Determination of Chemical Oxygen Demand (COD)

Chemical Oxygen Demand (COD): 

Chemical oxygen demand (COD) is a measure of the ability of chemical reactions to oxidize matter in an aqueous system.The chemical oxygen demand (COD) test is widely used as a means of measuring the organic strength of domestic and industrial wastes. This test allows measurement of a waste in terms of the total quantity of oxygen required for oxidation to carbon dioxide and water. The test is based on the fact that all organic compounds, with a few exceptions can be oxidized by the action of strong oxidizing agents under acid conditions. During the determination of COD, Organic matter is converted to carbon dioxide and water regardless of the biological assimilability of the substance. As a result, COD values are greater than BOD values, especially when biologically resistant organic matter (e.g., lignin) is present.

Determination of Chemical Oxygen Demand: 
Reagent:

1. Dilute sulfuric acid
2. Standard potassium permanganate
3. Standard Ammonium Oxalate

Testing Procedure:

1. Pipette 100 ml of the sample into a 250 ml Erlenmeyer flask.
2. Add 10 ml. diluted sulfuric acid and 10 ml of standard potassium permanganate.
3. Heat the flask in a boiling water bath for exactly 30 minutes, keeping the water in the bath above the level of the solution in the flask .The heating enhances the rate of oxidation reaction in the Flask.
4. If the solution becomes faintly colored, it means that most of the potassium permanganate has been utilized in the oxidation of organic matter. In such a case, repeat the above using a smaller sample diluted to 100 ml with distilled eater.
5. After 30 minutes in the water bath, add 10 ml of standard ammonium oxalate into the flask. This 10 ml ammonium oxalate , which is a reducing agent , is just equivalent to the 10 ml potassium permanganate ( oxidizing agent ) added earlier. The excess of reducing agent (ammonium oxalate) now remaining in the flask is just equivalent to the amount of the oxidizing agent (potassium permanganate) used in the oxidation of organic matter.
6. The quantity of ammonium oxalate remaining in the flask is now determined by titration with standard potassium permanganate. Titrate the content of the flask while hot, with standard potassium permanganate to the first pink coloration. Record the ml of potassium permanganate used.

Calculation: 
COD (mg /l) = [ml of MnO4 used in step (6) × (100)/ ml of sample used]