By Jim Skelter – Updated August 30, 2022

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COD (Chemical Oxygen Demand) quantifies the amount of oxidizable organic matter in water. The standard method involves boiling the sample for two hours with potassium dichromate, then titrating the remaining oxidant with ferrous ammonium sulfate. A high COD value signals elevated organic pollution.

Why COD Matters

Regulators and treatment facilities use COD to assess wastewater strength, set effluent limits, and monitor treatment plant performance. Accurate calculation is essential for compliance and process optimization.

Step‑by‑Step COD Calculation

1. Record the titrant volumes: Measure the volume of ferrous ammonium sulfate used to titrate the sample (a) and the blank (b) in milliliters.
2. Determine reagent normality: C is the normality of the ferrous ammonium sulfate solution.
3. Apply the formula: COD (mg L⁻¹) = [(a – b) × C × 8 000] ÷ V, where V is the sample volume in milliliters.
4. Convert to milligrams per liter: The result from the formula already yields COD in mg L⁻¹.

Example Calculation

Sample: 50 mL; a = 3.2 mL; b = 0.4 mL; C = 0.02 N.

COD = [(3.2 – 0.4) × 0.02 × 8 000] ÷ 50 = (2.8 × 0.02 × 8 000) ÷ 50 = (448) ÷ 50 = 8.96 mg L⁻¹.

Safety & Best Practices

• Handle potassium dichromate and other oxidants in a fume hood.
• Wear acid-resistant gloves and eye protection.
• Use calibrated burettes and volumetric flasks to ensure accurate titrant volumes.

For detailed protocols, consult the EPA’s COD standard operating procedure.