After wastewater treatment, the chemical oxygen demand (COD) is used to determine the amount of contamination in the water. The higher value of chemical oxygen demand implies that the water sample has a greater amount of organic pollutants. The COD test measures only substances that can be chemically digested. The determination of COD requires less time than the determination of Biological Oxygen Demand. COD is indicated when polluted water is harmful and organic matter cannot be detected by biological oxygen demand; it is also useful in effluent treatment plants.

Principle of COD Test 

• In the presence of sulfuric acid, silver sulphate, and mercury sulphate, potassium dichromate oxidises the organic matter contained in the water sample to produce carbon dioxide (CO2) and water (H2O).
• The amount of potassium dichromate utilised is determined by comparing the volumes of ferrous ammonium sulphate consumed during blank and sample titrations.
• The amount of potassium dichromate used in the reaction is equivalent to the amount of oxygen (O2) required to oxidise the wastewater’s organic content.

Objectives

• To calculate the chemical oxygen requirement of an unidentified sample

Solution Preparation

Preparation of Potassium dichromate (K2Cr2O7) Solution

1. Add 6,13 g of dried at 105 °C for at least two hours potassium dichromate to 800 ml of distilled water.
2. Shake the flask vigorously to dissolve the contents, then add 1000 ml of water and combine thoroughly.

Preparation of Silver sulfate-Sulfuric acid Solution

1. Dissolve 10 gm Silver sulfate (Ag2SO4) in 500 ml concentrated sulfuric acid and make up the solution to 1000 ml swirl the flask to mix well.
2. Allow the solution to stand for 24 hours before to use.

Preparation of Mercury sulfate Solution

• Dissolve with care 0.1 gm of HgSO4 in 5 ml of concentrated Sulfuric acid.

Preparation of Ferrous ammonium sulfate Solution (0.025 M)

1. Dissolve 9.8 grammes of ferrous ammonium sulphate in 100 millilitres of distilled water and 20 millilitres of sulfuric acid.
2. Cool the solution and dilute it with 1,000 millilitres of distilled water.
3. Determine the actual concentration of the solution in order to compute the chemical oxygen demand by standardising it.

Preparation of Ferroin Indicator

1. 400 ml of purified water should be mixed with 3.5 grammes of Iron Sulfate heptahydrate and 7.5 grammes of Phenanthroline monohydrate.
2. Mix thoroughly to dissolve, and add enough pure water to reach 500 ml.

Test for Chemical Oxygen Demand

1. Place 10 ml of sample in a flask with a circular bottom.
2. Add some glass beads to keep the solution from contacting the flask while it is being heated.
3. Mix 1 ml of Mercury sulphate (HgSO4) solution by rotating the flask.
4. Add 5 ml of a solution of potassium dichromate (K2CrO7).
5. Now add 15 ml of Silver sulfate-sulfuric acid solution slowly and cautiously.
6. Connect the reflex condenser and digest the contents for two hours on a hot plate.
7. After digestion, rinse the condenser with 25 ml of distilled water collected in the same flask.
8. Add 2-4 drops of ferroin indicator to the flask and titrate to the endpoint with 0.025 M ferrous ammonium sulphate solution.
9. Prepare the blank in the same manner as the sample, substituting distilled water for the sample.

Calculation

Determine the chemical oxygen requirement using the following formula:

COD = 8x1000xDFxMx(VB – VS)/Volume of sample (in ml)

Where,

• DF – Factor of Dilution (if applicable)
• M — Molarity of a standard solution of Ferrous Ammonium Sulfate
• VB – Volume consumed during titration with blank solution
• VS — Volume utilised during titration sample preparation

Example Calculation

Sample (VS) ferrous ammonium sulphate volume = 23,8 ml

Ferrous ammonium sulphate volume for Blank VB) = 25.6 ml

Dilution Factor (DF) = 1 (sample used as it is)

COD = 8x1000x1x0.025x(25.6-23.8)/10 = 8000×0.025×1.8/ 10 = 360/10 = 36 mg/lit or ppm