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

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

Apparatus:

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)

* 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

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

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

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

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.

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

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