Chemical oxygen demand (COD) is one of the most important indicators of organic pollution in water, and UV spectrophotometry provides a rapid, reagent-free, and environmentally friendly alternative to the classical dichromate method. By measuring ultraviolet absorbance at 254 nm and applying a validated correlation, this technique delivers COD results in minutes and is ideally suited to both laboratory analysis and continuous online monitoring. This article explains the principle, applicable standards, sample types, testing procedure, quality control, applications, and frequently asked questions of COD determination by UV spectrophotometry.
Technical Overview
Chemical oxygen demand (COD) quantifies the mass of oxygen required to chemically oxidize the organic matter and reducible inorganic matter in a water sample, expressed in milligrams per liter (mg/L). It is a core composite index in environmental monitoring, wastewater treatment, and industrial water management because it directly reflects the level of organic pollution in a water body.
The classical determination method uses potassium dichromate oxidation under strongly acidic conditions with silver sulfate as a catalyst and mercury sulfate as a chloride masking agent. Although accurate, this approach has significant drawbacks: it requires a lengthy reflux digestion of approximately two hours, consumes toxic and expensive reagents containing hexavalent chromium, mercury, and silver, generates hazardous waste, and is difficult to automate for real-time monitoring.
The UV spectrophotometric method determines COD indirectly by measuring the absorbance of a water sample at a wavelength of 254 nm (UV254). Many natural and anthropogenic organic compounds, including humic and fulvic acids, aromatic hydrocarbons, phenols, proteins, and other molecules with unsaturated bonds, strongly absorb ultraviolet light in this region. According to the Beer-Lambert law, the absorbance at 254 nm is linearly correlated with the concentration of UV-absorbing organic matter and, within a defined water matrix, with the COD value. The method is established as a standard technique in China under HJ 970-2018 (trial) and is closely related to the spectral absorption coefficient (SAC254) defined in DIN 38404-3.
Method Principle
The water sample is filtered through a 0.45 micrometer membrane to remove suspended solids, and the filtrate is placed in a 10 mm quartz cuvette. The absorbance at 254 nm is measured against an ultrapure water reference on a UV-visible spectrophotometer. The COD concentration is then calculated using a site-specific conversion relationship, typically expressed as COD equals a conversion coefficient multiplied by the absorbance at 254 nm. For turbid or colored samples, dual-wavelength turbidity compensation, for example an additional measurement near 350 to 365 nm, can be applied. Results may also be reported as the spectral absorption coefficient SAC254 in units of inverse meters, as specified in DIN 38404-3, or converted to equivalent COD values using a validated correlation curve.
Key Advantages
- Extremely rapid analysis, typically completed in seconds to minutes, with no high-temperature digestion step.
- No consumption of hexavalent chromium, mercury salts, silver salts, or concentrated acids, eliminating secondary pollution and reducing operating costs.
- Simple operation with minimal sample and reagent handling, well suited to automation and continuous monitoring.
- Low detection capability that is appropriate for clean water matrices such as source water and groundwater.
- Excellent suitability for online sensors and early-warning systems at water intakes, treatment plants, and river monitoring stations.
Limitations and Interferences
- The method is indirect: only organic compounds that absorb at 254 nm are detected, so UV-transparent substances such as methanol, ethanol, sugars, and some low-molecular-weight organic acids may cause the COD to be underestimated.
- The correlation between absorbance and COD is matrix specific, so a conversion coefficient must be established and periodically verified for each water body or process stream.
- Turbidity, color, iron, nitrate, and other UV-absorbing species can interfere; membrane filtration and wavelength compensation are required.
- High-strength industrial wastewater is generally not directly suitable without dilution and rigorous validation against a reference method.
Testing Samples
Surface Water
Rivers, lakes, reservoirs, and estuarine waters are the primary application scenarios. Their organic load is typically dominated by humic substances that absorb strongly at 254 nm, so the correlation with COD is usually stable and reliable. COD levels in these matrices commonly fall below 30 to 60 mg/L, well within the working range of the method.
Groundwater
Groundwater features a low and stable organic background, making it highly suitable for UV measurement after filtration. The method is effective for tracking contamination events such as landfill leachate infiltration, petroleum spills, or sewage leakage into aquifers.
