Water quality color testing, also known as chromaticity testing, quantifies the color intensity of water caused by dissolved organic matter, metals, dyes, and other substances, making it a fast and sensitive indicator of pollution and treatment performance. This article explains the testing principles, internationally recognized methods such as GB/T 11903 and ISO 7887, applicable sample types, and practical applications in environmental monitoring and industrial process control.
Technical Overview
Color, or chromaticity, is one of the most immediate sensory indicators of water quality. In natural waters, color mainly originates from humic and fulvic substances, tannins from decaying vegetation, iron and manganese compounds, and algal pigments. In industrial settings, color may be introduced by synthetic dyes, pulp and paper effluents, petrochemicals, pigments, and dissolved metals. While color itself is not always directly toxic, elevated chromaticity often signals the presence of organic pollution, reduces light penetration in receiving waters, degrades aesthetic acceptability, and can interfere with disinfection processes.
In water quality analysis, an important distinction is made between true color and apparent color. True color refers to the color of water after suspended solids and turbidity have been removed, typically by centrifugation, and it reflects the color of dissolved substances. Apparent color is the color of the original, unfiltered sample, which combines dissolved and particulate contributions. Most regulatory limits are based on true color. Chromaticity results are commonly expressed in platinum-cobalt degrees (°PC, also called Hazen units or color units, CU), in dilution multiples for heavily polluted samples, or as spectral absorption values in m⁻¹ according to ISO 7887.
Water color testing is widely performed by accredited third-party laboratories, environmental monitoring stations, water treatment plants, and industrial quality control departments. The principal reference standards include GB/T 11903 (Water Quality — Determination of Colority), ISO 7887 (Water Quality — Investigation and Determination of Colour), ASTM D1209 (Color of Clear Liquids by the Platinum-Cobalt Scale), and APHA Standard Methods 2120.
Testing Samples
Drinking Water and Source Water
Treated tap water, packaged drinking water, and raw water from reservoirs or rivers intended for potable use are tested against strict chromaticity limits, typically no more than 15 platinum-cobalt degrees under standards such as GB 5749 and WHO aesthetic guidance. Low-level color detection with high sensitivity is essential for these samples.
Surface Water and Groundwater
Rivers, lakes, reservoirs, wetlands, and groundwater sources are monitored for color as part of routine environmental quality assessment. Rising chromaticity in surface water often indicates eutrophication, organic loading, or upstream industrial discharge.
Municipal and Industrial Wastewater
Effluents from municipal treatment plants and industries such as textile dyeing, printing and dyeing, pulp and paper, tanning, food processing, chemical manufacturing, and pharmaceuticals are tested before discharge. Because these samples frequently exhibit strong and non-yellow hues, the dilution multiple method is often applied, with discharge limits commonly set at 50 to 80 dilution multiples depending on the discharge grade.
Process and Ultrapure Water
Boiler feed water, cooling water, pharmaceutical water systems, electronics rinsing water, and beverage production water are tested to verify that color-causing contaminants remain within process specifications, since even trace organics can affect product quality or equipment performance.
Testing Items
Typical color-related test items reported by testing laboratories include:
- Chromaticity (true color) in platinum-cobalt degrees (°PC, Hazen units, CU)
- Apparent color of the original sample
- Color expressed as dilution multiples for dark or complex industrial effluents
- Spectral color values in m⁻¹ determined at 436 nm, 525 nm, and 620 nm per ISO 7887 Method B
- Hue description (yellow, brown, green, blue, red, etc.) recorded during visual examination
- Supporting parameters such as pH and turbidity, which affect color measurement and interpretation
Testing Methods
Platinum-Cobalt Standard Colorimetric Method
Regarded as the reference method for clean, lightly colored water with yellowish tones, this approach is specified in GB/T 11903, ISO 7887 Method A, ASTM D1209, and Standard Methods 2120. A stock chromaticity solution of 500 degrees is prepared by dissolving 1.246 g of potassium chloroplatinate (equivalent to 0.500 g of platinum) and 1.000 g of cobalt chloride hexahydrate in distilled water with 100 mL of hydrochloric acid, diluted to exactly 1000 mL. Working standards of 0 to 70 degrees are prepared by serial dilution. The sample, with turbidity removed by centrifugation, is placed in a 50 mL Nessler tube and visually compared against the standard series against a white background under specified lighting. The result is reported as the matching standard concentration; samples exceeding 70 degrees are first diluted and the result multiplied by the dilution factor. The detection capability of the method is generally about 5 degrees.
