Colority (chromaticity) is a fundamental sensory and regulatory indicator of water quality, reflecting the presence of dissolved organic matter, metals, and industrial pollutants. This article presents a professional overview of water colority determination methods, including the platinum-cobalt colorimetric method, the dilution multiple method, and spectrophotometric techniques, together with the applicable ISO, ASTM, and GB standards. Whether you are verifying drinking water compliance or evaluating industrial wastewater discharge, understanding these testing approaches is essential for obtaining accurate and defensible results.
Technical Overview
Colority refers to the degree to which water exhibits coloration caused by dissolved substances, colloidal particles, or suspended matter. Natural water color typically originates from humic and fulvic acids, tannins, iron and manganese compounds, while industrial discharges may introduce dyes, lignin, phenolics, and colored organic chemicals. Colority is a required parameter in most drinking water and wastewater discharge regulations worldwide because it affects aesthetic acceptability and often correlates with the presence of chemical contaminants.
A key technical distinction in colority measurement is between true color and apparent color. True color is the color of water after turbidity has been removed, typically by filtration through a 0.45 micrometer membrane, and reflects only dissolved and colloidal colorants. Apparent color is measured on the unfiltered sample and includes the optical contribution of suspended solids. International and national standards, including ISO 7887, ISO 6271, ASTM D1209, and GB 11903, define the procedures, calibration scales, and reporting conventions used for colority determination.
The most widely applied techniques are the platinum-cobalt colorimetric method for clean, lightly colored water and the dilution multiple method for deeply colored wastewater. Spectrophotometric methods based on ISO 7887 provide an objective instrumental alternative. Accredited third-party laboratories routinely perform these tests for regulatory compliance, process control, and environmental monitoring purposes.
Testing Samples
Drinking Water and Treated Water
Tap water, bottled water, purified water, and finished water from treatment plants are the most common samples for colority testing. These waters are typically clear and lightly colored, making them well suited to the platinum-cobalt colorimetric method. Drinking water standards generally require colority not to exceed 15 degrees on the platinum-cobalt scale.
Surface Water and Groundwater
Rivers, lakes, reservoirs, and groundwater sources are monitored for colority as part of routine environmental quality assessment. Elevated colority in surface water often indicates humic substance loading, eutrophication, or upstream pollution, while colored groundwater may signal iron, manganese, or organic contamination. Samples containing visible turbidity should be filtered before true color determination.
Industrial and Municipal Wastewater
Effluents from textile dyeing, printing and papermaking, chemical synthesis, pharmaceuticals, tanning, food processing, and dye manufacturing frequently exhibit intense coloration that exceeds the range of visual comparison methods. These samples are analyzed using the dilution multiple method, in which the result is expressed as the number of times the sample must be diluted with clean water until the color becomes visually indistinguishable from the dilution water.
- Sample collection should use clean glass containers filled completely to exclude air.
- Samples should be stored at approximately 4 degrees Celsius in the dark.
- Analysis should be performed as soon as possible, preferably within 24 hours of sampling.
- Samples intended for true color measurement must be filtered through a 0.45 micrometer membrane prior to testing.
Testing Items
Colority testing services typically cover the following measurable items, each reported with its appropriate unit and method reference.
- True colority, expressed in degrees on the platinum-cobalt scale.
- Apparent colority of unfiltered samples, reported with a note on turbidity interference.
- Colority expressed as a dilution multiple for heavily colored wastewater.
- Spectral absorption coefficients at 436, 525, and 620 nanometers for spectrophotometric characterization.
- Color removal efficiency evaluation for decolorization treatment processes.
Testing Methods
Platinum-Cobalt Colorimetric Method
The platinum-cobalt method is the reference procedure for clean, lightly yellow-colored water and is specified in GB 11903, ISO 6271, and ASTM D1209. The principle is visual comparison of the sample against a graded series of platinum-cobalt color standards in matched 50 milliliter Nessler tubes, viewed vertically against a white background under adequate daylight-type illumination.