Drinking Water Sources and Treated Water
Raw water at treatment plant intakes, clarified, filtered, and finished water can be monitored continuously for UV254. This supports coagulant dosing optimization and tracking of disinfection by-product precursors, since UV254 correlates closely with natural organic matter reactivity.
Municipal Wastewater Effluent and Reclaimed Water
Secondary and tertiary effluent, as well as reclaimed water for reuse, can be screened rapidly by the UV method for process trending and compliance pre-screening, with periodic confirmation using the dichromate or fast digestion reference method.
Industrial Water Systems
Circulating cooling water, boiler make-up water, and general process water can be checked quickly for organic contamination events. Concentrated industrial wastewater with very high COD should be diluted appropriately or analyzed by reference oxidation methods.
Testing Items
Typical deliverables of a UV spectrophotometric COD testing program include:
- COD concentration determined by the UV absorption method, expressed in mg/L.
- UV254 absorbance measured with a 10 mm quartz cuvette, and the spectral absorption coefficient SAC254 expressed per meter.
- Development and validation of site-specific conversion coefficients linking UV254 to reference COD values.
- Comparative verification against dichromate-based COD determination according to HJ 828-2017, ISO 15705, or ASTM D1252.
- Correlation studies with related parameters such as TOC, DOC, and BOD5 for integrated water quality assessment.
- Data validation and performance assessment for online UV254 monitoring equipment.
Testing Methods
Applicable Standards
- HJ 970-2018, Water quality, Determination of chemical oxygen demand, UV absorption method (trial), the primary Chinese standard for this technique.
- DIN 38404-3, defining the measurement of the spectral absorption coefficient at 254 nm.
- HJ 828-2017, the dichromate reference method used for correlation development and result verification.
- ISO 15705 and ASTM D1252 as internationally recognized COD methods for comparative validation.
- National technical specifications for water quality sampling and preservation, such as the HJ 91 series and HJ 493.
Sample Collection and Pretreatment
Samples are collected in clean glass or polyethylene bottles, filled completely to exclude air, and analyzed as soon as possible. If immediate analysis is not feasible, samples should be stored refrigerated at approximately 4 degrees Celsius and protected from light, generally for no more than 24 to 48 hours. Acid preservation is normally avoided unless validated, because pH shifts can alter the UV absorbance of natural organic matter. Before measurement, each sample is filtered through a 0.45 micrometer membrane to remove suspended solids. Residual chlorine, if present, should be neutralized with sodium thiosulfate, and samples exceeding the working range must be diluted with ultrapure water.
Measurement Procedure
- Warm up the UV-visible spectrophotometer and verify wavelength accuracy and photometric stability at 254 nm.
- Rinse the 10 mm quartz cuvette with ultrapure water and zero the instrument using the ultrapure water reference.
- Rinse the cuvette with the filtered sample at least twice, then fill it and record the absorbance at 254 nm, along with the compensation wavelength if a dual-wavelength protocol is applied.
- Calculate the COD value using the established site-specific correlation coefficient, or read the result directly if the instrument has been pre-calibrated.
- Record sample temperature, pH, appearance, and any deviations, and clean the cuvette thoroughly between measurements.
Calibration and Correlation Development
For quantitative COD reporting, a correlation between UV254 absorbance and reference COD must be established for each specific water body. Paired measurements of UV254 and dichromate COD are performed on a statistically sufficient number of representative samples, typically at least twenty, covering the full concentration range of interest. Linear regression yields the conversion coefficient, and the correlation coefficient is required to meet the acceptance criteria of the applicable standard, commonly no less than 0.90 and preferably above 0.95 for regulatory use. Instrument performance can additionally be checked with potassium hydrogen phthalate standard solutions, and the correlation should be re-verified periodically or whenever the water matrix changes significantly, for example after seasonal shifts, rainfall events, or process modifications.
Quality Assurance and Quality Control
- Analyze a procedural blank of ultrapure water with every batch to monitor contamination and baseline drift.
- Run parallel duplicate samples and confirm that relative deviation is within method acceptance limits.
- Verify a fixed proportion of results, commonly around 10 percent, against the dichromate reference method.
- Perform calibration verification with certified or freshly prepared potassium hydrogen phthalate standards.
