Chemical oxygen demand is the test to determine the amount of oxygen required to oxidized organic matter in acid condition into carbon dioxide ( CO2) and water ( H2O ) using strong oxidizing agent.

Procedure:

1. Prepare standard solutions as under:

  • 0.25N K2Cr2O7
  • 0.25N FeSo4 (NH4)2
  • Ferroin indicator
  • HgSO4 powder
  • AgSO4


2. Find out required size of sample as under analysis:

  • Take 10 mL dichromate in COD tube.
  • Take about pinch of mercuric sulfate ( HgSO4)
  • Add 30 mL concentrated H2SO4 and cool it.
  • Add drop by drop sample till ting of green color is obtained.
  • Write down volume of sample till green color appears which is X.
  • The volume of sample to add will be 2/3 of X.


3. Testing process:

  • Clean all the glassware with water and distill water.
  • Take 10 mL 0.25N K2Cr2O7 in COD tube.
  • Add a pinch of HgSO4 and add 30 mL concentrated H2SO4.
  • Add sample as per size decided in above section (2).
  • Digest COD tube of solution at 140-150`C in COD soxhelt apparatus with air condenser.
  • Digest time is for 2 hours.
  • After digestion cool the tubes with tap water
  • Add 4-5 drops of ferroin indicator.
  • Titrate with 0.25 N FAS standard solution till blood red color obtained.



Calculation:

COD (mg/L) =(A - B) x N x 8 x 1000
                            mL of the sample


Where,

A = mL of ferrous ammonium sulfate used for blank
B = mL of ferrous ammonium sulfate used for sample
N= Normality of ferrous ammonium sulfate
8 = Mili equivalent weight of oxygen.
 
  • Note down the reading.