Chemical oxygen demand (COD)

CHEMICAL OXYGEN DEMAND (COD)

Chemical oxygen demand is the oxygen requirement of a sample for oxidation of organic and inorganic matter under specified conditions corrected for the influence of chlorides. The major advantage of the COD test is the short time required to carry out rather than the longer 5 days procedure for BOD. This test is recommended as a supplement to BOD test. In COD test, the sample is subjected to a chemical oxidation induced by chemical reagents and it is an artificial oxidation by which both biologically oxidisable  and biologically inert organic matter are oxidized.

Aim

         To determine the dissolved oxygen content of the given water sample.

Principle

         The organic matter of the sample is oxidized to water, CO2 and ammonia by reflux –condensing with a known excess of potassium dichromate in a 50 % sulphuric acid solution. The excess dichromate is titrated with standard solution of ferrous ammonium sulphate.

6 Fe2+ + Cr2O72- + 14H →     6 Fe3+ + 2 Cr3+ + 7H2O

Materials required

I.                    Mercuric sulphate crystals,

II.                 Sulphuric acid : Silver sulphate reagent – Dissolve 10.1 g silver sulphate (Ag2SO4) in 1000 mL sulphuric acid

III.               Potassium dichromate 0.25N:  Dissolve 12.258 g potassium dichromate (Previously dried at 1030C  for 2 hrs.) in distilled water and make up to 1000 mL in a volumetric flask. One mL of 0.25 N K2Cr2O7 = 2.0 mg Oxygen.

IV.              Ferroin indicator: Dissolve 695 mg ferrous sulphate (FeSO4.7H2O) and 1.485 g of 1,10 –Phenanthroline monohydrate together in distilled water and dilute to 100 mL and

V.                 0.1 N Ferrous ammonium sulphate : Dissolve 39.29 g ferrous ammonium sulphate in distilled water. Add 20 mL concentrated sulphuric acid, cool and make upto 1000 mL in a volumetric flask. Standardize it against 0.25 N potassium dichromate for standardization.

Pipette out 10 mL 0.25 N potassium dichromate solution into a 250 mL conical flask. Dilute to 100 mL and add 30 mL concentrated sulphuric acid and cool to room temperature. Add 2 to 5 drops of ferroin indicator and titrate with ferrous ammonium sulphate solution. The end point is the colour changes from blue green to reddish brown. 

Procedure

·         Pipette out 50 mL of sample  into a 500 mL conical flask and add 25 ml of standardized dichromate solution

·         Add carefully with gently mixing, 75 mL concentrated sulphuric acid – silver sulphate reagent.

·         Drop carefully 2 to 3 boiling chips into the flask and then attach a reflux condenser.

·         Reflux the mixture for 2 hrs with a small Bunsen flame. Cool the flask and rinse the inside of the condenser with a small amount of distilled water allowing the washing to run into the flask.

·         Dilute appropriately with distilled water. The final volume after dilution should be about 2.5 times the volume of sample + dichromate+ H2 SO4

·         Then add 2 to 3 drops of ferroin indicator and titrate the unreached dichromate with standardized ferrous ammonium sulphate in the burette. The end point is the sharp change of colour from blue green to reddish brown.

·         Simultaneously perform a blank using distilled water in place of sample.

·         From the titre value, calculate the COD

Calculation

         COD (mgL-1)            =   (V2-V3) x N x 8 x 1000

                                                                         V1

Where,

V1    = Volume of sample taken in mL

V2    = Blank titre value in mL

V3    = Sample titre value in mL 

N     = Normality of ferrous ammonium sulphate solution