Calculation of Chemical Oxygen Demand(C.O.D)

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Round bottom flask, water condenser, Burette, Pipette,Heating mantle.

Reagents:

* Mercuric sulphate (< 1gm)

* Sulfuric Acid (1 Kg H₂SO₄ + 5.5 gm Ag₂SO₄ ) dissolve for 24 hrs.

* 0.25 N Potassium Dichromate (12.259 gm K₂Cr₂O₇ dried at 103⁰C + 1 Liter D.W.)

* 0.1 N Ferrous Ammonium Sulphate (39.2 gm Fe (NH₄)₂(SO₄)₂.6H₂O + 20 ml Conc.H₂SO₄ + 1 Ltr D.W.)

* Glass beads.

Procedure

* Wash all the apparatus with distilled water

* Dry the round bottom flask by keeping in oven for 10-15 minute

* Follow the steps as per protocol mentioned below.

Protocol

Reagent Blank Sample

Hg₂SO₄ Pinch Pinch

K₂Cr₂O₇ 10 ml 10 ml

Distilled water 20 ml Sample + D.W = 20 ml

1 Kg H₂SO₄ + 5.5 gm Ag₂SO₄ 30 ml 30 ml

Note: – for accuracy in results take raw effluent Sample of 0.2 ml to 0.5 ml.

* While adding acid keep the flask in ice bath to avoid loss of volatile Substances due to heat liberation.

* Add 1 – 2 glass beads to avoid the bumping during heating.

* Keep the flask for refluxing for 2 hours.

* Cool the flask thoroughly.

* Titrate with 0.1 N FAS (Ferrous Ammonium Sulphate) using Ferroin indicator

* Continue to titrate till colour turns wine red which is the end point

Calculations:

COD as mg/l of oxygen consumed =(A-B) x Normality of FAS x 8000 / ml. of sample taken

Where

A = ml. of FAS used for titration of blank

B = ml. of FAS used for titration of sample

Standardization of FAS

* 0.05 N Potassium Dichromate

(2.452 gm Anhydrous K₂Cr₂O₇ in 1 Ltr of Distilled water)

* 0.25 N Potassium Dichromate

(12.259 gm Anhydrous K₂Cr₂O₇ in 1 Ltr of Distilled water)

* Whenever a new solution of FAS or Potassium Dichromate is prepared it has to be standardized using Potassium Dichromate as primary Standard.

* Weigh 39.2 gms of FAS and dissolve it in 1000ml of distilled water by adding 20.0ml of concentrated sulphuric acid drop wise by cooling.

* Transfer to 1000ml volumetric flask and dilute it up to mark to get 0.1N FAS solution

* Take 10 ml of 0.25N Potassium Dichromate & 90 ml of distilled water

* Then add 30 ml of concentrated Sulphuric Acid (addition should be done by cooling

Simultaneously)

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* Titrate this solution against freshly prepared FAS using Ferroin indicator

* Continue titrating till wine red colour appears which the end point is.

Calculations:–

FAS K₂Cr₂O₇

N₁V₁ = N₂V₂

Where N₁ = Normality of FAS

V₁ = Volume of FAS used for titration of Dichromate

N₂ = Normality of Potassium Dichromate

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V₂ = Volume of Potassium Dichromate taken for titration

By putting the above values the Normality of FAS can be calculated

Precision & Bias: For COD of 200mg/lit in absence of chloride the standard

Deviation was found to be + / – 13 mg /lit of O₂ consumed.

Note: * The chief limitation of the COD test is its inability to differentiate between Biologically Oxidisable

And Biologically Inert Organic matter.

* It does not provide any evidence of the rate at which the Biologically active material would be

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Stabilized under condition that exists in nature. ( Ref : Page 545 – Perry L. McCarty )

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(Reference – Table 3-10, Page – 72, Metcalf & Eddy)

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Check Procedure for C.O.D.

• Lightly crush & Dry Potassium Hydrogen Phthalate at 120⁰C, Dissolve 425mgs in Distilled water and Dilute to 1000ml.
• This Solution has a theoretical C.O.D. of 500 ppm.
• This solution to be used as sample to cross check the procedure as well as the chemicals being added whether it gives the correct result.

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(This solution could be preserved for 3 months under refrigerated condition.)

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