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 160OC 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.
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.
· 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.
· 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 160OC 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
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