Water Quality Colority Determination Experiment: Principles, Methods, and Technical Applications

Colority (chromaticity) is one of the most direct sensory indicators of water quality, revealing the presence of humic substances, dissolved metals, and industrial pollutants such as dyes and chemicals. This article provides a complete technical guide to the water colority determination experiment, covering the platinum-cobalt colorimetric method, the dilution multiple method, spectrophotometric procedures, relevant standards such as ISO 7887 and GB 11903, and practical applications in drinking water, wastewater, and industrial effluent testing.

Technical Overview

Colority, also known as chromaticity, describes the degree to which water departs from colorlessness and is produced by dissolved organic compounds, colloidal substances, and certain inorganic ions. Natural color in surface water mainly originates from humic and fulvic acids, tannins, iron and manganese compounds, while abnormal color in wastewater is typically caused by dyes, pigments, pulp and paper liquor, petroleum products, and chemical process effluents.

In water quality analysis, an essential distinction is made between two concepts:

• True color: the color of water after suspended matter (turbidity) has been removed by centrifugation or membrane filtration. It reflects only dissolved and colloidal color-producing substances.
• Apparent color: the color of the original, unclarified sample, which combines the effects of dissolved substances and suspended particles.

Colority is quantified in platinum-cobalt units (PCU), also called degrees (°) or Hazen units, defined by the color of a standard solution containing a specified concentration of chloroplatinate and cobalt salts. The water colority determination experiment is governed by several recognized standards, including ISO 7887 (Water quality — Examination and determination of colour), GB 11903 (Water quality — Determination of colority), ASTM D1209 (Standard Test Method for Color of Clear Liquids using the Platinum-Cobalt Scale), and Method 2120 of the Standard Methods for the Examination of Water and Wastewater published by APHA/AWWA/WEF.

Testing Samples

Drinking Water and Treated Water

Treated tap water, purified water, and packaged drinking water are routinely tested for colority as a routine sensory and treatment-performance indicator. Most national drinking water standards, including GB 5749, specify a colority limit of 15 degrees or less, with no other abnormal colors permitted.

Natural Surface Water and Groundwater

Rivers, lakes, reservoirs, and groundwater sources are monitored for colority to evaluate natural organic matter loading, seasonal variation, and the impact of upstream pollution. Elevated true color often signals humic substances that may increase disinfection by-product formation potential during treatment.

Industrial Wastewater and Process Effluents

Effluents from textile dyeing, printing and dyeing, pulp and papermaking, chemical synthesis, food processing, tanning, and pharmaceutical manufacturing frequently exhibit colority far above 50 degrees. These samples are usually tested using the dilution multiple method and checked against integrated wastewater discharge limits, which commonly range from 50 to 80 degrees depending on the discharge category.

Sample Collection and Preservation

• Collect samples in clean glass containers, fill completely, and seal without headspace to minimize oxidation.
• Refrigerate at approximately 4 °C during transport and analyze as soon as possible, ideally within 48 hours.
• Avoid chemical preservatives that may alter sample color; document pH and appearance on receipt.
• Use a representative sample volume of at least 100 mL to allow replicate measurements.

Testing Items

The core determination reported by this experiment is the colority of water, expressed in platinum-cobalt units (degrees). Depending on client requirements and regulatory context, the following items can be reported:

• True color (colority after turbidity removal) — the standard reporting basis for most regulatory comparisons.
• Apparent color (colority of the unfiltered original sample).
• Dilution multiple (expressed in "times") for highly colored industrial effluents exceeding the visual comparison range.
• Supporting observations such as visual description of the sample, pH, and turbidity to assist data interpretation.

Testing Methods

Platinum-Cobalt Standard Colorimetric Method

This is the reference method for clean or slightly colored water with colority below approximately 50 degrees. The principle is visual comparison of the sample against a graded series of platinum-cobalt standard solutions in matched 50 mL color-comparison tubes (Nessler tubes) against a white background.

• Stock standard preparation: dissolve 1.2456 g of potassium hexachloroplatinate(IV) (K2PtCl6) and 1.000 g of cobalt chloride hexahydrate (CoCl2·6H2O) in approximately 100 mL of hydrochloric acid and dilute to 1000 mL with distilled water. This stock solution corresponds to 500 degrees.
• Working standards are prepared by serial dilution, typically at 0, 5, 10, 15, 20, 25, 30, 40, 50, 60, and 70 degrees.
• The clarified sample is placed in a comparison tube, brought to the 50 mL mark, and viewed vertically from above alongside the standard series under uniform daylight or a standard light source.
• The reported result is the value of the standard most closely matching the sample; results between standards may be interpolated. The minimum detectable level of the method is approximately 5 degrees.

