Determination of Chromaticity in Surface Water: Principles, Standards, and Testing Methods

Surface water chromaticity (color) determination is a fundamental parameter in water quality assessment, providing valuable insight into the presence of dissolved organic matter, metals, and anthropogenic pollutants. This article offers a comprehensive technical overview of surface water color testing, covering applicable standards such as ISO 7887 and GB 11903, sampling requirements, the platinum-cobalt colorimetric method, dilution multiple techniques, and practical applications in environmental monitoring and treatment process control.

Technical Overview

Chromaticity, or color, is one of the most sensory indicators of surface water quality and serves as an early warning signal for contamination. The determination of color in surface water is primarily governed by the national standard GB 11903 (Water Quality — Determination of Colority) and the international standard ISO 7887 (Water Quality — Examination and Determination of Colour), with ASTM D1209 (Standard Test Method for Color of Clear Liquids Using the Platinum-Cobalt Scale) providing a complementary reference framework.

A critical technical distinction in color measurement is the difference between true color and apparent color:

• True color: the color of water after removal of suspended solids and turbidity, typically by centrifugation or membrane filtration. This reflects dissolved color-causing substances such as humic acids, fulvic acids, tannins, iron, and manganese.
• Apparent color: the color of the original, unfiltered water sample, which includes the optical contribution of suspended particulate matter.

Color in surface water originates from both natural and anthropogenic sources. Naturally occurring color results from the decomposition of vegetative matter (producing humic and fulvic compounds), geological leaching of iron and manganese, and algal proliferation. Anthropogenic sources include textile dye effluents, pulp and paper mill discharges, tannery wastewater, and runoff from agricultural and urban areas. Elevated color levels reduce light penetration in aquatic ecosystems, affect photosynthesis, and frequently correlate with elevated chemical oxygen demand (COD) and disinfection by-product precursors.

Color is typically expressed in degrees (°), equivalent to Hazen units or platinum-cobalt (Pt-Co) color units, where one degree corresponds to the color produced by 1 mg/L of platinum in the form of potassium chloroplatinate. Under GB 3838 (Environmental Quality Standards for Surface Water), color serves as a supplementary evaluation parameter, while GB 5749 (Standards for Drinking Water Quality) sets a limit of 15 degrees for true color at the tap.

Testing Samples

Rivers and Streams

Representative grab or composite samples are collected from flowing sections with adequate mixing. Sampling points are selected upstream and downstream of suspected pollution sources to characterize spatial color gradients. Samples are collected in clean glass containers, filled completely to exclude air, and stored at 4 °C.

Lakes and Reservoirs

Water bodies with stratification may require depth-profile sampling using Van Dorn or Niskin samplers. Algal blooms and seasonal turnover significantly influence color, so sampling frequency is typically increased during summer months and after rainfall events.

Estuarine and Brackish Waters

Color determination in estuarine environments must account for tidal dilution and salinity-driven flocculation of humic substances. True color measurement after filtration is strongly recommended to eliminate the strong interference from suspended sediment.

Drinking Water Sources

Raw water intake points are monitored regularly to optimize coagulation, adsorption, and oxidation treatment processes. Source water with true color exceeding 15–20 degrees generally requires enhanced treatment to meet potable water standards.

Receiving Waters of Industrial Discharges

Waters downstream of industrial outfalls, particularly from textile, dyeing, papermaking, and chemical manufacturing facilities, are tested to verify compliance with discharge permits and to assess environmental impact.

Testing Items

The core testing items for surface water chromaticity determination include:

• True color: measured after centrifugation at 3,000 rpm for 30 minutes or filtration through a 0.45 μm membrane filter, reported in degrees (Pt-Co scale).
• Apparent color: measured on the original unfiltered sample, reported in degrees, used for rapid field assessment.
• Dilution multiple: reported for intensely colored samples where color exceeds the visual comparison range, expressed as the dilution factor at which color becomes just indistinguishable from the blank.
• Supporting parameters: turbidity, suspended solids, COD, UV254 absorbance, iron and manganese content, and pH, which assist in identifying color origin and interpreting results.

Testing Methods

Platinum-Cobalt Standard Colorimetric Method

This is the reference method specified in GB 11903 for clean or lightly colored surface water. A stock chromaticity standard is prepared by dissolving 1.246 g of potassium chloroplatinate (K2PtCl6) and 1.000 g of cobalt chloride hexahydrate (CoCl2·6H2O) in distilled water with 100 mL of hydrochloric acid, diluted to 1,000 mL, yielding a 500-degree standard. A graded series of working standards (0, 5, 10, 15, 20, 25, 30, 35, 40, 50, 60, and 70 degrees) is prepared by dilution. The sample is visually compared against the standard series in 50 mL Nessler tubes against a white background under daylight or standardized illumination. The result is reported as the matching standard concentration, with a detection range suitable for true color values up to 70 degrees.

Dilution Multiple Method

For heavily colored surface water samples exceeding the platinum-cobalt comparison range, the dilution multiple method is applied. The sample is serially diluted with colorless distilled water until the color becomes just perceptible compared with the dilution blank in Nessler tubes. The dilution factor at this endpoint is reported as the color value. This method is particularly useful for receiving waters impacted by dye-laden industrial effluents and provides a practical index for pollution tracking and treatment efficiency evaluation.

Spectrophotometric Method

ISO 7887 specifies an instrumental approach in which the true color of the filtered sample is determined by measuring spectral absorbance at 436 nm, 525 nm, and 620 nm using a 50 mm or 100 mm optical path length cuvette. The results may be expressed as spectral color coefficients or converted to equivalent Pt-Co values. Instrumental methods eliminate operator subjectivity, improve reproducibility, and enable continuous online monitoring when implemented with flow-through probes.

Sample Preparation and Interference Control

Proper pretreatment is essential for accurate color determination:

• Turbidity interference: suspended matter is removed by centrifugation or membrane filtration; filtration is avoided when the filter may leach or adsorb color.
• Sample preservation: samples are collected in glass containers, protected from light, refrigerated at 4 °C, and analyzed as soon as possible, preferably within 48 hours.
• pH effects: since humic color intensity varies with pH, the pH of samples and standards should be consistent; record pH alongside color results.
• Temperature: comparisons should be performed at room temperature, as temperature affects both color perception and standard stability.

Quality Assurance and Quality Control

Reliable results depend on rigorous QA/QC practices, including the use of freshly prepared or commercially certified Pt-Co standards, procedural blanks, duplicate analyses with acceptance criteria of relative deviation within 10%, periodic verification of Nessler tube optical uniformity, participation in interlaboratory proficiency testing, and validation of visual observers or replacement with spectrophotometric measurement where objective quantification is required. Accredited third-party laboratories routinely apply these controls under ISO/IEC 17025 management systems.

Applications

Surface water chromaticity determination supports a wide range of environmental and industrial applications:

• Environmental monitoring: routine surveillance of rivers, lakes, and reservoirs under national surface water quality programs, enabling trend analysis and pollution source tracing.
• Drinking water treatment: characterization of raw water color to design and optimize coagulation-flocculation, activated carbon adsorption, and advanced oxidation processes.
• Wastewater compliance verification: confirmation that treated effluent meets color-related discharge requirements before release into receiving surface waters.
• Ecological assessment: evaluation of light attenuation effects on primary productivity and aquatic habitat quality in lakes and slow-moving rivers.
• Industrial process control: monitoring of process water and recycled water in pulp and paper, textile, and food and beverage operations.
• Environmental impact assessment: baseline color characterization and post-project verification for construction and industrial developments near water bodies.

FAQ

What is the difference between true color and apparent color in surface water testing?

True color is measured after suspended solids are removed by centrifugation or filtration and reflects only dissolved color-causing substances. Apparent color is measured on the original, unfiltered sample and includes the optical effect of turbidity. Regulatory limits for drinking water apply to true color.

Which standards govern surface water color determination?

The principal standards are GB 11903, which specifies the platinum-cobalt comparison method and the dilution multiple method; ISO 7887, which specifies visual examination, spectrophotometric, and optical filter procedures; and ASTM D1209, which defines the platinum-cobalt scale for clear liquids.

What units are used to express water color?

Color is expressed in degrees (°), equivalent to platinum-cobalt units or Hazen units, where one degree equals the color of a solution containing 1 mg/L platinum as chloroplatinate with cobalt chloride. For highly colored samples, results are reported as dilution multiples.

How are turbid surface water samples handled?

Turbidity is removed by centrifugation or 0.45 μm membrane filtration prior to true color measurement. Because filtration can alter dissolved color in some waters, centrifugation is often preferred when compatible with the analytical protocol.

What is the recommended sample holding time for color determination?

Samples should be collected in glass containers, stored in the dark at 4 °C, and analyzed within 48 hours. Biological activity and chemical changes in stored samples can cause color drift, so prompt analysis is strongly recommended.

Why is surface water color testing important?

Color is a rapid, sensitive indicator of organic pollution, metal contamination, and algal activity. Elevated color affects aesthetics, light penetration, treatment chemical demand, and the formation of disinfection by-products, making it a key parameter in integrated water quality management.