Chemical Oxygen Demand Lab Report

Chemical Oxygen Demand (COD) is a measure of the capacity of water to consume oxygen during the chemical decomposition of the organic matter and oxidation of inorganic chemicals such as ammonia and nitrite.

Test Procedure

The COD was determined by the dichromate reflux method. The appropriate amount of sample was introduced into commercially available digestion solution containing potassium dichromate, sulphuric acid and mercuric sulphate and the mixture was then incubated for 120 min at 150º C in a COD reactor (HACH, USA). COD concentration was measured colorimetrically using a spectrophotometer (DR 2010, HACH, USA). Chemical Oxygen Demand (COD) is used as a measure of oxygen requirement of a sample that is susceptible to oxidation by strong

Add 5 to 7 glass boiling beads.

3. Add 1g of mercuric sulphate, 5 ml of concentrated sulphuric acid solution, and mix until the HgSO4 is in solution. The function of the mercuric sulphate is to bind or complex chlorides. One gram may not be required if the chloride concentration is low (caution: always add acid slowly down the side of the flask while mixing to avoid overheating).

4. Accurately add 25 ml of 0.25 N potassium dichromate (K2Cr2O7) and mix.

5. Add while mixing, 70 ml of concentrated sulphuric acid solution.

6. After through mixing, attach the flask to the reflux condenser, apply heat, and reflux for 2 hours. Refluxing time can be decreased depending on the ease of oxidation of organic materials. This time may be determined by refluxing for periods from 15 minutes to 2 hours and comparing the results.

7. A reagent blank containing 50 ml of distilled water treated with the same reagent as the sample should be refluxed with each set of samples.

8. Cool the apparatus to room temperature after the refluxing period. Wash down the interior of the condenser and flask twice with approximately 25 ml portions of distilled water.

9. Remove flask from the condenser and dilute to a final volume of approximately 350ml with distilled