Determination of COD in Sewage


To determine the COD of the given sewage sample by Open Reflux method.


Reflux apparatus of capacity 500m1 with a condenser, Burner or hot plate, 250 or 500mL Erlenmeyer flask with standard (24/40) tapered glass joints, Friedrich’s reflux condenser (12 inch) with standard (24/40) tapered glass joints, Volumetric pipettes (10, 25, and 50ml capacity), Burette, 50ml with 0.1ml accuracy, Burette stand and clamp, Analytical balance, accuracy 0.001gm, Spatula, Volumetric flasks (1000ml capacity), glass beads.


  • 0.25 N Std Potassium-dichromate (0.04167 M), Conc. Sulphuric acid reagent, (0.25M) Std Ferrous ammonium sulphate , Ferroin indicator, Mercuric sulphate crystals, analytical grade and Standard Potassium hydrogen phthalate (KHP)


The organic matter gets oxidized completely by K2Cr2O7 in the presence of H2SO4 to produce CO2 and H2O. The excess K2Cr2O7 remaining after the reaction is treated with Ferrous Ammonium Sulphate .The dichromate consumed gives the oxygen required for oxidation of the organic matter.



Dilute 10 ml standard K2Cr2O7 to about 100 ml, add 30 ml conc.H2SO4, cool. Add 2 drops of ferroin indicator and titrate with FAS.


Place 0.4 gm HgSO4 in a reflux flask. Add 20 m1 sample or an aliquot of sample diluted to 20m1 with distilled water. Mix well and add glass beads followed by 10 m1 Std K2Cr2O7. Add slowly 30 m1 H2SO4 containing Ag2SO4, mixing thoroughly. Connect the flask to the condenser and mix the contents before heating. Reflux for a minimum period of 2 hours, cool and wash down the condenser with distilled water. Dilute to about 150 m1, cool and titrate excess K2Cr2O7 with 0.1 N FAS using Ferroin indicator. Sharp colour change from blue green to wine red indicates the end point. Reflux the blank in the same manner using distilled water instead of the sample.


COD (mg/l) = [ ( a – b) × 8000 × N ] / ml of sample.

Where a & b = ml of FAS for blank and sample respectively.

N = Normality of FAS (0.1 N )


Trial NoInitial ReadingFinal readingDifference



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