Water color is one of the most immediate sensory and regulatory parameters in drinking water, surface water, and industrial effluent monitoring, providing a rapid indication of dissolved organics, metals, and process contamination. This article reviews the principal standards for water color determination—including ISO 7887, ASTM D1209, and GB 11903—together with sampling requirements, platinum–cobalt and dilution-based test methods, typical regulatory limits, and practical application guidance.
Technical Overview
Color is a fundamental physical characteristic of water, defined as the visual sensation produced when light passes through or is reflected by a water sample. Although color is rarely toxic by itself, it is a powerful diagnostic indicator: yellowish-brown tints typically signal natural organic matter such as humic and tannic substances, greenish hues may indicate algal activity, and unusual colors often reveal industrial dyes, pulp and paper liquor, petroleum products, or corrosion products such as iron and manganese. Because consumers judge water quality largely by appearance, color is a mandatory monitoring parameter in drinking water regulations and wastewater discharge permits worldwide.
A key technical distinction in all color determination standards is between true color, which is the color of light absorbed by dissolved substances after turbidity has been removed, and apparent color, which includes the contribution of suspended matter. Results are expressed in platinum–cobalt color units, where one unit corresponds to the color produced by 1 mg/L of platinum in the form of potassium chloroplatinate, also historically called Hazen units (HU) or degrees. Strongly colored wastewaters are commonly reported as dilution multiples, and optical methods may report spectral absorption coefficients in reciprocal meters.
- ISO 7887:2011 – Water quality, investigation and determination of colour, covering visual comparison, spectrophotometric true colour determination, and light absorption measurement
- ASTM D1209 – Standard Test Method for Color of Clear Liquids (Platinum–Cobalt Scale)
- Standard Methods 2120 (APHA/AWWA/WEF) – Color, including visual, single-wavelength, and multiple-wavelength tristimulus procedures
- GB 11903-89 – Water quality, determination of colourity, specifying the platinum–cobalt colorimetric method and the dilution multiple method
- GB 5749-2022 and GB 8978-1996 – Chinese drinking water and integrated wastewater discharge standards defining color limits
- JIS K 0102 – Japanese industrial water and wastewater testing methods including color determination
The World Health Organization sets no health-based guideline value for color but notes that most consumers detect color above approximately 15 true color units, while the EU Drinking Water Directive requires color to be acceptable to consumers and show no abnormal change. Accredited third-party laboratories routinely perform color testing against these standards as part of compliance, characterization, and process-control programs.
Testing Samples
Samples for color determination should be collected in clean borosilicate glass containers, typically 100 to 500 mL, filled with minimal headspace and free of residual cleaning agents. Because biological activity and chemical changes can alter color, samples should be cooled to 2–5 °C, protected from light, and analyzed as soon as possible, preferably within 24 hours. The sample pH must be measured and reported alongside the color result, since the color intensity of dissolved organic matter varies with pH.
Drinking Water and Treated Water
Tap water, treated municipal supplies, packaged drinking water, and source waters destined for potable use are tested to verify compliance with sensory limits, typically 15 platinum–cobalt units or equivalent in national regulations such as GB 5749-2022. These samples are usually clear and only lightly colored, making them well suited to the platinum–cobalt visual comparison method.
Surface Water and Groundwater
Rivers, lakes, reservoirs, and groundwater are monitored for color to track natural organic matter, seasonal turnover, algal events, and contamination from runoff or leachate. Moderately colored samples may be measured visually against platinum–cobalt standards after turbidity removal, while more strongly colored waters are evaluated spectrophotometrically.
Industrial Wastewater and Effluent
Effluents from textile dyeing, printing and dyeing finishing, pulp and paper, chemical manufacturing, tanning, food processing, and pharmaceutical production frequently exhibit intense coloration that far exceeds the visual comparison range. These samples are characterized using the dilution multiple method of GB 11903-89 or spectrophotometric procedures, and results are compared against discharge permit limits commonly set at 50 to 80 dilution multiples depending on the receiving water category.
High-Purity and Process Water
Purified water, water for injection, boiler feed water, and food and beverage process water are tested for color to detect organic breakthrough, resin or activated carbon exhaustion, and corrosion events. Although color levels are expected to be near zero, periodic verification confirms treatment system integrity and supports quality management systems.
Testing Items
Color testing programs typically include the following reported parameters, selected according to the applicable standard and the nature of the sample:
- Color value in platinum–cobalt units (degrees, PCU, HU) by visual comparison against calibrated standards
- True color, determined after removal of turbidity by centrifugation or membrane filtration
- Apparent color, determined on the unfiltered sample including suspended matter
- Color expressed as a dilution multiple for strongly colored industrial effluents
- Spectral absorption coefficients at 436 nm, 525 nm, and 620 nm in m⁻¹, as specified in ISO 7887
- Tristimulus color values, dominant wavelength, and luminance purity by multiple-wavelength spectrophotometry per Standard Methods 2120D
- Supporting measurements of pH and turbidity, which are required for correct interpretation of the color result
Testing Methods
Platinum–Cobalt Visual Comparison Method
This classical procedure, harmonized across ISO 7887 Method A, GB 11903-89, ASTM D1209, and Standard Methods 2120B, is the reference technique for clear, yellow-hued waters. A stock standard of 500 platinum–cobalt units is prepared by dissolving 1.246 g of potassium chloroplatinate and 1.000 g of cobalt chloride hexahydrate in approximately 100 mL of hydrochloric acid and diluting to 1 L; the stock is traceable and commercially available as a certified reference solution. Graded working standards, for example 0, 5, 10, 15, 20, 25, 30, 40, 50, 60, and 70 units, are placed in matched 50 mL Nessler tubes, and the sample is compared against the series against a white background under diffuse daylight or a standardized illuminant. The result equals the matching standard, with intermediate values estimated to the nearest unit; samples above the range are diluted and the result multiplied by the dilution factor. The method is generally applicable between about 5 and 70 units, with a practical visual detection threshold of approximately 5 units.