- Regularly inspect cuvette cleanliness, optical surfaces, stray light performance, and wavelength accuracy.
- Maintain control charts of absorbance and correlation coefficients to detect long-term drift and matrix changes.
Applications
Drinking Water Safety and Early Warning
Continuous UV254 monitoring at raw water intakes provides real-time warning of sudden organic pollution events, such as spills or algal blooms, allowing operators to adjust treatment rapidly and safeguard finished water quality.
Wastewater Treatment and Water Reclamation
Treatment plants use UV-based COD trending to evaluate the performance of biological units, detect shock loads, optimize aeration and chemical dosing, and screen effluent quality before discharge or reuse, reducing reliance on slow and hazardous wet-chemistry tests.
Environmental and Regulatory Monitoring
Automatic monitoring stations on rivers, lakes, and cross-boundary water bodies integrate UV254 sensors to deliver high-frequency organic pollution data that complement periodic laboratory COD analysis, improving spatial and temporal coverage of water quality networks.
Industrial Water and Process Management
Industries apply the method to circulating cooling systems, boiler feed water, and process streams for rapid detection of organic contamination, leakage, and fouling precursors, supporting predictive maintenance and water stewardship programs.
Emergency Response and Spill Investigation
The speed and simplicity of UV measurement make it a first-response tool during pollution incidents, enabling rapid screening of affected areas and guiding the collection of confirmatory samples for reference method analysis.
Research and Integrated Assessment
Researchers use UV254 and its COD equivalent to study natural organic matter characterization, disinfection by-product formation potential, stormwater impacts, and relationships among COD, BOD, and TOC in hydrological and ecological investigations.
FAQ
What is COD determination by UV spectrophotometry?
It is an indirect technique that measures the ultraviolet absorbance of a filtered water sample at 254 nm and converts the absorbance value into an equivalent COD concentration using a validated, matrix-specific correlation. It reflects the UV-absorbing fraction of organic pollution rather than the total chemically oxidizable matter.
How does it differ from the classical dichromate method?
The dichromate method chemically oxidizes nearly all reducible matter through a two-hour acidic digestion with toxic reagents, whereas the UV method is a direct optical measurement completed within minutes without hazardous chemicals. The dichromate technique remains the reference for regulatory compliance, while the UV approach excels in speed, safety, cost, and suitability for online monitoring.
Which types of water are suitable for this method?
The method is best suited to relatively clean and stable matrices such as surface water, groundwater, drinking water sources, and treated effluent, generally at COD levels up to approximately 60 mg/L. High-strength industrial wastewater requires dilution and rigorous correlation validation, or alternatively should be measured by dichromate or fast digestion spectrophotometric methods.
Why do samples need to be filtered before measurement?
Suspended particles scatter and absorb light, causing turbidity interference that distorts the absorbance reading at 254 nm. Filtration through a 0.45 micrometer membrane isolates the dissolved UV-absorbing fraction and substantially improves accuracy and repeatability. Online instruments address the same problem with optical turbidity compensation algorithms.
Does the method detect all organic compounds?
No. Substances that are transparent at 254 nm, such as methanol, ethanol, simple sugars, and certain small organic acids, contribute little to the absorbance signal. Consequently, the method should be applied to matrices where UV-absorbing organics dominate, and the correlation should be checked whenever the composition of the organic load may have changed.
How is the conversion coefficient between absorbance and COD established?
Paired UV254 and reference dichromate COD measurements are performed on at least twenty representative samples spanning the concentration range. Linear regression yields the conversion coefficient, which must then be verified periodically, typically seasonally or after significant changes in the water source or treatment process, to ensure continued accuracy.
Can the method be used for continuous online monitoring?
Yes. UV254 is one of the most widely deployed online water quality parameters. Modern probes use multi-wavelength optics, automatic cleaning, and turbidity compensation to deliver near-real-time surrogate COD data, which is valuable for process control and early warning when supported by periodic laboratory validation.
Where can UV spectrophotometric COD testing be performed?
Testing can be conducted by accredited third-party laboratories, environmental monitoring stations, and utility or industrial laboratories equipped with a UV-visible spectrophotometer, validated quartz cuvettes, and the necessary quality control systems. Cooperation between online monitoring operators and accredited laboratories is recommended for correlation development and data verification.