Dilution Multiple Method

For intensely colored industrial wastewater and heavily polluted surface water, the sample is diluted stepwise with colorless dilution water until the color becomes just indistinguishable from the dilution water when compared in the same tubes. The total dilution factor at that endpoint is reported as the colority in "times" (multiples). This approach is suitable for samples with colority well above 50 degrees, such as dyeing effluent, and aligns with the procedures described in GB 11903.

Spectrophotometric and Instrumental Methods

Instrumental approaches reduce operator subjectivity and improve data traceability. ISO 7887:2011 defines complementary procedures including visual comparison (Method A), absorbance measurement at the mercury-line wavelengths of 436 nm, 525 nm, and 620 nm using a 100 mm path-length cuvette (Method B), and a transmittance-based color determination (Method C). A calibration curve prepared from platinum-cobalt standards allows conversion of absorbance into degrees (PCU). Spectrophotometers should be wavelength-calibrated and zeroed with colorless dilution water before use.

Pretreatment for Turbidity Removal

Because suspended solids cause light scattering and bias color readings, true color measurement requires prior clarification: centrifugation of the sample or filtration through a 0.45 µm membrane filter is recommended. The filter must be pre-rinsed to avoid leaching color, and highly adsorptive filter media should be avoided when dissolved dyes are suspected. The pretreatment step must be recorded in the test report.

Quality Control Measures

• Verify the stock standard against certified reference material or freshly prepared solutions at defined intervals; store protected from light.
• Analyze reagent blanks and duplicate samples; visual comparisons should be performed by trained operators under consistent lighting.
• Use matched, scrupulously clean comparison tubes free of scratches and staining.
• Report the pretreatment applied (filtered/unfiltered), the method used, and the result with appropriate units to ensure regulatory comparability.

Applications

Colority determination data are applied across municipal, industrial, environmental, and research contexts:

• Drinking water treatment plants use colority to monitor coagulation, sedimentation, and activated carbon performance and to verify compliance with the 15-degree limit of drinking water standards.
• Wastewater treatment facilities track colority in influent and effluent to evaluate decolorization efficiency of biological, chemical, and adsorption processes.
• Textile, dyeing, papermaking, chemical, and food-processing industries use colority testing for process control, cleaner-production audits, and discharge permit compliance.
• Environmental monitoring agencies apply colority as a screening indicator for surface water pollution investigations and pollution-source tracing.
• Universities and accredited third-party laboratories use the experiment for environmental science education, method validation, and contract testing services.

FAQ

What is the difference between true color and apparent color?

True color is measured after suspended solids have been removed by centrifugation or 0.45 µm filtration and reflects dissolved and colloidal colorants. Apparent color is measured on the original sample and is influenced by both dissolved substances and particle scattering. Regulatory colority limits almost always refer to true color.

Why must turbidity be removed before measurement?

Suspended particles scatter and absorb light, which visually mimics or masks color and makes the sample appear darker or cloudier than its true color. Removing turbidity ensures the reading reflects chemically dissolved colorants and yields comparable, defensible results.

What is the detection range of the platinum-cobalt visual method?

The visual comparison method has a minimum detectable level of approximately 5 degrees and is best suited for samples up to about 50 degrees. Samples with higher colority should be tested by the dilution multiple method or measured instrumentally after appropriate dilution.

How should samples be preserved before colority testing?

Samples should be collected in glass containers, sealed without headspace, kept at approximately 4 °C, and analyzed within 48 hours. Avoid adding acids or other preservatives that may shift sample color, and always clarify the sample immediately before measurement.

Which standard should my laboratory follow?

Laboratories in China typically apply GB 11903 together with the GB 5749 drinking water limit, while international testing commonly follows ISO 7887:2011, ASTM D1209 for clear liquids on the platinum-cobalt scale, or Standard Methods 2120. The chosen standard and any pretreatment must be stated in the test report for data comparability.

Can colority be measured instrumentally instead of visually?

Yes. Spectrophotometric measurement of absorbance at 436 nm, 525 nm, and 620 nm using a 100 mm cuvette, calibrated with platinum-cobalt standards as described in ISO 7887, provides objective and traceable results and is particularly valuable for low-color drinking water where visual discrimination is difficult.