Chemical Oxygen Demand (COD)

November 05, 2016 3:25 pm

Chemical Oxygen demand (COD) is widely used as means of measuring the pollution strength of domestic and industrial wastes. This test allows measurement of a waste in terms of total quantity of oxygen required for oxidation to carbon dioxide and water.

Definition

Chemical Oxygen Demand (COD) is the amount of oxygen required for chemical degradation of organic matter present in the waste water.

Range

Analysis Procedure:[IS 3025-58 (2006)]

1. Take 20 ml of sample in a COD tube and 20 ml of distilled water in another tube
2. Add 10 ml Potassium Dichromate
3. Add 30 ml of sulphuric acid (conc. H2SO4)
4. Add pinch of silver sulphate & mercuric sulphate
5. Reflux the tubes in the COD digester at 150⁰C for 2 hrs.
6. After cooling to room temperature, transfer the contents to the conical flask
7. Fill the burette with freshly prepared Ferrous Ammonium Sulphate (FAS)
8. Titrate the contents in the flask against FAS using Ferroin indicator
9. Continue the titration till color changes to reddish brown.

Calculation

• COD mg/L = (A-B) X N X 8 X 1000 / ml
• of sample where,
• A: Volume of FAS for blank
• B: Volume of FAS for sample
• N: Normality of FAS

Alternate Procedure

The COD test is precise and accurate for samples with a COD of 50 mg/l or greater. For more dilute samples it is preferred that a more dilute dichromate solution be used so that a significant relative difference between the quantity of dichromate added and that remaining after refluxing results.

With dilute samples, care must be taken to avoid sample contamination and good analytical techniques must be used if reasonably accurate results are to be obtained.

It is also important in any modification that the volume of concentrated sulfuric acid to volume of sample plus dichromate solution be maintained at a 1:1 ratio. If it is smaller, the oxidizing power of the solution will decrease significantly, while if it is larger, the blank consumption of dichromate becomes excessive.

Interference

Straight-chain aliphatic compounds, aromatic hydrocarbon fatty acids, chlorides, nitrites and iron are the main interfering radicals.

The interference caused by chlorides sulphate addition of other reagents.

Sulphamic acid is added to potassium dlchromate solution to avoid interference caused by nitrite (N02).

Silver sulphate is added to concentrated H2SO4, (22’ g/4 kg acid) as a catalyst. This accelerates the oxidation of straight-chain aliphatic and aromatic compounds.

Precision & accuracy

Precision and accuracy both depends upon the COD value. For the high COD values (>_400 mg/l) precision up to 2 percent is expected from a good analyst. As the COD value goes on decreasing, precision also becomes poorer and poorer that is percentage goes .on increasing. Precision for the low COD samples maybe improved by using alternate method where diluted reagents are used.

Conclusion

COD test is therefore an important parameter in determine the quality of water. It is relatively fast and reproducible.

Disclaimer

The details given are for general understanding. References should be taken from respective authorized agencies.