The stock standard solution is prepared by dissolving potassium chloroplatinate and cobalt chloride hexahydrate in hydrochloric acid and diluting to volume, producing a 500-degree color standard. Working standards covering the range of 0 to approximately 70 degrees are prepared by quantitative dilution. One degree of colority corresponds to the color produced by 1 milligram of platinum per liter in the presence of a defined cobalt concentration. The method detection capability typically extends down to about 5 degrees, and results are reported to the nearest whole degree. Turbidity must be removed before testing because suspended particles interfere with visual comparison.
Dilution Multiple Method
The dilution multiple method, specified in GB 11903, is designed for industrial wastewater and municipal effluent whose color exceeds the platinum-cobalt comparison range. The sample is progressively diluted with colorless dilution water, and each dilution is compared against the dilution water itself. The result is the dilution factor at which the color first becomes just imperceptible, reported as a dimensionless multiple. This method is simple, robust, and directly aligned with wastewater discharge standards that express chromaticity limits as dilution multiples.
Spectrophotometric Method
ISO 7887 defines an instrumental procedure in which a filtered sample is measured in a spectrophotometer using a 100 millimeter path length cuvette at wavelengths of 436, 525, and 620 nanometers. The results are expressed as spectral absorption coefficients, which objectively characterize color across the visible spectrum and enable precise trend monitoring. Instrumental variants of the platinum-cobalt method, in which absorbance is measured near 455 to 465 nanometers against a calibration curve of platinum-cobalt standards, are also widely used in accredited third-party laboratories because they reduce operator subjectivity and support digital record keeping.
Applications
Colority determination data support a broad range of regulatory, operational, and scientific applications across the water sector.
- Compliance verification of drinking water quality against national standards with a typical limit of 15 degrees platinum-cobalt colority.
- Wastewater discharge compliance testing against integrated discharge standards, where chromaticity limits are commonly set at dilution multiples of 50 or 80 depending on the discharge grade.
- Process control at water treatment plants, where coagulation, adsorption, and oxidation dosages are optimized based on color removal performance.
- Evaluation of decolorization technologies in textile, dyeing, and papermaking industries, including coagulation, activated carbon adsorption, and advanced oxidation processes.
- Environmental impact assessments, pollution source tracing, and surface water quality monitoring programs.
- Research and development of treatment materials and processes, where objective spectral absorption data quantify color removal efficiency.
FAQ
What is the difference between true color and apparent color?
True color is measured after removing turbidity, usually by filtration through a 0.45 micrometer membrane, and reflects only dissolved and colloidal colorants. Apparent color is measured on the original unfiltered sample and includes the optical effect of suspended particles. Regulatory limits for water supplies generally refer to true color, while wastewater assessments may report apparent color with appropriate qualification.
Which method should be used for deeply colored wastewater?
Heavily colored industrial wastewater that exceeds the visual comparison range should be tested by the dilution multiple method, where the result is reported as the number of dilutions required to reach visual colorlessness. This approach is specified in GB 11903 and aligns directly with discharge permit limits expressed as dilution multiples.
What are the typical regulatory limits for colority?
Drinking water standards commonly limit colority to 15 degrees on the platinum-cobalt scale, with stricter values of 5 degrees or lower applied to bottled and purified water. Wastewater discharge standards typically specify chromaticity limits of 50 for first-grade discharge and 80 for second-grade discharge expressed as dilution multiples, although exact values depend on the applicable jurisdiction and receiving water body.
How should samples for colority testing be collected and preserved?
Samples should be collected in clean glass containers, filled completely to minimize headspace, stored at approximately 4 degrees Celsius in the dark, and analyzed as soon as possible, preferably within 24 hours. Prolonged storage can cause biological or chemical changes that alter the color of the sample.
What interferences affect colority measurement?
Turbidity is the principal interference for visual comparison methods and must be removed by settling or filtration before true color determination. Instrumental measurements are additionally affected by cuvette cleanliness, wavelength accuracy, and the presence of bubbles. Using calibrated standards, matched glassware, controlled illumination, and validated procedures in accredited third-party laboratories minimizes these errors.