Standard Operating Prucedure for the Measurement of Chemicsl Oxygen Demand

Chemical Oxygen Demand (COD): A Practical Laboratory Guide

Chemical Oxygen Demand (COD) is a key parameter used to assess the level of organic pollution in water and wastewater. It measures the amount of oxygen required to chemically oxidize organic and inorganic matter present in a sample. In this blog, we explain the COD test procedure, its principle, reagents, and calculations in a clear and laboratory-friendly way.

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Purpose of COD Testing

The purpose of this standard operating procedure is to describe the laboratory method for determining Chemical Oxygen Demand in water and wastewater samples.

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Scope

This procedure is applicable to laboratories engaged in the analysis of:

• Drinking water
• Surface water
• Industrial effluents
• Domestic and municipal wastewater

where COD measurement is required for monitoring, compliance, or research purposes.

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Roles and Responsibilities

• Laboratory Chemist: Responsible for sample preparation, digestion, titration, and calculation of COD values.
• Technical Manager: Reviews the analytical procedure and verifies results.
• Quality Manager: Ensures proper implementation of the SOP and compliance with quality standards.

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Principle of COD Determination

The COD test is based on the oxidation of organic matter by potassium dichromate (K₂Cr₂O₇) in a strongly acidic medium:

• The sample is refluxed with a known excess amount of potassium dichromate in the presence of concentrated sulfuric acid.
• Organic matter present in the sample gets oxidized during digestion.
• After refluxing, the remaining (unreduced) potassium dichromate is titrated with ferrous ammonium sulphate (FAS).
• The amount of dichromate consumed is directly proportional to the oxygen required to oxidize the sample constituents and is expressed as mg/L of oxygen.

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Instruments and Equipment Required

• Heating mantle
• Reflux condenser
• Burette (50 mL)
• Round-bottom flask (500 mL)
• Pipettes (50 mL)
• Measuring cylinders

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Reagents Used

1. Potassium Dichromate Solution (0.25 N)

Reagents required:

• Potassium dichromate (K₂Cr₂O₇): 12.259 g
• Distilled water: up to 1000 mL

Preparation: Dissolve 12.259 g of potassium dichromate in distilled water, transfer to a 1000 mL volumetric flask, and make up to the mark with distilled water. Store in an amber-colored bottle.

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2. Ferrous Ammonium Sulphate (FAS) Solution (0.25 N)

Reagents required:

• Ferrous ammonium sulphate hexahydrate [Fe(NH₄)₂(SO₄)₂·6H₂O]: 98 g
• Concentrated sulfuric acid: 20 mL
• Distilled water: up to 1000 mL

Preparation: Dissolve 98 g of ferrous ammonium sulphate in distilled water. Add 20 mL of concentrated sulfuric acid carefully, cool the solution, and dilute to 1000 mL with distilled water. Standardize daily against 0.25 N potassium dichromate.

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3. Ferroin Indicator

Reagents required:

• 1,10-Phenanthroline monohydrate: 1.485 g
• Ferrous sulphate heptahydrate (FeSO₄·7H₂O): 0.695 g
• Distilled water: up to 100 mL

Preparation: Dissolve 1.485 g of 1,10-phenanthroline monohydrate and 0.695 g of ferrous sulphate in distilled water and dilute to 100 mL. Store in a dark bottle.

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4. Concentrated Sulfuric Acid

Reagent required:

• Concentrated H₂SO₄ (analytical grade)

Used to provide the acidic medium necessary for oxidation during reflux digestion.

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5. Silver Sulphate

Reagent required:

• Silver sulphate (Ag₂SO₄)

Used as a catalyst to enhance oxidation of certain organic compounds, especially straight-chain fatty acids.

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6. Mercuric Sulphate

Reagent required:

• Mercuric sulphate (HgSO₄)

Added to eliminate chloride interference during COD determination.

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Step-by-Step Test Method

1. Pipette 20 mL of the sample into a 500 mL round-bottom reflux flask.

   For samples with high COD, take a smaller volume and dilute it to 20 mL with distilled water to ensure proper digestion and clear endpoint.

2. Add approximately 0.5 g of mercuric sulphate and a pinch of silver sulphate to the flask.

3. Add 10 mL of 0.25 N potassium dichromate solution.

4. Carefully add 30 mL of concentrated sulfuric acid with gentle swirling to mix the contents.

5. Attach a reflux condenser and reflux the mixture on a heating mantle for a minimum of two hours.

6. Allow the mixture to cool and wash down the condenser with about 80 mL of distilled water.

7. Cool the solution to room temperature.

8. Titrate the excess potassium dichromate with 0.25 N ferrous ammonium sulphate using 4–5 drops of ferroin indicator.

9. Observe the color change from bluish-green to reddish-brown, indicating the endpoint.

10. Record the burette reading for the sample.

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Blank Determination

Perform a blank test using 20 mL of distilled water and the same quantities of reagents under identical conditions. Record the burette reading for the blank.

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Calculation of Chemical Oxygen Demand

COD (mg/L) = {(B - A) x N x8000}\{Sample Volume (mL)}

Where:

• B = Volume of FAS used for the blank (mL)
• A = Volume of FAS used for the sample (mL)
• N = Normality of ferrous ammonium sulphate

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Final Notes

COD analysis is a robust and widely accepted method for estimating organic pollution in water and wastewater. Accurate reagent preparation, proper refluxing time, and careful titration are essential for reliable results. When performed correctly, COD testing provides valuable insights into treatment efficiency and environmental impact.

🧪 Clean chemistry leads to cleaner water!