Experimental determination of COD by iodometric titration

Principle:

 A known volume of water sample is taken and refluxed with known volume of K₂Cr₂O₇ in conc. H₂SO₄ in presence of Ag₂SO₄ and HgSO₄. The unused potassium dichromate is titrated against FAS, using ferroin as indicator. It is back titre value. Blank titration is carried out without water sample; this titre value is blank titre value. The difference between the two titre value curresponds to the amount of potassium dichromate required to oxidize the impurities present in water sample.

Procedure:

Back titration:

·         A known volume waste water sample is pipette out into conical flask.

·         Add 10 ml of K₂Cr₂O₇ solution using pipette. About 1 test tube full of conc. H₂SO₄ followed by 1g of Ag₂SO₄ and 1g of HgSO₄ is added to it.

·          The contents are refluxed for 30 minutes, on a water bath. The solution is cooled to room temperature, 3 drops of ferroin indicator is added to the flask.

·         It is titrated against standard FAS solution till the color changes from bluish green to redish brown. Volume of FAS consumed is noted ( ‘V₁’ ml)

Blank titration:

·         The above procedure is repeated by taking the same volume of distilled water in place of the waste water sample.

·         Volume of FAS consumed is noted (‘V2’ ml) and COD is calculated.

Calculations:

The volume of waste water sample = V ml

Normality of FAS used= N

Volume of FAS from back titration = V₁ ml

Volume of FAS from blank titration = V₂ ml

Amount of K₂Cr₂O₇ consumed interms of FAS = (V₂ - V₁) ml

Therefore normality of oxygen =    (V₂ - V₁) x N

                                                        V

                                               = N₁

 Hence, COD= N₁ x Equivalent weight of oxygen

= N₁ x 8 x 1000 mg/dm³ = …………….. mg/dm³