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Chemical Oxygen Demand (COD): Assessing Pollution Levels in Water Samples

Chemical Oxygen Demand (COD) is one of the most widely monitored parameters in environmental analysis. It provides a fast and reliable indication of the amount of organic and inorganic pollutants present in water samples, supporting laboratories, industries, and environmental agencies in evaluating treatment efficiency and regulatory compliance.

The Meaning of COD - Chemical Oxygen Demand

COD represents the amount of oxygen needed to chemically oxidize organic matter and certain inorganic compounds present in water. It is one of the most common indicators of contamination because it quantifies the overall oxidizable load in a single, fast measurement: the higher the COD, the greater the concentration of oxidizable substances.

Elevated COD values reflect the presence of pollutants that can reduce dissolved oxygen in natural waters, threatening aquatic ecosystems and indicating inefficiencies in treatment processes.

For this reason, COD determination is widely applied in wastewater treatment, environmental monitoring, food and beverage production, and chemical manufacturing. In these contexts, COD is used to track influent and effluent quality, support regulatory compliance, evaluate the effectiveness of purification steps, and monitor pollution trends over time. Its broad applicability and rapid execution make it a practical parameter for both routine analysis and process optimization.

How to Determine COD in Water Samples

How to Determine COD in Water Samples
The COD test is based on the chemical oxidation of the sample using a strong oxidizing agent (typically potassium dichromate) in the presence of sulfuric acid and a silver catalyst. The mixture is heated at a fixed temperature, usually 160 °C, for a defined digestion time to ensure complete oxidation of the oxidizable material. This process is commonly performed in sealed glass vials for safety, consistency, and ease of handling.

After digestion, the amount of dichromate that has not reacted with the sample is measured. In colorimetric methods, the change in color intensity correlates with the COD value, while titrimetric methods quantify the remaining oxidant through a precise titration step. The final result is expressed in mg O₂/L.

Reliable COD determination requires precise temperature control, uniform heating, correct reagent handling, and adherence to standardized methods such as ST-COD, ISO 15705, EPA 410.4, and IRSA–CNR No. 5130.

Solutions for COD Sample Digestion

Efficient COD analysis begins with accurate and reproducible sample digestion. Temperature stability, uniform heating, and safe reagent handling are essential to obtain reliable results, regardless of whether the lab performs colorimetric or titrimetric determinations. Velp offers two different solutions, designed to support different workflows, test-tube types, and regulatory requirements.

Dry Block Heaters for COD Analysis

COD digestion requires highly stable heating conditions, as even slight temperature fluctuations can compromise oxidation efficiency and overall accuracy. The ECODryBlock is designed to ensure controlled, uniform heating across all positions, reaching up to 165 °C and maintaining it with an homogeneity and stability of ±0.5 °C.

This makes it ideal for environmental laboratories working with diverse water matrices. In addition, programmable temperature ramps allow the system to reach the target temperature smoothly, ensuring reliable digestion.

Micro Digestion Method

Micro Digestion Method
For laboratories using the micro digestion approach, the ECODryBlock system (SB101A0700) with the COD-W24-D160-A Block (A00000494) accommodates 24 vials of 16 mm diameter, working with reduced sample and reagent volumes. 

An alternative setup uses the COD-W20-COMBO Block (A00000530), which offers greater flexibility by hosting 18 vials of 16 mm and 2 vials of 22 mm in the same unit.

In both options, a transparent insulating lid, included in the system, promotes even heat distribution and supports stable digestion conditions across all positions.

Test Tubes Digestion Method

Test Tubes Digestion Method
For laboratories following the IRSA–CNR No. 5130 procedure, the ECODryBlock system (SC101A0700), with the COD-W04-D420-A Block (A00000529), allows simultaneous digestion of four large-volume samples (200 mL).

Beyond COD, this setup can also be used for wet digestion procedures aimed at determining metallic and nonmetallic elements in a variety of organic and inorganic matrices, including environmental samples, soils, sediments, feed, alloys, and fabrics.

Required accessories:
  • COD test tubes Ø 42x200 mm, 200 ml with cone NS 29/32 (A00000145)
  • Sample rack (A00000532)
  • Air condenser with ground cone (A00001041)
  • PTFE sheaths for 29/32 cones (A00001042)
  • Antisplash bell (A00001045)

Digestion Units for COD Analysis

Digestion Units for COD Analysis
Laboratories performing test-tube COD procedures and other wet digestion applications in environmental, agricultural, industrial, or food testing can rely on the DK 6, DK 20 (semi-automatic), and DKL 20 (automatic) digestion units. 

These systems offer uniform heating, long-term temperature stability, programmable temperature ramps, and can be configured specifically for routine COD workflows. For higher throughput and reduced operator workload, the DKL model introduces automation to the digestion step.

Required accessories:
  • COD test tubes Ø 42x200 mm, 200 ml with cone NS 29/32 (A00000145)
  • Sample rack for COD (A00000408 or A00000237)
  • Air condenser with ground cone (A00001041)
  • PTFE sheaths for 29/32 cones (A00001042)
  • Antisplash bell (A00001045)

Ready to expand your lab’s analytical capabilities?

Contact our specialists to discover how Velp solutions can support and optimize your COD analysis workflow!

 
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