COD(chemical oxygen demand)-
Chemical oxygen demand is a measure of the oxygen required to oxidize all compounds, both organic and inorganic in water.
Reagents preparation for chemical oxygen demand:
- 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.
- Ferro indicator solution: Weigh 1.485 gm of 1,10-Phenanthroline monohydrate and 695 mg ferrous sulphate dissolved in distilled water and dilute to 100 ml in volumetric flask.
- 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.
- Mercuric Sulphate (powder)
- Sulphuric acid reagent:
- Prepare sulphuric acid reagent by dissolving 22 gm of silver sulphate in 2.5 liter of conc. sulphuric acid.
- Potassium Hydrogen Phthalate standard:
- 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.
- 0.1N Potassium Dichromate Solution:
- 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 in 100 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 consumed in titration
W= weight of potassium dichromate taken.
Determination of chemical oxygen demand(COD):
- Pipette 20 ml of sample in 250 ml of refluxing flask.
- Add approximately 400 mg (a pinch) of mercuric sulphate.
- Add 10 ml of potassium dichromate by pipette.
- 30 ml of conc.sulphuric acid reagent by measuring cylinder. Acid should be added in controlled manner with mixing of the sample.
- If the sample color changes to green, dilute the sample and repeat the procedure for diluted sample.
- Connect the reflux flask through the condenser and reflux for a minimum period of 2hrs at 150°C.
- 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.
- 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
- Reflux in same manner a blank with distilled water 20ml and follow the procedure from previously. Let the titrate value be ‘B’ ml.
- Calculate the COD (mg/lt) as follows:
COD= (B-V) × N(FAS)×8000)/vol. of sample(ml)