Determination of Color and Chromaticity Pollutants in Water Quality: Methods, Standards, and Applica

Color, or chromaticity, is one of the most immediate sensory indicators of water pollution and a mandatory parameter in drinking water, surface water, and industrial effluent monitoring worldwide. This article provides a comprehensive technical overview of the determination of color pollutants in water, covering sampling requirements, platinum-cobalt colorimetry, dilution multiple methods, spectrophotometric techniques, applicable standards such as ISO 7887 and GB 11903, and practical guidance for laboratories and industry professionals.

Technical Overview

Color in water originates from dissolved organic matter such as humic and fulvic acids, tannins, lignins, iron and manganese compounds, industrial dyes, pigments, pulp and paper effluents, textile wastewater, and certain algae or plankton residues. Although color itself is not always directly toxic, it serves as a rapid and cost-effective indicator of the presence of organic pollutants, metals, or anthropogenic contamination, and it strongly affects the aesthetic acceptability of water for drinking, recreation, and industrial use.

In water quality analysis, a distinction is made between true color, which is the color of water after removal of turbidity (typically by filtration through a 0.45 µm membrane), and apparent color, which is measured on the original unfiltered sample and includes the contribution of suspended solids. Color is quantified either in degrees (°) or Hazen / platinum-cobalt color units (PCU), derived from comparison with standard solutions of potassium chloroplatinate and cobalt(II) chloride hexahydrate, or by the dilution multiple method for highly colored effluents.

Regulatory limits are stringent for potable supplies. For example, the Chinese national drinking water standard GB 5749 specifies a color limit of no more than 15 platinum-cobalt degrees, while the integrated wastewater discharge standard GB 8978 restricts chroma of discharged effluents, typically to 50–80 dilution degrees depending on the discharge category and receiving water body. Comparable requirements appear in WHO drinking water guidelines, the EU Drinking Water Directive, and the U.S. National Secondary Drinking Water Regulations (15 color units).

Testing Samples

Drinking Water and Bottled Water

Treated tap water, packaged drinking water, mineral water, and purified water are routinely tested for true color to verify compliance with potable water standards and to monitor the performance of coagulation, sedimentation, filtration, and activated carbon adsorption processes.

Surface Water and Groundwater

Rivers, lakes, reservoirs, ponds, and wells are analyzed to characterize natural background color caused by humic substances, to detect contamination events, and to support water quality classification according to environmental standards such as GB 3838 for surface water.

Municipal Wastewater and Reclaimed Water

Influent and effluent from sewage treatment plants, as well as recycled or reused water, are monitored for chroma to evaluate decolorization efficiency and to confirm compliance with discharge or reuse criteria.

Industrial Effluents

Highly colored wastewaters from dyeing and textile finishing, printing and ink formulation, pulp and paper milling, tanning, food and beverage processing, pharmaceutical production, petrochemical refining, and pigment manufacturing require determination by the dilution multiple method because their chroma frequently exceeds the range of direct visual comparison.

Process and Cooling Water

Boiler feed water, cooling circulation water, and ultrapure process water in power generation, electronics, and pharmaceutical facilities are checked for color development, which can signal corrosion, organic fouling, or resin breakdown.

Testing Items

Typical test items and reported parameters for color determination include:

• True color (filtered sample), expressed in platinum-cobalt degrees (Hazen units / PCU)
• Apparent color (unfiltered sample), expressed in platinum-cobalt degrees
• Chroma by dilution multiple method, expressed in degrees (times of dilution to the threshold of perceptibility)
• Spectrophotometric absorbance values at characteristic wavelengths (for example 436 nm, 525 nm, and 620 nm) and calculated tristimulus or single-wavelength color numbers
• Color change after treatment processes, reported as percent color removal or decolorization efficiency
• Supporting parameters for data interpretation, such as turbidity, pH, suspended solids, total organic carbon (TOC), chemical oxygen demand (COD), and specific ultraviolet absorbance (SUVA254)

Testing Methods

Platinum-Cobalt Standard Colorimetric Method

This is the reference method for lightly colored, yellowish-hue waters such as drinking water, groundwater, and treated effluents. A series of platinum-cobalt standard color solutions (for example 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 70 degrees) is prepared, and the sample is visually compared against the standards in matched Nessler-type tubes against a white background. The standard whose color most closely matches the sample is recorded as the color value. If the sample exceeds the highest standard, it is diluted before comparison and the result is multiplied by the dilution factor. The method is specified in GB 11903, ISO 7887 (Method A), ASTM D1209, and Standard Methods 2120B, with a typical working range of approximately 5 to 70 degrees and a minimum detectable chroma of about 5 degrees. Turbidity must be removed before measurement because suspended particles cause interference; samples should be analyzed as soon as possible, preferably within the holding time defined by the applicable standard.

