Chemical Oxygen Demand Test

To determine COD (mg/L) of a sample given.

Introduction

The standard test for COD involves digesting the sample with strong sulphuric acid solution in the presence of chromium and silver salts.  These potentially hazardous chemicals clearly need to be handled with extreme care.  Recent health and safety concerns have made it necessary to find a means of carrying out this vitally important test whilst at the same time minimizing the risk to laboratory staff.

Theory

Chemical Oxygen Demand (COD) estimates the amount of organic matter in wastewater, industrial effluent and water from a variety of sources. Chemical oxygen demand (mg/L COD) is defined as the amount of oxygen consumed per liter under the condition of the experiment. During the oxidation of organic materials by dichromate in sulfuric acid with a silver compound as a catalyst, carbon is oxidized to form carbon dioxide and hydrogen in the organic compound is converted to water. A mercuric compound is normally added to reduce interference from the oxidation of chloride ions by the dichromate. In this experiment, the dichromate is reduced to green chromic ion (Cr+3) by oxidizable organic compounds. The determination of COD can be performed in the field or in the laboratory with various colorimeters and the spectrophotometer. However, most of the digestion experiment will be performed in the laboratory unless adequate reduced chromium (chromic iron) or the amount of unrelated dichromate can be measure. The COD analysis is relative fast compared with the BOD analysis. The COD of a waste is, in general, higher than the BOD because more compounds can be chemically oxidized than biologically oxidized.

Apparatus

  1. Blender, 2-speed 120 VAC
  2. Pipet, Tensette @, 0.1 – 1.0 mL
  3. Pipet, Volumetric, Class A, 2.00 mL
  4. COD Reactor (HACH)
  5. Test Tube
  6. Spectrophotometer 

Working Procedure

  1. Homogenize 100 mL of sample for 30 second in a blender (For sample containing large amount of second of solids, increase the hormogenization time).
  2. For the 200 – 1500 mg/L range or to improve accuracy and reproducibility of the order ranges, pour the hormogenized sample into a  250 mL beaker and gently stir with a magnetic stir plate.
  3. Turn on the COD reactor.
  4. Per-heat to 15 °C. Place the safety shield in front of the reactor.
  5. Remove the caps from two COD Digestion Reagent Vial (Be sure to use vial for the appropriate range).
  6. Hold one vial at a 45 – degree angle. Use a clean volumetric pipet (Ten Sette Pipet) to add 2.00 mL of sample to the vial. This is the prepared sample.
  7. Use a second vial at a 45 – degree angle. Use a clean volumetric pipet Ten Sette Pipet) to add 2.00 mL of deionized water to the vial. This is the blank.
  8. Caps the vials tightly. Rinse them with deionized water and wipe with a clean paper towel.
  9. Hold the vials by the cap over sink. Invert gently several times to mix. 
  10. Caution   :   The sample vial will become very hot during mixing.
  11. Please the vials in the preheated COD Reactor. Heat the vials for two hours.
  12. Turn the reactor off (Wait about 20 minutes for the vials to cool to 120 °C or less.
  13. Invert each vial several times while still warm. Lace the vials into a rack and cool to room temperature.
  14. Touch”HACH Programs”.
  15. Select Programs “431 COD ULR “ (Ultra-Low range) or “430 COLD LR” (Low Range) or “435 COD HR” (High Range / High Range Plus).
  16. Touch “Start”.
  17. Clean the outside of vials with a damp towel followed by a dry one to remove fingerprint or other marks.
  18. Place the blank into the cell holder.
  19. Touch “Zero”. The display will show: 0 mg/L COD.
  20. When the timer beeps, place the sample vials into the cell holder.
  21. Result will appear in mg/L COD.

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