Determination of COD

Chemical oxygen demand (COD) is a measure of the amount of dissolved oxygen needed to oxidize organic chemicals in water. The initial COD of black water needs to be determined, and the following procedure utilizing a COD digester is employed:

Materials for COD test:

  1. Potassium dichromate (K2Cr2O7)
  2. Mercuric sulphate (HgSO4)
  3. Sulfuric acid (H2SO4)
  4. Ferrous ammonium sulphate (FAS)
  5. Ferron indicator
  6. Potassium hydrogen phosphate (KHP)
  7. 250 ml Erlenmeyer flask
  8. Reflux condenser
  9. COD tubes
  10. Measuring cylinder
  11. Analytical balance
  12. COD tube stands
  13. Hand gloves

Preparation of chemicals:

  1. Preparation of 0.25 N potassium dichromate solution: Dissolve 12.25 gm potassium dichromate powder in 1000 ml distilled water in a 1000 ml volumetric flask.
  2. Preparation of Ammonium Ferrous Sulphate solution: Dissolve 24.5 gm Ammonium Ferrous sulphate powder in 250 ml distilled water in a 250 ml volumetric flask. Add 5ml Sulphuric acid.
  3. Preparation of Potassium Hydrogen Phosphate solution: Dissolve 0.425 gm of dried KHP powder in 1000 ml distilled water in a 1000 ml volumetric flask. (Dry the powder for 2 hours in an oven)

Procedure:

  1. Place 0.4 gm AgSo4 (silver sulphate) in each COD tube.
  2. Add 20 ml sample to each COD tube.
  3. Add 20 ml distilled water to each COD tube.
  4. Add 1-2 pieces of stones (crushed beads).
  5. Add 10 ml (0.25 N) potassium dichromate solution.
  6. Slowly add 30 ml concentrated sulfuric acid while swirling. Add more K2Cr2O7 solution if the solution turns green.
  7. Place COD tubes in the COD apparatus and set the temperature to 150°C.
  8. Digest the samples for 2 hours at 150°C.
  9. After 2 hours, switch off the apparatus, remove the COD tubes, and cool the samples to room temperature.
  10. Transfer the samples to conical flasks and add 150 ml distilled water.
  11. Perform titration with 0.1 N FAS solution using Ferron indicator.
  12. Calculate COD using the formula provided.

Formula of COD:

COD = {(a-b) X N X 8000}/ml of sample solution Where:

  • A: Reading of blank solution
  • B: Reading with sample solution
  • N: Normality of FAS solution
  • Dilution factor = (ml of sample + distilled water) / ml of sample

Conclusion:

The experiment demonstrates the synthesis of silver sulfate from silver with a yield of 7.5 grams from 5 grams of silver. This method requires a quarter less amount compared to the market price of standard silver sulfate. Future work will focus on the recovery of silver sulfate from waste silver chloride.