Determination of Chemical Oxygen Demand (COD)

1. Objective

To determine the Chemical Oxygen Demand (COD) of the given sample(s).

2. Apparatus Required

• Conical flasks
• Pipettes
• Burettes
• Measuring Cylinders
• COD reflux apparatus (flask + condenser + heating mantle)

Reagents Required

• Potassium Dichromate (K₂Cr₂O₇)
• Concentrated Sulphuric Acid (H₂SO₄) with Silver Sulfate (Ag₂SO₄) Catalyst
• Ferrous Ammonium Sulfate (FAS), approximately 0.1 N to 0.125 N
• Distilled Water
• Ferroin indicator

3. Theory

Chemical Oxygen Demand (COD) is the measure of the oxygen required to chemically oxidize both biodegradable and non-biodegradable organic matter present in water.

Why COD instead of BOD?

While BOD (Biochemical Oxygen Demand) determines the amount of oxygen required by biological organisms to break down organic material, COD is often preferred for several reasons:

• The COD test can be completed in 2–3 hours, whereas a standard BOD test requires 5 days (BOD₅). This makes COD ideal for rapid monitoring and process control.
• BOD tests rely on living organisms, which can be poisoned by toxic substances in the wastewater. The chemical oxidation in COD is unaffected by such toxins.

Guideline Values

Water Type	COD Value
Unpolluted River Water	< 10 mg/L
Treated Wastewater	< 250 mg/L
Industrial Effluent	100 – 1000 mg/L

4. Principle

In this method, the sample is refluxed with a known excess amount of Potassium Dichromate (K₂Cr₂O₇) in the presence of concentrated H₂SO₄ and a Silver Sulfate (Ag₂SO₄) catalyst. Organic matter gets oxidized to carbon dioxide (CO₂) and water (H₂O), and the remaining (unreacted) dichromate is titrated against Ferrous Ammonium Sulfate (FAS).

COD is computed from the amount of dichromate consumed by the sample. Blanks are also treated and titrated to get the correct value of COD.

Chemical Reaction

Why use Mercuric Sulfate (HgSO₄)?

Mercuric Sulfate is added to the sample to eliminate interference caused by chloride ions (Cl^–). Without it, chloride ions would react with the silver sulfate catalyst (precipitating as AgCl) and also be oxidized by the potassium dichromate, leading to erroneously high COD results. HgSO₄ binds with chloride ions to form a stable, soluble mercuric chloride complex (HgCl₂), preventing them from interfering.

5. Procedure

1. Culture tubes and caps were washed with 20% H₂SO₄ before use to prevent contamination.
2. The sample (2.5 mL) was placed in a culture tube, and K₂Cr₂O₇ digestion solution (1.5 mL) was added.
3. Sulphuric acid reagent (3.5 mL) was carefully run down the inside of the vessel so an acid layer was formed under the sample digestion solution layer. Tubes were tightly capped and inverted several times to mix completely.
4. The tubes were placed in an Autoclave at 121°C for 2 hours (or a block digester preheated to 150°C and refluxed for 2 hours behind a protective shield).
5. The vessels were cooled to room temperature and placed in a test tube rack.
6. 1 to 2 drops of Ferroin indicator were added, and the solution was stirred rapidly on a magnetic stirrer while being titrated with standardized 0.1 N FAS.
7. The end point was observed as a sharp color change from blue-green to reddish-brown.
8. In the same manner, a blank containing the reagents and a volume of distilled water equal to that of the sample was refluxed and titrated.

6. Observation and Calculation

Observation Table

Table 1: Titration Data for Blank and Sample

Type	Reading No.	Initial (mL)	Final (mL)	Difference (mL)	Average Titre (mL)
Blank	B1	22.7	24.2	1.5	1.50
	B2	24.3	25.8	1.5
Sample	S1	25.8	27.2	1.4	1.25
	S2	27.3	28.4	1.1

Given Data

• Sample type: Disk Filter
• V_sample = 2.5 mL
• Normality of FAS (N) = 0.1 N
• Dilution Factor = 1

COD Formula

Where:

• A (Blank Titre) = Volume of FAS used for blank = 1.5 mL
• B (Sample Titre) = Volume of FAS used for sample = 1.25 mL
• N = Normality of FAS = 0.1 N
• V_sample = Volume of the sample = 2.5 mL
• 8000 = Milliequivalent weight of oxygen (8) × 1000 mL/L

Calculation Steps

7. Result

The Chemical Oxygen Demand (COD) of the given disk filter sample was found to be:

8. Discussion and Conclusion

The measured COD of 80 mg/L indicates that the disk-filtered sample contains a moderate concentration of chemically oxidizable organic matter. When compared to standard guideline values:

• It is significantly higher than unpolluted river water (< 10 mg/L), suggesting the water carries some organic load.
• It is within the typical discharge limits for treated wastewater (< 250 mg/L).

Since the sample was passed through a disk filter, much of the suspended solid organic matter would have been removed. The remaining COD represents dissolved organic matter. This value suggests the filtration process was effective enough to bring levels within dischargeable limits, or the original source was not heavily polluted industrial waste.

9. Engineering Significance

1. Rapid Monitoring: The COD test is completed in 2–3 hours whereas BOD takes 5 days; hence, COD is widely used for day-to-day plant control as a rapid monitoring tool.
2. Process Design: The COD value is used to calculate oxygen requirements, the size of aeration tanks, and sludge production in activated sludge processes.
3. Waste Assessment: Industries like distilleries, food processing, pharmaceuticals, and textiles use COD analysis to assess waste strength before treatment.
4. Regulatory Compliance: Effluent discharge standards often specify COD limits. Regular monitoring ensures industries meet environmental regulations, and pollution of rivers and lakes is prevented.

10. Precautions

1. Standardize FAS: Standardize the FAS solution daily or just before use because it is unstable in air.
2. Acid Addition: Add sulfuric acid slowly while swirling to prevent sudden heating and splashing.
3. End Point Observation: The endpoint should be observed carefully: a distinct color change from blue-green to reddish-brown.
4. Glassware Cleanliness: Use clean, grease-free glassware to avoid contamination.