ENVIRONMENTAL BIOTECHNOLOGY

Experiment 5

Aim of the Experiment : To determine the chemical oxygen demand (COD) for a given polluted sample

Principle

Chemical Oxygen Demand (COD) is the measure of Oxygen consumed during the oxidation of the oxidizable organic matter by a strong oxidizing agent. All organic compounds, with a few exceptions, can be oxidized by the action of strong oxidizing agents under acidic conditions. Potoassium Dichromate in the presence of sulphuric acid is generally used as an oxidizing agent for determination of COD. During the determination of COD, such organic matter is completely converted to CO2 and H2O when the sample is refluxed with K2Cr2O7 and H2SO4 in presence of mercuric sulphate to neturalize the effect of chlorides, and silver sulphate (catalyst). The excess of potassium dichromate is titrated against ferrous ammonium sulphate (FAS) using ferroin as an indicator. The amount of K2Cr2O7 used is proportional to the oxidizable organic matter present in the sample.

The dichromate consumed gives the chemically produced O2 required for oxidation of the organic matter in mg/L unit.

Materials required

• 01N Ferrous Ammonium Sulphate [(NH4)2Fe(SO4)2·6H2O],

• 0.025N Potasium Dichromate (K2Cr2O7),

• Conc. Sulphuric Acid (H2SO4),

• Ferroin Indicator,

• Mercuric Sulphate (HgSO4),

• Silver Sulphate (Ag2SO4)

Procedure:

Sample: Take 0.5 g (pinch) HgSO4 in a 250 – 500 mL COD flask (Round bottom flask with a ground joint for Liebig reflux condenser) as shown in the figure (Next Page). Add 20 mL diluted water sample in to the flask. Add 10 mL of 0.025 N K2Cr2O7 solutions and carefully add 30 mL of conc. sulphuric acid containing (a pinch) Ag2SO4. Mix and add glass bids/porcelain pieces to mixture to prevent bumping.

Attach the flask to condenser (as shown in the figure) and reflux for 2 hours on heating mental. Remove the flask, cool and wash the condenser with distilled water, cool to room temperature.

Add 2 – 3 drops of ferroin indicator, mix thoroughly and titrate with 0.01 N FAS till color changes from bluish green to wine red as end point. Carry out the blank with 20mL of distilled water instead of water sample and same quantity of the chemicals.

Observation

Part I (Blank)

Burette : 0.01 N FAS solution

Flask : 10 mL 0.025N K2Cr2O7 + 20 mL distilled water + 30 mL Conc. H2SO4 + 0.5 g of HgSO4 + a pinch of Ag2SO4

Indicators : 1 – 2 drops of Ferroin indicator

End point : Bluish Green to Wine Red Color

Part II (Sample)

Burette: 0.01 N FAS solution

Flask: 10 mL 0.025N K2Cr2O7 + 20mL Diluted Water Sample + 30 mL Conc. H2SO4 + 0.5 g of HgSO4 + a pinch of Ag2SO4

Indicators: 1 – 2 drops of Ferroin indicator

End point : Bluish Green to Wine Red Colour

Calculation:

COD, mgL−1 = (b − a) × Normality of K2 Cr2 O7 × 1000 × 8

mL of Water Sample

Where,

a = mL of titrant with sample

b = mL of titrant with blank

=( − ) × 0.025 × 1000 × 8

20

Result:

COD of the given polluted water sample = _mgL-1