Dilution Multiple Method
Specified in GB 11903-89 and analogous international practices, the dilution multiple method addresses industrial wastewater whose color cannot be matched to platinum–cobalt standards. The sample is diluted stepwise with colorless dilution water, and each dilution is observed against a white background until the color becomes just barely perceptible compared with the dilution water. The color of the original sample is reported as the dilution multiple at this threshold, with typical reporting ranges extending from around 10 to 2500 times. This method is the basis for most dye, printing, and pulping effluent compliance determinations under discharge standards such as GB 8978-1996.
Spectrophotometric Methods
Instrumental procedures remove the subjectivity of visual matching and extend the working range. ISO 7887 Method B determines true color spectrophotometrically after turbidity removal, calibrating absorbance measurements against platinum–cobalt standards to report results in Hazen units. ISO 7887 Method C measures light transmission at 436 nm, 525 nm, and 620 nm and reports spectral absorption coefficients in m⁻¹, calculated as 2.303 multiplied by absorbance and divided by the optical path length in meters; this approach is widely used in European waterworks practice. Standard Methods 2120C provides a single-wavelength spectrophotometric procedure, while 2120D employs multiple-wavelength absorbance data to compute CIE tristimulus values, yielding dominant wavelength, hue, and luminance purity for waters whose color differs substantially from the yellow platinum–cobalt scale.
Sample Pretreatment and Interference Control
Turbidity is the principal interference in color testing because suspended particles scatter light and inflate the perceived color. True color determination requires removal of turbidity, with centrifugation generally preferred since filtration media can adsorb dissolved color; membrane filtration through a 0.45 µm filter is acceptable but the treatment must be reported. Other controls include the use of matched colorless glassware, consistent illumination conditions, prompt analysis to avoid biological color changes, and verification of residual chlorine or other oxidants that may alter color during storage. Quality assurance measures include certified platinum–cobalt reference solutions, duplicate determinations, wavelength and photometric accuracy checks for spectrophotometers, and participation in interlaboratory comparisons by accredited third-party laboratories.
Applications
Color determination under recognized standards supports a wide range of regulatory, operational, and investigative purposes:
- Municipal drinking water compliance testing against sensory limits such as the 15 platinum–cobalt unit limit in GB 5749-2022
- Wastewater discharge permit verification for textile, dyeing, printing, pulp and paper, tanning, and chemical industries
- Environmental impact assessment and receiving water quality monitoring for rivers, lakes, and coastal zones
- Treatment process optimization, including coagulant dosing, activated carbon performance, and membrane fouling diagnosis
- Food and beverage process water and product water quality control
- Pharmaceutical purified water system monitoring and deviation investigations
- Aquaculture and irrigation water suitability evaluation
- Product certification, buyer specifications, and due-diligence testing conducted by accredited third-party laboratories
In each context, color data are most valuable when the sampling conditions, pretreatment, method, and reporting units are clearly documented, enabling meaningful comparison with regulatory limits and historical trends.
FAQ
What is the difference between true color and apparent color?
True color is the color contributed solely by dissolved substances and is measured after turbidity has been removed by centrifugation or filtration. Apparent color includes the light-scattering effect of suspended particles and is measured on the unfiltered sample. Regulatory drinking water limits refer to true color, so turbidity removal and documentation of the pretreatment step are essential.
What units are used to express water color?
Results from platinum–cobalt methods are reported in degrees, platinum–cobalt units (PCU), Hazen units, or true color units, which are essentially equivalent and defined by 1 mg/L platinum as chloroplatinate. Strongly colored wastewaters are reported as dilution multiples, while ISO 7887 Method C reports spectral absorption coefficients at 436 nm, 525 nm, and 620 nm in reciprocal meters.
What is the regulatory limit for color in drinking water?
The Chinese drinking water standard GB 5749-2022 sets a maximum allowable color of 15 platinum–cobalt units. The World Health Organization recommends that color remain below approximately 15 true color units for consumer acceptability, and the EU Drinking Water Directive requires color to be acceptable to consumers with no abnormal change.
How should turbid samples be handled?
Turbidity should be removed before measuring true color, preferably by centrifugation because filter media can adsorb dissolved color; membrane filtration at 0.45 µm is an accepted alternative provided it is reported. If turbidity is not removed, the result must be reported as apparent color.
How long can samples be stored before color analysis?
Samples should be collected in glass containers, cooled to 2–5 °C, protected from light, and analyzed as soon as possible, ideally within 24 hours. No chemical preservative is added because preservatives may alter the color being measured, and the sample pH should always be recorded with the result.
Which method should be used for darkly colored industrial wastewater?
Wastewaters from dyeing, printing, and pulping operations typically exceed the 5 to 70 unit range of the visual platinum–cobalt method, so the dilution multiple method of GB 11903-89 is applied, supported where necessary by spectrophotometric procedures. Discharge standards such as GB 8978-1996 commonly set permit limits of 50 to 80 dilution multiples depending on the discharge category.
Are color results from different standards directly comparable?
Platinum–cobalt results from ISO 7887, ASTM D1209, GB 11903, and Standard Methods 2120 are broadly comparable because they share the same platinum–cobalt scale and reference standard preparation. However, waters whose hue differs markedly from the yellow platinum–cobalt scale, or results reported as dilution multiples or absorption coefficients, cannot be converted to platinum–cobalt units without repeating the test by the target method.