characteristic values.

b. Application: This method is applicable to potable and

surface waters and to domestic and industrial wastewaters.

c. Interference: See 2120C.1c.

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 Z1 to produce a final color result.*

Calibrate calculation algorithm software against platinum-cobalt

standard reference.

b. Spectrophotometer cells, 1 cm.

c. Filtering apparatus and filter: See 2120 B.2c.

3. Procedure

a. Sample collection: See 2120B.5a.

b. Sample preparation: Filter turbid samples according to

2120B.5b.

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 2120B.7.

5. Reference

1. AMERICAN SOCIETY FOR TESTING AND MATERIALS. 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

HACH COMPANY. 1999. Hach DR/4000 Spectrophotometer Procedures

* Hach DR/4000 Spectrophotometer, Program No. 1666, or equivalent. Manual, 9th ed. Hach Co., Loveland, Colo.

TABLE 2120:II. COLOR HUES FOR DOMINANT 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 significance of “c”.

COLOR (2120)/Tristimulus Spectrophotometric Method

6
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 is

expressed 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 significantly different from, but not excluding, platinum-cobalt standards.

c. Interferences: See 2120C.1c.

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

tristimulus (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 filter: See 2120B.2c.

3. Procedure

a. Sample collection: See 2120B.5a.

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.5b.

c. Spectrophotometric measurement: Let spectrophotometer

warm up in accordance with the manufacturer’s instruction. Set

instrument to pre-programmed calibration curve for ADMI

Weighted Ordinate Method. Zero instrument and take measurements of original and pH-adjusted samples according to manufacturer’s directions. Express results as prescribed in 2120D.5

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 2120B.7.

5. References

1. MCLAREN, K. 1970. The Adams-Nickerson colour-difference formula. J. Soc. Dyers Colorists 86:354.

2. ALLEN, W., W.B. PRESCOTT, R.E. DERBY, C.E. GARLAND, J.M. PERET &

M. SALTZMAN. 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

HACH COMPANY. 1999. Hach DR/4000 Spectrophotometer Procedures

* Hach DR/4000 Spectrophotometer, Program No. 1660, or equivalent. Manual, 9th ed. Hach Co., Loveland, Colo.

COLOR (2120)/ADMI Weighted-Ordinate Spectrophotometric Method

7