Determination of Chemical Oxygen Demand (COD) by Spectrophotometry: Principle, Procedure, and Applic

Chemical Oxygen Demand (COD) is a core indicator of the organic pollution load in water and wastewater, and spectrophotometry has become one of the most efficient techniques for its determination. By combining sealed-tube dichromate digestion with photometric detection, the method delivers rapid, sensitive, and reproducible COD measurements across a wide concentration range. This article outlines the underlying principle, applicable sample types, testing procedures, relevant standards, quality control requirements, and typical applications of the spectrophotometric COD method.

Technical Overview

Chemical Oxygen Demand (COD) is defined as the mass concentration of oxygen, expressed in milligrams per liter (mg/L), that is equivalent to the amount of oxidizable matter in a water sample consumed during oxidation by a strong chemical oxidant under specified acidic and heated conditions. Because most organic compounds and certain inorganic reducing species are oxidized almost completely under these conditions, COD serves as a comprehensive index of the pollution load in domestic sewage, industrial effluents, and natural waters, and it is one of the most frequently monitored parameters in environmental regulation worldwide.

The spectrophotometric determination of COD is based on the classical potassium dichromate oxidation chemistry. A measured aliquot of sample is digested with a known excess of potassium dichromate (K2Cr2O7) in a concentrated sulfuric acid medium. Silver sulfate (Ag2SO4) is added as a catalyst to ensure efficient oxidation of straight-chain aliphatic compounds, while mercury sulfate (HgSO4) is used to complex chloride ions and suppress their interference. As the digestion proceeds, hexavalent chromium, Cr(VI), is reduced to trivalent chromium, Cr(III), in a quantity proportional to the amount of oxidizable matter present in the sample.

After digestion, the concentration of chromium species is determined photometrically in accordance with the Beer–Lambert law. For higher COD concentrations, the absorbance of the green Cr(III) produced is measured at approximately 600 nm; for lower concentrations, where the change in Cr(VI) can be quantified more sensitively, the decrease in absorbance of the orange dichromate ion is measured at approximately 440 nm. The COD value is then read directly from a calibration curve prepared with standard solutions of potassium hydrogen phthalate (KHP).

Relevant Standards

• ISO 15705 – Water quality, determination of the chemical oxygen index, small-scale sealed-tube method.
• HJ/T 399-2007 – Water quality, determination of the chemical oxygen demand by the fast digestion–spectrophotometric method.
• GB 11914-89 – Water quality, determination of the chemical oxygen demand by the dichromate titration method, commonly used as the reference method for comparison.
• ASTM D1252 – Standard test methods for chemical oxygen demand of water, including a colorimetric procedure (Method C).
• EPA Method 410.4 – Colorimetric determination of COD following digestion.

Advantages Over the Classical Titration Method

• Significantly reduced consumption of reagents and sample volume, which lowers analytical cost and minimizes hazardous mercury- and chromium-bearing waste.
• Sealed-tube digestion prevents loss of volatile organic compounds and eliminates acid fumes in the laboratory environment.
• Shorter digestion time and simplified endpoint measurement, enabling high-throughput batch analysis of dozens of samples simultaneously.
• Digital photometric readout removes the operator-dependent endpoint judgment associated with visual titration, improving precision and reproducibility.
• The technique is readily adapted to benchtop photometers, dedicated COD analyzers, and automated on-line monitoring systems.

Testing Samples

The spectrophotometric COD method is applicable to a broad spectrum of aqueous samples. Correct sampling, homogenization, and preservation are prerequisites for valid results, because COD is defined on a whole-sample basis.

Surface Water and Groundwater

Rivers, lakes, reservoirs, and groundwater typically exhibit low to moderate COD levels. The low-range photometric measurement at 440 nm provides adequate sensitivity for these matrices, and the results support water quality classification and pollution source tracing.

Domestic and Municipal Wastewater

Raw sewage, primary and secondary clarifier streams, and final effluents from municipal treatment plants are routinely monitored. COD data are used to characterize influent organic loading, evaluate treatment efficiency, and verify compliance with discharge limits.

Industrial Wastewater

Effluents from petrochemical, pharmaceutical, pulp and paper, printing and dyeing, food and beverage, fermentation, tannery, and chemical manufacturing industries often contain high organic loads. These samples are usually diluted to fall within the working range, and matrix effects are verified through spike recovery tests.

High-Chloride Samples

Marine waters, brines, and certain industrial effluents contain elevated chloride concentrations, which consume dichromate and cause a positive bias. Such samples can still be analyzed, provided that mercury sulfate is added at the required ratio or that the sample is appropriately diluted.

Testing Items

The single reported test item is the chemical oxygen demand, expressed in mg/L of oxygen (O2). Typical performance characteristics of the spectrophotometric method are as follows:

• Working range without dilution of approximately 15 to 1000 mg/L, depending on the selected wavelength and optical configuration.
• Extension to higher concentrations, for example several thousand mg/L, through quantitative sample dilution.
• Method detection limits on the order of 10 to 15 mg/L for standard 2 mL test portions.
• Relative standard deviations generally within 10 percent for homogeneous samples, depending on the matrix and concentration level.

Results are compared against applicable discharge or environmental quality standards, and the method reports total COD, including both dissolved and particulate oxidizable matter.

Testing Methods

Reagents and Equipment

• Potassium dichromate standard solution of defined concentration.
• Sulfuric acid–silver sulfate solution as the catalytic digestion medium.
• Mercury sulfate for chloride masking.
• Potassium hydrogen phthalate (KHP) for the preparation of calibration standards.
• Sealed digestion tubes or ready-to-use reagent cuvettes.
• Thermostatically controlled digestion reactor or block heater.
• Spectrophotometer or dedicated COD photometer capable of measurement at 440 nm and 600 nm, matched to the tube path length.

