Chemical Oxygen Demand (COD) Determination by UV-Visible Spectrophotometry

Chemical Oxygen Demand (COD) determination by UV-visible spectrophotometry is a rapid, high-throughput analytical technique for quantifying the oxygen-equivalent content of oxidizable organic matter in water and wastewater. Based on the sealed-tube dichromate digestion principle and standardized under frameworks such as ISO 15705 and HJ/T 399-2007, the method combines hot acid digestion with photometric detection to deliver accurate COD results in a fraction of the time required by conventional titration. This article provides a technical overview of the method, applicable sample types, testing items, procedural details, quality control requirements, and typical industrial and environmental applications.

Technical Overview

Chemical Oxygen Demand (COD) is a key aggregate pollution index that expresses the mass of oxygen, in milligrams per liter, required to chemically oxidize reducing substances, primarily organic compounds, in a water sample. The UV-visible spectrophotometric method determines COD by digesting the sample with a known excess of potassium dichromate in a strong sulfuric acid medium, catalyzed by silver sulfate, inside a sealed digestion tube. During digestion, oxidizable matter reduces orange hexavalent chromium (Cr(VI)) to green trivalent chromium (Cr(III)). The concentration of chromium species before and after digestion is then measured photometrically, and the change in absorbance is correlated to the COD value through a calibration curve.

Two measurement strategies are typically applied depending on the concentration range. In the high-range mode, the residual Cr(VI) is measured at approximately 440 nm, where the decrease in absorbance is proportional to the amount of oxidized matter. In the low-range mode, the Cr(III) produced is measured at approximately 600 nm, which offers higher sensitivity for samples with low pollution levels. This photometric approach is described in international and national standards including ISO 15705 (small-scale sealed-tube ST-COD method), HJ/T 399-2007 (fast digestion spectrophotometric method), and related colorimetric procedures referenced in ASTM D1252.

Compared with the classical reflux titration method specified in HJ 828-2017 and GB 11914, the spectrophotometric technique offers significant advantages, including shortened digestion time, reduced reagent consumption, smaller sample volume, lower operator exposure to hot concentrated acid, excellent batch-processing capability, and easy integration with automated or online monitoring systems. These characteristics make it the preferred routine method for laboratories processing large numbers of wastewater samples.

Testing Samples

Municipal and Domestic Wastewater

Influent, aeration basin mixed liquor, and final effluent from municipal wastewater treatment plants are routinely tested to evaluate treatment efficiency and verify compliance with discharge limits. Samples containing suspended solids are homogenized before subsampling to ensure representative results.

Industrial Wastewater

Effluents and process waters from petrochemical, chemical, pharmaceutical, food and beverage, pulp and paper, textile dyeing, fermentation, and electroplating industries are analyzed. High-strength industrial samples are typically diluted into the calibrated working range before digestion.

Surface Water and Groundwater

Rivers, lakes, reservoirs, and groundwater samples with relatively low pollution levels can be assessed using the low-range measurement mode, supporting surface water quality classification and contamination screening programs.

Cooling, Circulating, and Process Waters

Industrial cooling towers, boiler feed systems, and recirculating process streams are monitored for organic contamination, fouling potential, and treatment chemical performance.

Testing Items

The core testing item is the COD value, expressed in mg/L of oxygen, determined as CODCr under dichromate digestion conditions. Typical calibrated working ranges are:

• Low range: approximately 10 to 150 mg/L, measured via Cr(III) absorbance at about 600 nm, suitable for treated effluent and surface water.
• High range: approximately 100 to 1,000 mg/L, measured via residual Cr(VI) absorbance at about 440 nm, suitable for raw wastewater.
• Extended range: samples above the calibrated range are quantified after appropriate dilution, allowing determination up to several thousand mg/L with demonstrated dilution integrity.
• Method detection limit: typically on the order of 10 mg/L or lower, depending on instrument configuration, cuvette path length, and the governing standard.

Testing Methods

Sample Collection and Preservation

Samples are collected in glass containers and homogenized if particulate matter is present. They are acidified with concentrated sulfuric acid to pH ≤ 2 to inhibit microbial degradation, stored refrigerated at approximately 4 °C, and analyzed as soon as practicable, commonly within 7 to 28 days depending on the applicable standard. Residual chlorine, if present, is removed with an appropriate reducing agent prior to preservation.

