1. COD
Closed Reflux, Colorimetric method
Two reagents need for COD
• Digestion solution
• Sulfuric acid reagent
Digestion solution preparation ( High range)
Required Chemicals and apparatus
Pottasium Dichromate (K2Cr2O7)
Mercuric Sulphate ( HgSO4)
conc. Sulfuric acid
Distilled water
2. Apparatus
• 1000 ml Volumetric Flask
• Glass rod
• Funnel
About 10.216g K2Cr2O7 ( Primary standard grade, previously dried at
150 °C for 2h)
was added in to the 1000 ml volumetric flask. Then 500 ml distilled
water was added in to it and mixed well. After that 167 ml conc.
Sulfuric acid was added gently in to it. About 33.3 g HgSO4 was added
and dissolved. Solution was cooled to room temperature and dilute to
1000 ml by using distilled water.
3. Sulfuric acid Reagent preparation
Required Chemicals and apparatus
• Silver Sulfate ( Ag2SO4)
• Sulfuric acid
Apparatus
• 500 ml beaker
• Glass rod
4. • Add Ag2SO4 powder, to Conc. Sulfuric acid at the rate of 5.5g
Ag2SO4/Kg H2SO4. Let stand 1 to 2 d to dissolve.
• Conversion of H2SO4 mass to volume
Density=
𝑚𝑎𝑠𝑠
𝑉𝑜𝑙𝑢𝑚𝑒
1.83 gcm-3 =
1000 𝑔
𝑉𝑜𝑙𝑢𝑚𝑒
= 546.44 ml
5.5 g Ag2SO4 was added in to the 500 ml beaker and 546.44 ml H2SO4
was added in to it. Let stand 1 to 2 days to dissolve.
5. Vial Preparation
• 2.5 ml waste water sample was added in to the COD vial. 1.5 ml
digestion solution was added in to it. After that 3.5 ml sulfuric acid
reagent was added in to the vial and mixed well.
Blank preparation
2.5 ml distilled water was added in to the COD vial. 1.5 ml digestion
solution was added in to it. After that 3.5 ml sulfuric acid reagent was
added in to the vial and mixed well.
6. Digest the sample
• COD digester machine was preheated at 150 °C. After that vials were
placed in to the machine and maintained 150 °C for 2h. Finally vials
were cooled to room temperature and read the value using
colorimeter.
• Reference:
