Chemical Oxygen Demand (C.O.D.)

Chemical Oxygen Demand (C.O.D) 

 

Aim:To determine chemical oxygen demand (C.O.D.) of the given water sample as per IS: 3025 (Part58)

Apparatus: C.O.D. digester, COD vials, Glassware

Chemicals: 

• Potassium dichromate
• Sulfuric acid
• Ferrous Ammonium Sulphate (FAS)
• Silver Sulphate
• Mercury sulphate
• Ferroin indicator
• Organic free distilled water

Preparation of reagents:

• Standard Potassium Dichromate reagent (used as digestion solution). It is prepared by:
• 4.913 g K2Cr2O7 + 33g Mercuric sulphate + 167 ml conc.H2SO4 + make-up this mixture to 1000 ml using distilled water. This is standard potassium dichromate solution used for dilution

• Sulphuric acid reagent - catalyst solution. It is prepared by:
• 5.5g silver sulphate crystals + 500 ml conc.H2SO4 for 24 hours to dissolve cilver sulfate crystals completely 

• Standard Ferrous Ammonium Sulphate Solution
• Dissolve 39.2g Ferrous ammonium Sulphate crystals in 1000ml distilled water

Procedure:

• Take two COD vials with stopper (One for sample and one for blank)
• Add 2.5 ml sample and 2.5 ml D.W to each vial respectively
• To each vial, add 1.5 ml potassium dichromate solution (digestion solution)
• Add 3.5 ml sulphuric acid reagent (catalyst solution)       [COD vials are HOT now]
• Cap the vials TIGHTLY and place them in the digester with the temperature set to 150C for 120 minutes. Start the digester.
• After two hours, transfer the cooled digested solution to a conical flask
• Add few drops of ferroin indicator to the flask. The solution turns bluish green
  
• Fill the burette with Ferous Ammonium sULPHATE (FAS)
• Titrate the solution in the flask with FAS (in the burette)
• Appearance of reddish brown colour indicates end point of the titration. Note the volume of FAS consumed.
  

COD is determined using the formula:

COD in mg/L = [(A-B) * N * 8 * 1000] / [volume of sampletaken]

(OBSERVATIONS)

Result:

 

 

Notes:

1. COD does not differentiate between biologically available and inert organic matter.
2. COD is a measure of total quantity of oxygen needed to oxidise ALL organic matter into carbondioxide and water
3. COD values are ALWAYS GREATER THAN BOD values
4. Potassium dichromate is considered the best oxidising agent
5. Mercuric sulphate is added to reduce the interference of chlorides
6. For industrial wastewater COD is almost 2.5 times BOD
7.