Friday, October 2, 2020

Chemical Oxygen Demand

Determination of chemical oxygen demand of a water sample.

 Chemical Oxygen Demand

The chemical oxygen demand (COD) test measures the oxygen demand of the waste in account of the total amount of oxygen required for oxidation of the waste to carbon dioxide and water. The test is based on the fact that all organic compounds, with a few exceptions, can be oxidized by the action of strong oxidizing agents under acid conditions. 

Organic matter + Oxidizing agent = CO2 + H2O 

The reaction in above equation involves conversion of organic matter to carbon dioxide and water regardless of the biological assimilability of the substance. For example, glucose and lignin (biologically inert substance) are both oxidized completely by the chemical oxidant. In many cases  COD values are greater than Biochemical oxygen demand (BOD) values, when biologically resistant organic matter is present. Thus, there are a limitations of The chemical oxygen demand (COD) test is inability to differentiate between biodegradable and non-biodegradable organic matter. Another limitations that, inability to provide evidence of the rate at which the biologically active material would be stabilized under conditions that exist in nature. The time required for the evaluation of chemical oxygen demand (COD) test is very short, that is the major advantage of The chemical oxygen demand (COD) test. The  time required for The chemical oxygen demand (COD) test is about 3 hours rather than the 5-days required for the measurement of BOD. This is the reason for The chemical oxygen demand (COD)  is a substitute for the BOD test in many instances

Chemical oxygen demand title image

PRINCIPLE:

Potassium dichromate or potassium permanganate is usually used as the oxidizing agent in the determination of chemical oxygen demand (COD). In this class potassium permanganate would be used in the determination of chemical oxygen demand (COD). Potassium permanganate is selective in the reaction and attacks the carbonaceous and not the nitrogenous matter. In any method of measuring chemical oxygen demand (COD), an excess of oxidizing agent must be present to ensure that all organic matter is oxidized as completely as possible within the power of the reagent. This requires that a reasonable excess be present in all samples. It is necessary, therefore, to measure the excess in some manner so that the actual amount can be determined. For this purpose, a solution of a reducing agent (e.g., ammonium oxalate) is usually used.


APPARATUS:

  • Beaker (250 mL)
  • Dropper
  • Stirrer
  • Digestion vessels
  • COD Digester
  • Burette
  • pipette
  • A Round bottom reflux flask.

REAGENT:

  • Diluted sulfuric acid solution
  • Standard potassium permanganate solution
  • Standard Ammonium Oxalate solution.

PROCEDURE :-

Preparation of sample-

  1. Take about 10ml of distilled water in a round bottom reflux flask. Add 1ml of Mercury sulfate (MgSO4) and 5ml of Potassium Dichromate (K2Cr2O7) to the flask. The Potassium Dichromate used to oxidize all organic compound in the water sample
  2. Add about 15ml of diluted sulfuric acid to the reflux flask.
  3. Transfer the prepared solution to a Digestion vessel and place the Digestion vessel to a COD Digester for 2 hours. After 2 hours, move the Digestion vessel from COD Digester and rest it in a place and allow to cool for 30 minutes.

Titration of Blank solution-

  1. Transfer the solution to a conical flask and add about 2-4 drops of Ferroin indicator, The colour of the solution will changes to lite blue.
  2. Take 0.025M Ferrous Ammonium Sulfate to the burette and set the initial reading of burette to zero.
  3. Now start the titration until the colour of the solution taken in conical flask from blue to colourless. And take the final reading of burette as marked 'A' in the observation table.

Titration of the Water sample-

  1. Take 0.025M Ferrous Ammonium Sulfate to the burette and set the initial reading of burette to zero.
  2. The prepared solution taking in conical flask , and add 2-4 drops of Ferroin indicator, The colour of the solution will changes to lite blue. Then titrate the solution to the Ferrous Ammonium Sulfate taking in burette.
  3. Now start the titration until the colour of the solution taken in conical flask from blue to colourless. And take the final reading of burette as marked 'B' in the observation table.

OBSERVATION :- 

BOD table image



COD Formula image



PRECAUTION :- 

  • The complete Chemical oxygen demand test of a given water sample to be done in taken care.
  • During the test , the burette reading should be noted by observe as perpendicular position of eye to the burette.
  • Special care should be taken at the time of handle of COD Digester.



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