Due to gradually increasing population the number of industries is also increasing. This results in increased pollution. The chemically oxidisable organic substances discharged in water depletes the amount of O2. Estimation of BOD alone could not give the exact idea of pollutants present in water, therefore, COD is estimated. COD refers to the oxygen consumed by the oxidisable organic substances. The values of COD cannot be compared directly with that of BOD. The chemical oxidants such as potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4) are used to measure the oxidisability of the organic matter of water where the oxidants oxidise the constituents (or the hydrogen but not nitrogen). Then potassium iodide (KI) is added.
The excess amount of oxygen reacts with KI and liberates iodine. The excess amount of oxygen liberates an equal amount of iodine. By using a starch indicator, iodine is titrated with sodium thiosulfate and amount is estimated.
- Water sample
- Conical flasks (100 ml capacity)
- K2Cr2O7 solution (0.1N, 3.67 g/l )
- H2SO4 solution 2M (10.8 ml conc. H2SO4/1000 ml distilled water)
- Sodium thiosulfate 0.1M (15.811 g/2litre distilled water)
- Starch solution (1%)
- Water bath
- Distilled water control blank (3)
- Titration assembly
- Pour 50 ml of water sample in a conical flask (100 ml capacity).
- Similarly, take 50 ml distilled water in a flask.
- Pour 5 ml K2Cr2O7 solution separately in both the flasks
- Incubate the flasks at 100oC for one hour keeping in a water bath.
- Thereafter, remove the flasks to cool for 10 minutes.
- Mix 5 ml KI solution, and 10 ml of H2SO4 solution in each flask.
- Transfer 0.1 M sodium thiosulfate solution in burette fitted in titration assembly, and titrate with both the samples in flasks till pale yellow colour disappears. In each case note the amount of sodium thiosulfate solution used.
- Add 1 ml of starch solution to both the flasks. The Colour turns blue.
- Again titrate with sodium thiosulfate as above till complete disappearance of blue colour.
Note the volume of sodium thiosulfate used for both the water samples.
Calculate the COD (mg/l) of water by using the following formula: