Now emerge the above screw caped glass tube in the hot paraffin oil.
Maintaining the Paraffin oil temperature 140 to 150 degree centigrade and digest the sample for minimum 2 hours . (If strong bubbling observed in to the screw cap glass tube solution then disconnecting heating as it may lead to explosion of glass tube  sample may contain volatile  constituent and hence this method is not suitable for the above sample discontinue & discard the glass tube solution after cooling. And follow the para 2.

Para 2
After 2 hours solution take out and place it in a fast stand set it cool slowly.
When the solution reach room temperature remove cap and add 2 drop COD TK-3. Mix well and start adding  and counting the number of drops while mixing until the color change from bluish green to redish brown  (Say ‘X” Drop)
Carry out balance titration using 2 ml distilled water in a 10 ml test jar . Repute above step  and put COD TK-4. Note down the number of drop of COD TK4 require to get the solution color change from bluish green to redish brown. (Say “Y”Drops).

COD PPM = X-YX40)

If the expected COD of the test sample is more than 800 PPM then take 20 ml of sample  in 100 ml volumetric flask and diluted up to 100 ml with distilled water. Use 2 ml above diluted solution and follow direction as above. Multiply the result  obtained by 5.


Caution  : Reagent COD TK 2 is acidic , handle carefully and avoid contact with skin.