Dilution Multiple Method
This method is used for industrial wastewater and heavily polluted surface water whose color cannot be meaningfully quantified on the platinum-cobalt scale, particularly when the hue is not yellow. The sample is diluted stepwise with colorless dilution water until the color becomes just imperceptible to the observer compared with the dilution water. The chromaticity is reported as the total dilution factor at that endpoint, and the residual hue is recorded descriptively. ISO 7887 Method C provides an equivalent procedure for strongly colored water samples.
Spectrophotometric Determination
For objective, instrument-based quantification, ISO 7887 Method B measures the absorbance of the centrifuged sample in a 50 mm path length cuvette at 436 nm, 525 nm, and 620 nm. The results are converted into spectral color values expressed in m⁻¹, which correlate with the visual platinum-cobalt scale but eliminate observer variability. Tristimulus filter spectrophotometry, described in Standard Methods 2120, offers a similar instrumental alternative.
Sampling, Pretreatment, and Quality Control
Representative samples are collected in clean glass containers, filled completely to exclude air, and stored cool (approximately 4 °C) and protected from light. Analysis should be performed as soon as possible, ideally within 48 hours, because biological and chemical changes can alter color. Turbidity is removed by centrifugation rather than filtration where possible, since filter media may adsorb color-causing substances. Quality control measures include freshly prepared calibration standards, duplicate visual comparisons, blind reference samples, and consistent observer viewing conditions to ensure reproducible results.
Applications
Water color testing supports a broad range of regulatory, environmental, and industrial objectives:
- Drinking water compliance verification against national and international aesthetic limits
- Wastewater discharge supervision and pollution source tracing for regulatory authorities
- Performance evaluation of water treatment processes such as coagulation, adsorption with activated carbon, and advanced oxidation
- Process control in textile dyeing, pulp and paper, tanning, and chemical manufacturing to optimize decolorization treatment
- Environmental impact assessment and routine monitoring of rivers, lakes, and reservoirs
- Quality assurance of pharmaceutical water, food and beverage process water, and ultrapure water systems
- Research and development of new decolorization technologies, membranes, and treatment reagents
FAQ
What is chromaticity in water testing?
Chromaticity is a quantitative measure of the color intensity of water, produced mainly by dissolved organic and inorganic substances. It is typically reported in platinum-cobalt degrees for lightly colored water or in dilution multiples for heavily colored wastewater.
What is the difference between true color and apparent color?
True color is measured after suspended matter has been removed by centrifugation and reflects only dissolved color-causing substances. Apparent color is measured on the original sample without pretreatment and includes the effect of suspended particles. Regulatory limits are generally based on true color.
What are the typical regulatory limits for water color?
Common limits include 15 platinum-cobalt degrees for drinking water under GB 5749 and approximately 15 true color units recommended by the WHO for aesthetic acceptability. For industrial wastewater discharge, limits of 50 and 80 dilution multiples apply to different discharge grades under GB 8978. Applicable limits should always be confirmed against the specific standard governing the sample matrix.
Why must turbidity be removed before measurement?
Suspended particles scatter light and create a false color impression during visual comparison, biasing the result upward. Centrifugation is preferred because filtration can remove some dissolved or colloidal color-causing substances and distort the true color value.
Which method should be selected for a given sample?
The platinum-cobalt colorimetric method is suitable for clean water with a yellowish tint up to about 70 degrees. The dilution multiple method is appropriate for dark, strongly colored, or non-yellow industrial effluents. Spectrophotometric determination provides objective results for laboratories seeking to minimize observer variability and is well suited to automated monitoring programs.
How should samples be preserved before color testing?
Samples should be collected in clean glass bottles, filled without headspace, kept at approximately 4 °C in the dark, and analyzed as soon as possible, preferably within 48 hours, to prevent biological and chemical changes that alter color.