Introduction
Chemical Oxygen Demand (COD) is one of the most important parameters in water and wastewater analysis. It measures the amount of oxygen required to chemically oxidize organic matter present in a sample.
COD testing plays a crucial role in:
- Environmental monitoring
- Industrial effluent control
- Wastewater treatment efficiency
🧪 Principle of COD Test
The COD test is based on oxidation of organic matter using a strong oxidizing agent.
⚗️ Reaction Mechanism:
Organic compounds react with potassium dichromate (K₂Cr₂O₇) in acidic medium:
Organic Matter + K₂Cr₂O₇ + H⁺ → CO₂ + Cr³⁺ + H₂O
Cr(VI) (Orange) → Reduced to → Cr(III) (Green/Blue)
Heat is applied to accelerate the reaction.
🧰 Apparatus & Reagents
🔧 Apparatus:
- COD Reflux Digestion Unit (Heating Mantle)
- Condenser Setup
- Burette for titration
🧴 Reagents:
- Potassium Dichromate (K₂Cr₂O₇)
- Sulfuric Acid (H₂SO₄)
- Ferrous Ammonium Sulfate (FAS)
- Ferroin Indicator
⚙️ Experimental Parameters
Reflux Time: 2 Hours
Sample Volume: ~20 mL
Catalyst: Silver Nitrate (AgNO₃)
Chloride Masking: Mercury Sulfate (HgSO₄)
🧮 COD Calculation Formula
After digestion, the sample is titrated using FAS.
📊 Formula:
COD (mg/L) = (A-B)*N*8000 / Sample Vol. (Ml)
Where:
A = mL of FAS for Blank
B = mL of FAS for Sample
N = Normality of FAS
⚠️ Interferences and Their Treatment
🚫 Major Interference:
Chloride ions (Cl⁻)
✅ Solution:
Add Mercury Sulfate (HgSO₄) before digestion
It masks chloride interference up to a certain level
🌍 Applications of COD Analysis
COD testing is widely used in:
- 🌊 River Water Assessment
Determines pollution levels in natural water bodies
- 🏭 Industrial Effluent Monitoring
Ensures compliance with discharge standards
- 🏢 Wastewater Treatment Plants
Evaluates treatment efficiency
- 🌱 Environmental Remediation
Helps in pollution control strategies
- 📊 Importance of COD in Industry
Quick and reliable measurement
Indicates total organic load
Helps in process optimization
Essential for regulatory compliance
✅ Conclusion
The Open Reflux Method (IS 3025-58:2006) remains a standard and reliable technique for COD determination.
It provides: ✔ Accurate measurement of organic pollution
✔ Essential data for environmental protection
✔ Support for industrial and municipal wastewater management