Chemical Oxygen Demand IS 3025

In environmental chemistry, the chemical oxygen demand (COD) is an indicative measure of the amount of oxygen that can be consumed by reactions in a measured solution.

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Principle

Most types of organic matter are oxidized by a boiling mixture of chromic and sulfuric acids. A sample is refluxed in strongly acid solution with a known excess of potassium dichromate. After digestion, the remaining unreduced dichromate is titrated with ferrous ammonium sulphate to determine the amount of dichromate consumed and the oxidizable matter is calculated in term of oxygen equivalent. The contents are refluxed for 2 hours.

2K2Cr2O7 + 8H2SO4 → 2K2SO4 + 2Cr2 (SO4)3 + 8H2O + 3O2
C6H12O6 + 6O2 → 6CO2 + 6H2O

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Apparatus & Glasswares

1. COD reflux apparatus with condenser.
2. Volumetric Flask
3. Pipette
4. Burette

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Interference

• Straight-chain aliphatic compounds, aromatic hydrocarbon fatty acids, chlorides, nitrites and iron are the main interfering radicals.
• The interference caused by chlorides can be eliminated by the addition of mercuric sulphate to the sample prior to addition of other reagents. About 0.4 g HgS04 is adequate to complex 40 mg CI ions in the form of poorly ionized HgCl.
• Nitrite exerts a COD of 1.1 mg O2/mg NO2- -N, hence 120 mg of sulphamic acid is added to potassium dichromate solution to avoid interference caused by nitrite (NO2).When 20 ml sample and 10 ml dichromate is taken, this can take care of concentrations up to 6 mg/L NO2-N.
• Silver sulphate is added to concentrated H2SO4(22 g/4 kg acid) as a catalyst. This accelerates the oxidation of straight-chain aliphatic and aromatic compounds.
• For complete and better oxidation of organic matter it is necessary to maintain the final concentration of H2SO4 at 18 N.

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Reagent

• Standard potassium dichromate, 0.25 N:

  Dissolve 12.259 g of K2Cr2 O7 previously dried at 150°C for 2 h, in distilled water and dilute up to 1000 mL.

• Sulfuric acid – silver sulphate reagent:

  Add 25g Ag2SO4 to 2500 mL of conc. H2SO4 and keep overnight for dissolution.

• Standard Ferrous Ammonium Sulphate (FAS), 0.25 N:

  Dissolve 98g FAS in about 400mL distilled water and add 20mL of conc. H2SO4, Cool and dilute it to 1000 mL. Standardize the solution daily against the standard K2Cr2 O7.

• Standard Ferrous Ammonium Sulphate (FAS), 0.1 N:

  Dissolve 39.2 g FAS in about 400mL distilled water and add 20mL of conc. H2SO4, Cool and dilute it to 1000 mL. Standardize the solution daily against the standard K2Cr2 O7.

  
  Standardization of FAS:
  • Take 10 mL of K2Cr2 O7 in conical flask and dilute to 100 mL with distilled water.
  • Acidify by 30 mL of H2SO4 and cool.
  • Titrate with FAS using 2-3 drops of ferroin indicator.
  
  

  Normality of FAS =$\frac{N of K2Cr2O7 \times 10}{ mL of FAS }$

• Ferroin indicator:

  Dissolve 1.485 g of 1, 10- phenanthroline monohydrate, together with 695 mg of FeSO4.7H2O in water and dilute to 100 mL.

• Mercuric sulphate (HgSO4) :

  Analytical grade crystals

• Potassium hydrogen phthalate (KHP):

  Dissolve 0.425 g (425 mg) lightly crushed dried potassium hydrogen phthalate in distilled water and dilute to 1000 mL. This solution has a theoretical COD of 500 μg O2/mL. This solution is stable when refrigerated, up to 3 months in the absence of visible biological growth.

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Procedure

1. Take a clean 150 mL capacity flat bottom COD tubes.
2. Add 10mL of 0.25 N K2Cr2O7 and add 0.4g of HgSO4 in the flask. Add slowly 30mL H2SO4 containing Ag2SO4 mixing thoroughly. This slow addition along with swirling prevents fatty acids to escape out due to high temp.
3. Add sample with mixing when slight green color appear, stop the addition of sample. And remaining amount of distilled water up to 20 mL of total volume of sample.
4. Connect the flask to condenser. Mix the contents before heating, improper mixing will result in bumping and sample may be blown out.
5. Reflux for a minimum of 2 hours. Cool and wash down the condenser with distilled water.
6. Solution takes in flask. Add distilled water dilute for a minimum of 150 mL.
7. Cool and titrate excess K2Cr2O7 with 0.1N FAS using ferroin indicator sharp color changes from blue green to wine red indicates end point or completion of the titration.
8. Reflux blank in the same manner using distilled water instead of sample.

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Chemical Oxygen Demand is calculated as

COD(mg/L)=$\frac{(A-B) \times N \times 8 \times 1000}{ mL of the sample }$

Where,

• A = mL of FAS used for blank.
• B = mL of FAS used for sample
• N=Normality of FAS