Chemical Oxygen Demand (COD) in Water | Biotechnology

In this article we will discuss about the principle, requirements and procedure for determination of Chemical Oxygen Demand (COD) in water.

Since there are more amounts of various chemically oxidisable organic substances en­tering water systems, BOD alone does not give a clear picture of organic matter content of water.

Principle:

A known volume of potassium dichromate oxidises the organic matter (C and H and not N₂) and then excess O₂ is allowed to react with potassium iodide to liberate iodine in amounts equal to the excess oxygen which is estimated titrimetrically with sodium thiosulphate solution using starch as indicator.

Requirements:

1. Water sample.

2. Potassium dichromate solution:

0.1 (N): 3.676 g K₂Cr₂O₇ in one litre of distilled water.

3. Sulphuric acid 2(M):

10.8 ml of conc. H₂SO₄ in 100 ml distilled water.

4. Potassium Iodide solution (10%).

5. Sodium thiosulphate 0.1(M):

15-811 g of sodium thiosulphate in 2 litres of dis­tilled water.

6. Starch solution (1%).

7. Water bath.

8. Pipette.

9. 100 ml conical flasks.

10. Water blanks (3).

Procedure:

1. Take water sample in triplicate along with distilled water.

2. Add 5 ml each of K₂Cr₂O₇ to all the six flasks and keep them in boiling water bath (100°C) for one hour and then al­low samples to cool.

3. Add 5 ml of potassium iodide in all six flasks and then 10 ml of H₂SO₄.

4. Titrate contents of each flask with 0.1 (M) Na₂S₂O₄ until pale yellow col­our appears.

5. Now add 1 ml of starch solution to all six flasks, the solution turns blue.

6. Titrate with 0.1 (M) sodium thiosulphate- end point blue colour disappears.

Note down the burette reading be­fore and after titration and find out the volume of thiosulphate used for each sample. Take three readings and their mean for each sample. Find out the vol­ume of thiosulphate used for each blank.

Calculations:

COD of sample mg/litre = A)

Where C = Concentration of titrant (m.mol/litre).

B = Volume in ml of titrant used for sample.

A=Volume in ml of titrant used for blank.

S = Volume in ml of water sample taken.