Digestion Procedure

A measured volume of homogenized sample is added to a ready-made or laboratory-prepared digestion reagent containing potassium dichromate, sulfuric acid, silver sulfate catalyst, and mercuric sulfate. Mercuric sulfate complexes chloride ions and suppresses their positive interference for chloride concentrations up to approximately 1,000 mg/L. The sealed tube is heated in a digester or block heater at a controlled temperature of 150 to 165 °C, typically for 15 minutes to 2 hours as specified by the selected protocol, then cooled to room temperature before measurement.

Spectrophotometric Measurement

The cooled digest is measured in the UV-visible spectrophotometer at the wavelength corresponding to the selected range: approximately 440 nm for the high range based on residual Cr(VI), or approximately 600 nm for the low range based on generated Cr(III). Pre-measured commercial reaction tubes may be read directly in the instrument; laboratory-prepared digests are transferred to matched cuvettes. Turbid digests are allowed to settle or centrifuged before reading.

Calibration and Quantification

The instrument is calibrated using COD standard solutions prepared from potassium hydrogen phthalate (KHP) traceable to certified reference materials, covering at least five concentration points across the working range. A linear regression of absorbance versus concentration is established, with a correlation coefficient typically required to be at least 0.999. Sample COD values are calculated from the regression equation and adjusted for any dilution factor.

Quality Control and Interference Control

Each analytical batch includes a method blank, calibration verification standards, duplicate samples, and matrix spike recoveries. Acceptance criteria commonly require duplicate relative percent deviation within 10 percent and spike recovery between 90 and 110 percent. Known interferences and their controls include:

• Chloride above approximately 1,000 mg/L consumes dichromate and causes a positive bias; it is controlled by mercuric sulfate complexation, dilution, or alternative validated procedures.
• Nitrite exerts a small positive demand and may be corrected or masked where significant concentrations are expected.
• Turbidity and color are largely eliminated by the hot acid digestion and settling of the digest.
• Aromatic hydrocarbons and pyridine-type compounds are only partially oxidized by dichromate and may be under-represented relative to theoretical oxygen demand.

Applications

UV-visible spectrophotometric COD determination is widely applied across environmental management and industrial operations, including:

• Compliance monitoring of wastewater discharge against national and local effluent limits, including requirements referenced in GB 8978 and GB 18918.
• Surface water and groundwater quality assessment in support of environmental quality standards such as GB 3838.
• Process control at wastewater treatment plants, including aeration optimization, organic loading evaluation, and effluent stability tracking.
• Industrial pollution source tracing, pretreatment assessment, and cleaner production audits.
• Environmental impact assessments, pollution remediation verification, and academic or contractual research projects.
• Validation and cross-checking of online COD analyzers installed at discharge outfalls.

Accredited third-party laboratories routinely perform this method under ISO/IEC 17025 management systems, providing legally defensible data for regulatory reporting, litigation support, and supply chain environmental verification.

FAQ

What is the difference between the spectrophotometric COD method and the classical titration method?

Both methods use hot dichromate oxidation. The titration method, described in HJ 828-2017 and GB 11914, requires a two-hour open reflux followed by ferrous ammonium sulfate titration and is often used as the reference or arbitration method. The spectrophotometric method uses sealed-tube digestion with photometric endpoint detection, which is faster, generates less waste, and supports high-throughput batch analysis with equivalent accuracy when properly validated.

How is chloride interference managed in this method?

Mercuric sulfate in the digestion reagent complexes chloride ions and suppresses their oxidation by dichromate, effectively tolerating chloride up to about 1,000 mg/L. Samples with higher salinity, such as seawater or certain brines, must be diluted, corrected, or analyzed by a chloride-insensitive alternative method after validation.

How should COD samples be preserved before analysis?

Samples should be acidified with sulfuric acid to pH ≤ 2 immediately after collection, stored at approximately 4 °C in full glass containers, and analyzed promptly, typically within 7 to 28 days depending on the governing standard.

Can the spectrophotometric method be used for regulatory reporting?

Yes. When performed according to recognized standards such as ISO 15705 or HJ/T 399-2007 within a validated quality system, the results are acceptable for routine regulatory monitoring. For disputed results, the classical dichromate titration method is generally designated as the arbitration procedure.

What concentration ranges can be measured?

Typical calibrated ranges are 10 to 150 mg/L in the low-range mode and 100 to 1,000 mg/L in the high-range mode, with higher concentrations determined accurately after validated dilution. Selection of the appropriate range prevents large extrapolation errors.