Sample Preparation and Preservation

Samples are collected in glass containers, homogenized in a blender if they contain suspended solids or floating oil, and preserved by acidification with concentrated sulfuric acid to pH 2 or below. Preserved samples are refrigerated at approximately 4 °C and analyzed as soon as practicable, in accordance with the holding times specified by the applicable standard or regulatory framework.

Digestion Procedure

A homogeneous aliquot, typically 2.0 mL, is transferred into the digestion tube, and the dichromate reagent and sulfuric acid–silver sulfate solution are added in the prescribed sequence and volumes. The tube is capped tightly, inverted several times to mix, and heated in the digestion reactor. Commonly specified conditions include 148 °C ± 3 °C for 120 minutes under ISO 15705 or 165 °C ± 2 °C for 15 minutes under HJ/T 399-2007. After heating, the tube is removed, cooled to room temperature, and inspected; any turbidity or precipitation indicates an invalid digestion requiring repeat analysis of a diluted aliquot.

Calibration and Photometric Measurement

A calibration curve is established using at least five concentration levels of KHP standard processed through the complete digestion procedure together with a reagent blank. The digested standards and samples are wiped clean and placed in the spectrophotometer, and absorbance is measured at 600 nm for the high range or 440 nm for the low range. COD concentrations of the samples are obtained from the regression equation of the calibration curve, with dilution factors applied where necessary. The correlation coefficient of the calibration curve is typically required to be 0.999 or better.

Quality Control

• A reagent blank is digested and measured with every analytical batch.
• Duplicate analyses are performed at a defined frequency, with the relative percent deviation within the stated acceptance criteria.
• Matrix spike recoveries, commonly required to fall between 80 and 120 percent, verify the absence of matrix effects.
• Certified reference materials or independent standard solutions are analyzed to confirm accuracy and calibration validity.
• Calibration verification standards are measured periodically during extended sample sequences.

Interferences and Countermeasures

• Chloride ions are oxidized by dichromate and produce a positive bias; mercury sulfate is added at approximately a 10:1 mass ratio to chloride to complex it, permitting samples with chloride up to about 1000 mg/L to be analyzed. More saline samples require dilution.
• Nitrite and other inorganic reductants such as ferrous iron and sulfide contribute to the apparent COD; where significant, their contribution is evaluated or chemically suppressed, for example with sulfamic acid for nitrite.
• Aromatic hydrocarbons, pyridine, and related compounds are not oxidized quantitatively and may yield low results; this is an inherent characteristic of dichromate chemistry shared with the reference titration method.
• Color and turbidity of the original sample are largely destroyed or masked during digestion, but strongly colored digests or particulate carryover invalidate the photometric reading and necessitate repetition.

Applications

• Discharge compliance monitoring of municipal and industrial effluents against statutory COD limits.
• Process control at wastewater treatment plants, including influent loading assessment, aeration optimization, and evaluation of biological treatment performance.
• Water quality surveys of rivers, lakes, and receiving water bodies for environmental impact assessment and pollution source identification.
• Sector-specific monitoring in petrochemical, pharmaceutical, food and beverage, pulp and paper, textile, and chemical production facilities.
• Analysis of landfill leachate, anaerobic digestion liquor, and other high-strength waste streams requiring dilution-based measurement.
• Support for research and engineering studies, including treatability tests and correlation between COD and biochemical oxygen demand (BOD).
• Integration into automated on-line COD monitoring stations for continuous regulatory and process surveillance.

FAQ

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

Both methods use the same dichromate oxidation chemistry, but the spectrophotometric method quantifies the digested chromium species photometrically instead of titrating the residual dichromate with ferrous ammonium sulfate. It consumes smaller reagent volumes, avoids open reflux, supports batch analysis, and eliminates subjective endpoint judgment, while remaining statistically comparable to titration for most routine matrices.

What concentration range can be measured?

Typical sealed-tube photometric systems cover approximately 15 to 1000 mg/L without dilution, using the 600 nm measurement for higher concentrations and the 440 nm measurement for lower concentrations. Samples above the range are quantitatively diluted and re-analyzed, allowing the determination of very high strength wastes.

How is chloride interference handled?

Mercury sulfate is included in the digestion system to complex chloride ions, generally allowing samples containing up to about 1000 mg/L of chloride to be analyzed without significant bias. For more saline matrices, dilution reduces the chloride below the tolerance threshold, and the dilution factor is applied to the final result.

Can turbid or colored samples be analyzed?

Yes. Samples are homogenized before subsampling, and the strong acid digestion oxidizes or decolorizes most natural color and organic turbidity. However, if the digestate is cloudy or contains a precipitate, the photometric reading is invalid and the analysis must be repeated with an appropriate dilution.

How should samples be preserved before testing?

Samples should be acidified with sulfuric acid to pH 2 or below, stored at approximately 4 °C in tightly sealed glass containers, and analyzed as soon as possible within the holding time prescribed by the governing standard or regulatory program.

Are spectrophotometric COD results acceptable for regulatory reporting?

When the method is performed in accordance with recognized standards such as ISO 15705, HJ/T 399-2007, or ASTM D1252 within accredited third-party laboratories or properly validated in-house laboratories, photometric COD results are widely accepted for compliance and reporting purposes. In some jurisdictions, the classical titration method may be stipulated as the referee method for dispute resolution, and photometric results are validated against it through periodic correlation studies.

Why might spectrophotometric results differ slightly from titration results?

Small discrepancies can arise from differences in digestion temperature and time, chloride masking efficiency, sample homogeneity, dilution error, and matrix effects at extreme concentration levels. Consistent use of calibrated photometers, fresh standards, and rigorous quality control minimizes such differences, and inter-method comparisons typically agree within about 10 percent for well-characterized samples.