Water Analysis : To Estimate Chemcial Oxygen Demand (COD) Of The Given Sample

INTRODUCTION:

Chemical Oxygen Demand is used as a measure of oxygen equivalent to organic matter content of the sample susceptible to oxidation by strong chemical oxidant. It is useful in monitoring and control of pollution of liquid wastes. The process is carried out at 160^OC in presence of concentrated sulfuric acid. Another parameter used for pollution control is BOD, which also estimates the dissolved oxygen content, but uses biological system. Both the parameters can be correlated. However, COD is used when BOD cannot remove certain wastes like petrochemical waste, volatile alcohol, pyridine and others.

PRINCIPLE:

Boiling mixture of chromic and sulfuric acids oxidize most types of organic matter. K₂Cr₂O₇ reacts with organic matter in presence of heat and the acid oxidizes the organic matter so that the residual K₂Cr₂O₇ can be measured.

REAGENTS:

·         Standard potassium dichromate solution (0.0147M): Dissolve 12.259 gms of K₂Cr₂O₇ in water and dilute to 1 liter.

·         Sulfuric acid reagent: Add 5.5 gms of AgSO4 /kg of H₂SO₄

·         Ferroin indicator

·         Standard ferrous ammonium sulfate (0.025 M) Mix 98 gms FAS and 20 ml of H₂SO₄ and make the volume to 1 liter.

PROCEDURE:

·         Take 25 ml of sample in COD tube or round bottom flask

·         Add 12.5 ml of K₂Cr₂O₇ mix, and add 37.5 ml of concentrated H₂SO₄ solution slowly from the edge of the container.

·         Attach the condenser on the top of the tube.

·         Allow it to react at 160^OC for 1 hour.

·         Cool the mixture and add ferroin indicator (2-3 drops)

·         Titrate it against 0.025M FAS and note the readings.

·         Calculate the amount of organic matter in mg/L by the following formula

CALCULATION:

COD  =             (A-B) x M x 8000      .

 mL of sample

Where, A =Titration reading of Blank

B = Titration reading of Sample

M = Morality of FAS