Chemical oxygen demand (COD) determination procedure and calculation

Chemical oxygen demand (COD) determination procedure and calculation

This procedure covers determination of chemical oxygen demand (COD) and the calculation for the analysis of waste-water in different stages of Effluent Treatment Plant (ETP) in lab. COD is a measure of the oxygen required to oxidize all compounds, both organic and inorganic in water.
Reagents preparation:

1. Standard potassium dichromate solution (0.25N): weigh 12.259gm of previously dried potassium dichromate at 103°C to 105°C for 2 hrs and dissolve in distilled water and make up to 1000 ml in volumetric flask.
2. Ferro indicator solution: Weigh 1.485 gm of 1,10Phenanthroline monohydrate and 695 mg ferrous sulphate dissolved in distilled water and dilute to100 ml in volumetric flask.
3.  0.1N Ferrous Ammonium Sulphate (FAS):Weigh 39.212gm of FAS and dissolve in distilled water. Add 20 ml concentrated sulphuric acid, cool and dilute to 1000 ml in volumetric flask.
4. Mercuric Sulphate (powder)
5.  Sulphuric acid reagent:
6. Prepare sulphuric acid reagent by dissolving 22 gm of silver sulphate in 2.5 liter of conc. sulphuric acid.
7.  Potassium Hydrogen Phthalate standard:
8. Dissolve 0.425 gm of KHP, previously dried at 120°C for 2 hrs, in 1 liter distilled water. KHP has theoretical COD of 1.176 O₂/mg and this solution has theoretical COD of 500 microgram of O₂/ml of 500ppm.
9.   0.1N Potassium Dichromate Solution:
10. Dissolve 0.5gm of Potassium dichromate in 100ml of water

Standardization of FAS:

• Take an iodine flask containing 90 ml of water.
• Take 10 ml of 0.1N Potassium dichromate solution (prepare 0.1N Potassium dichromate solution by dissolving about 0.5 gm potassium dichromate in100 ml of water.
• Add 12 ml of conc.sulphuric acid. Cool the content in the flask to laboratory temperature.
• Add 3 to 5 drops of ferrion indicator solution.
• Titrate against 0.1N ferrous ammonium sulphate solution until the color changes from blue to red.
• Note down the initial burette reading (I) and note down the final burette reading.

Calculate the normality of FAS as follows.
Normality of FAS = W/ (TV×0.04904×10).
Where TV= (F-I) volume of ferrous ammonium sulphate solution consume in titration
W= weight of potassium dichromate taken.
Determination of  chemical oxygen demand, COD:

1. Pipette 20 ml of sample in 250 ml of refluxing flask.
2. Add approximately 400 mg (a pinch) of mercuric sulphate.
3. Add 10 ml of potassium dichromate by pipette.
4. 30 ml of conc.sulphuric acid reagent by measuring cylinder. Acid should be added in controlled manner with mixing of the sample.
5. If the sample color changes to green, dilute the sample and repeat the procedure for diluted sample.
6. Connect the reflux flask through the condenser and reflux for a minimum period of 2hrs at 150°C.
7. Add 80 ml distilled water through condenser cool it to room temp and titrated with standard sulphate using 2 to 4 drops of ferrion indicator.
8. End point is the sharp color change from blue green to brick red, even though blue green reappear within minutes. Let the titrate value be ‘V’ ml
9. Reflux in same manner a blank with distilled water 20ml and follow the procedure from previously. Let the titrate value be ‘B’ ml.
10. Calculate the COD (mg/lt) as follows

COD= ((B-V)×N(FAS)×8000)/vol of sample(ml).