Determination of chemical oxygen demand of water

Chemical Oxygen Demand indicates the amount of oxygen required to oxidize all soluble and particulate organic substances in water under specific conditions. All organic compounds may convert into carbon dioxide, water and ammonia. It is a significant parameter to determinate water characteristic. The COD value depends on oxidizing agent, pH, temperature, and period of time. It is expressed as ppm or milligrams per liter (mg/l) of oxygen. There are several methods for determination of Chemical Oxygen Demand but the wet chemical method (dichromate method) is the most common Method.

Principle of chemical oxygen demand

In dichromate method the organic matter in water oxidize with the strong oxidizing agent potassium dichromate under acidic conditions. Normally sulfuric acid is used to achieve the acidity in this COD test method. An additional amount of potassium dichromate are taken, to sure completely oxidation of all organic matter. After complete the oxidation, measure the excess amount of potassium dichromate by titrating with ferrous ammonium sulfate. During the titration the ferroin is used as indicator. At the end point the color of the ferroin changes from blue-green to a reddish brown. The consumption of the dichromate is proportional to the amount of organic matters that are present in the water.
3[CH2O] + 16H+ + 2Cr2O72- → 4Cr3+ + 3CO2 + 11H2O

It is a fast applicable parameter for industrial wastewater, water controlling plant, sewage, rivers, lakes or aquifers but not applicable for drinking water as the lower content of oxidizable organic matter. Frequently, a silver compound like Silver sulfate is used as a catalyst to promote oxidation of certain organic compounds such as linear aliphatic compounds, aromatic compounds and pyridine. Some inorganic materials like chloride, nitrite may interfere with the result of COD. Chloride interference can eliminate by adding Mercuric sulfate whereas nitrite interference can eliminate by adding sulfamic acid.

COD testing equipment and reagents

  1. Water sample
  2. Distilled water
  3. Standard Potassium dichromate solution (0.25N)
  4. Sulfuric acid reagent
  5. Standard ferrous ammonium sulfate (0.25 N)
  6. Ferroin indicator
  7. Mercuric sulfate
  8. Silver sulfate
  9. Water bath
  10. Titration apparatus
  11. 500ml conical flask (Erlenmeyer Flask)
  12. Burette
  13. Pipettes
  14. Pipette bulb
  15. Wash bottle

COD test procedure

  1. Collect the sample water from a source.
  2. Take 500 ml conical flask/ reflux flask and filled with 50ml of sample water.
  3. Place several boiling stones in the reflux flask.
  4. Determinate the chloride ion. If it contains a significant amount then, add 1 g of HgSO4 and 5.0 ml conc. H2SO4; then swirl the mixture until the mercuric sulfate dissolves.
  5. After that place the reflux flask in an ice bath and slowly add 25ml of potassium dichromate solution (K2Cr2O7) with swirling.
  6. Now add 75 ml of a mixture of sulfuric acid-silver sulfate solution to the cooled reflux flask with swirling.
  7. Apply heat to the mixture under reflux in water bath at 150°C temperature for two hours.
  8. Then allow the flask to cool about room temperature and wash the condenser with distilled water. Transfer the mixture into 500 ml Erlenmeyer flask.
  9. Then titrate the excess dichromate with 0.25 N ferrous ammonium sulfate, Fe(NH4)2SO4 solution by using 2 to 4 drops of ferroin indicator. At the end point the color turns from blue-green to reddish brown and take the burette reading. Let the reading is ‘B’ ml.
  10. Run a blank titration similarly. Let the reading is ‘A’ ml.

Chemical oxygen demand calculation

COD in mg/L= 8000 (A-B) N / V

A is the volume of FAS used in the blank sample, in milliliters.
B is the volume of FAS in the original sample, in milliliters.
N is the normality of FAS solution.
V = milliliters of sample used for the test.

Relation between BOD and COD

Now a days, water is contaminant by various type of organic substances. Chemical Oxygen Demand is an important water quality parameter as like BOD. But we cannot get a clear picture of all type of organic substances from the BOD values alone whereas COD values gives a better result. Because, biodegradable and non-biodegradable substances are includes in COD values. Consequently, COD values are larger than BOD. Moreover, the measurement of COD requires a shorter time about 2-3 hours. On the other hand bod requires longer time period 5 days. The higher the chemical oxygen demand, the higher the amount of pollution in the water.

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