Introduction to COD-
- COD Chemical Oxygen Demand is the amount of oxygen required to chemically oxidize the organic matter, using strong oxidizing agents under acidic conditions.
- COD is widely used for evaluating the amount of organic matter present in water or wastewater.
- COD method finds the amount of oxygen required for degradation of both biodegradable and non-biodegradable organic matter of water sample.
- It is said to be an alternative to BOD, as COD is faster takes only 3 hours to get completed along with that it provides accuracy.
Principle of COD-
- The water sample is digested with a strong chemical oxidizing agent like Potassium Dichromate, potassium permanganate or Potassium iodate in presence of sulphuric acid at a specific temperature for specific period of time. After the oxidation of water sample, the amount or concentration of organic matter can be calculated by titrating excess dichromate present in water sample with Ferrous Ammonium Sulphate.
- The result may be affected by traces of organic material present in the glassware that used during the COD experiment.
- Distilled water that is used for solution preparation must be free from organic matter.
- During digestion process, the high temperature may cause the evaporation of volatile substances.
Reagents used –
- Standard Potassium Dichromate 0.250N – To prepare standard Potassium Dicrhromate, add 12.258 g of K2Cr2O7 in 1000ml of Distilled water.
- Sulphuric Acid Reagent – Dissolve 10g of silver sulphate in 500 ml of concentrated sulfuric acid and make the volume up to 1000 ml..
- Standard Ferrous Ammonium Sulfate 0.250 N (FAS) – Prepare FAS by dissolving 98g of Ferrous Ammonium Sulphate Fe(NH4)2(SO4)2 6H2O in distilled water, add 20mL of concentrated sulphuric acid and dilute the mixture up to 1 liter.
- Mercury Sulfate
- Ferroin Indicator – Dissolve 1.48 g of 1-10-(ortho) phenanthroline monohydrate with 0.7 g of FeSO4 7H2O in 100 mL of water
Apparatus required –
- COD Digester
- COD tubes with stand.
- Wash bottle.
- Conical flask.
- Burette stand.
- Pipette with elongation tube.
- In COD tubes, take 20ml of water sample. Before pipetting the sample, rinse the pipette with sample to obtain accurate result
- To prepare blank, take 20ml of distilled water in COD tubes.
- Add 10 ml of Potassium Dichromate (K2Cr2O7) in sample and blank tubes
- Add 30 ml of concentrated Sulphuric Acid in sample and Blank tubes
- Add one spatula of Mercury Sulfate in each tube.
- Add glass beads to the tubes in order to prevent bumping of solution.
- Connect the reflex condenser with COD tubes and keep the tubes into the COD Digester (150°C).
- Wash the condenser with 60 ml of distilled water and collect the same in COD tubes.
- Allow it to cool for 2 hours.
- Titrate the solution in the flask against 0.1N of FAS using few drops of Ferroin Indicator.
- Titrate the content until the blue color disappear to form brick red or reddish brown colour
It is expressed in milligrams per litre (mg/l), which indicates mass of oxygen consumed per litre of solution.
COD = (B – A) × N × 8000/ sample volume in ml
A = volume of FAS used for sample (in ml).
B = volume of FAS used to blank (in ml).
N = Normality of FAS.
Why Potassium Dichromate suits best for COD?
- It oxidizes almost all organic matter.
- It is relatively cheap.
- It is stable for room temperature.
- It is easy to purify.
- This reagent contains no hazardous metal.
Why COD values are greater than BOD values?
COD oxidizes biological and non-biological organic matter whereas BOD oxidizes only biodegradable organic matter.
- It takes less time than BOD.
- It oxidizes biological and chemical compounds expect few.
- It can be standardize more easily.
- Due to short time for analysis, errors can be corrected on the same day.
- It is unable to differentiate the “Biological and non biological organic matter.
- COD fails to oxidize some organic compounds like acetate, phenol, toluene and Benzene etc.
- Used majorly in analysis of Industrial wastewater.
- Used widely as means for measuring organic strength of “domestic and industrial waste water”.
- The most common application of COD is in quantifying the amount of oxidizable pollutants found in surface water (e.g. lakes and rivers).
- The oxidant used is a chemical compound.
- Acidic conditions are required to perform.
- High COD value indicates the presence of greater amount of oxidizable organic material in sample, which reduces DO levels.
Journal on industrial wastewater treatment by k.l moed
Environmental Engineering volume 1 and 2 by S.K. garg.
MLA. Metcalf & Eddy, Inc. Wastewater Engineering : Treatment and Reuse. Boston :McGraw-Hill, 2003.
Standard Methods for the Examination of Water and Wastewater, 14th Edition, p 550, Method 508 (1975).
Annual Book of ASTM Standards, Part 31, “Water”, Standard D 1252-67, p 473 (1976).