Dilution Multiple Method

For darkly colored industrial wastewater whose hue differs from the platinum-cobalt yellow standard, the dilution multiple method specified in GB 11903 and ISO 7887 is used. The sample is serially diluted with colorless dilution water until even experienced observers can just no longer perceive any color difference from the dilution water in matched tubes against a white background. The chroma is reported as the dilution factor in degrees. Multiple observers are recommended, and the reported value is often the geometric mean of individual threshold results. This method is not applicable to samples with very low chroma and is generally used when color exceeds approximately 50 degrees.

Spectrophotometric Method

Instrumental color measurement eliminates the subjectivity of visual comparison. ISO 7887 (Method B) and U.S. EPA Method 110.2 determine color by measuring the absorbance of the filtered sample at 436 nm, 525 nm, and 620 nm; the results may be expressed as spectral color values or combined into a single color figure. Dedicated colorimeters and benchtop spectrophotometers calibrated with platinum-cobalt, APHA/Pt-Co, or Gardner scales can report Hazen units directly. Spectrophotometric determination offers better precision, objective and traceable records, automated data capture, and suitability for online or at-line color monitoring in treatment plants.

Quality Assurance and Interference Control

• Samples should be collected in clean glass containers, protected from light, cooled during transport, and analyzed promptly, since biological activity and chemical changes can alter color over time.
• True color determination requires filtration through a 0.45 µm membrane filter; the filter must be rinsed to remove extractable color before use.
• Turbidity, floating oil, foam, and air bubbles must be eliminated to avoid false elevation of apparent color.
• pH should be recorded because the color of humic substances and many dyes is pH-dependent; ISO 7887 recommends reporting the pH with the result.
• Calibration standards must be freshly verified, and visual comparisons should be performed under standardized daylight or a certified light box by trained, color-normal observers.
• Method blanks, duplicate determinations, and participation in interlaboratory comparisons are recommended to ensure reliable, defensible data.

Applications

• Drinking water utilities: verifying compliance with the 15-degree limit of GB 5749 and equivalent WHO or EU limits, and optimizing coagulant dosing for humic substance removal.
• Environmental compliance: discharge permit monitoring for textile, dyeing, papermaking, tanning, and chemical plants against GB 8978 chroma limits.
• Wastewater treatment design and operation: evaluating the performance of decolorization technologies such as coagulation-flocculation, adsorption with activated carbon, advanced oxidation (ozone, Fenton, UV/H2O2), and membrane processes.
• Surface water quality assessment: supporting water body classification, eutrophication studies, and pollution source tracing.
• Food, beverage, and pharmaceutical industries: incoming water qualification and process water quality control where color affects product appearance.
• Aquaculture and aquarium management: monitoring color as an indicator of dissolved organics and algal metabolites.
• Research and product development: accredited third-party laboratories provide color determination within broader water quality characterization programs for environmental impact assessments, remediation projects, and regulatory submissions.

FAQ

What is the difference between true color and apparent color? True color is the color contributed solely by dissolved substances after removal of suspended matter by 0.45 µm filtration, while apparent color is measured on the original sample and includes turbidity effects. Regulatory limits for drinking water generally refer to true color.

Which unit is used to express water color? Color is most commonly expressed in degrees on the platinum-cobalt (Hazen, PCU) scale, where one unit corresponds to the color produced by 1 mg/L platinum in the form of potassium chloroplatinate with cobalt chloride. Highly colored wastewaters may instead be reported as dilution degrees.

Why can colored water fail even when chemically clean? Color is primarily an aesthetic and sensory parameter; even harmless humic coloration can cause consumer rejection, increase disinfectant demand, and promote formation of disinfection by-products, which is why strict limits apply.

When should the dilution multiple method be used instead of platinum-cobalt colorimetry? The dilution multiple method is preferred when the sample chroma exceeds the range of visual comparison standards or when the hue is markedly different from the yellow platinum-cobalt standard, which is typical of dye and textile effluents.

How are turbidity and pH interferences handled? Turbidity is removed by settling, centrifugation, or membrane filtration before true color measurement, and pH is recorded and, where required by the standard, adjusted to a defined value before determination.

Can color be measured continuously online? Yes, online colorimeters using absorbance measurement at defined wavelengths enable real-time monitoring of raw water, treatment process streams, and final effluent, supporting rapid response to color excursions.

How long can samples be stored before color testing? Because color can change through biological or chemical reactions, analysis is recommended as soon as possible, ideally within a few hours of collection, and always within the holding time stipulated by the governing standard method.