Chemical Oxygen Demand (COD)
The chemical oxygen demand is defined as the amount of oxygen expressed in gram per liter consumed under specified conditions in the oxidation of organic and oxidisable inorganic materials. It is quick method in comparison to BOD for the determining the total organic and oxidisable inorganic pollution in a sample of water. Pollution like cellulose which are not measurable by BOD test since it can’t be bio-degraded by the oxygen dissolved in water; are easily measured by COD test. In COD test, acidified K2Cr2O7 is used instead of oxygen which is stronger oxidizing agent than oxygen.
The COD evaluation is carried out by adding a known excess of aq. Cr2O7-2 to the water sample and then estimating the unconsumed Cr2O7-2 by titrating it with Fe2+ solution [Mohr’s Salt Solution, FeSO 4(NH3)2SO4.6H2O].
When water sample is high polluted with organic wastes, the oxygen demand will exceed the maximum equilibrium solubility of O2 in water and such a water sample may not have any dissolved oxygen. The basic reaction involved in the COD test is
Cr2O7-2 + 14H+ + 6e– ———–> 2Cr3+ + 7H2O
O2 + 4H+ + 4e– —————-> 2H2O
From above reaction,
1 mol of Cr2O7-2 consumed = 6/4=1.5 moles of oxygen
Let amount of Cr2O7-2 initially taken = x mol
And amount of Cr2O7-2 left (unused) =y mol
Then amount of Cr2O7-2 consumed = (x-y) mol
x-y mol of Cr2O7-2 consumed =1.5 (x-y) mol oxygen
Dissolved Oxygen (DO)
The amount of oxygen dissolved in sample of water as uncombined form is known as dissolved oxygen. The fresh water has 5 to 7 mg per liter of dissolved oxygen. This level of DO is essential for supporting aquatic lives including micro organisms. The aerobic decay of organic waste matters by micro organisms keeps the water in fresh and clean condition.
If the DO decreases due to water pollution, then anaerobic decay may occurs which forms bad odor and indicates the polluted water.
When the water is polluted with large amounts of organic matter, a lot of dissolved oxygen would be rapidly consumed in the biological degradation. The decrease in the DO affect the aquatic life and may completely damage the ecosystem of the water.
DO is consumed by oxidation of organic substance in the presence of certain micro organisms or by reducing agent such as
CH2O + O2 —————> CO2 + H2O
2SO3-2 + O2 —————–> 2SO4-2
Determination of DO plays a key role in the control of the water pollution. It can be determined by iodometric titration method. In this method, dissolved oxygen is allowed to react with I– solution to form I2 which is then titrated with standard Na2S2O3 solution (hypo solution).It is carried by addition of Mn+2salt in strongly alkaline medium. The basic reaction in the titration is,
Mn+2 + ½ O2 + 2OH– ————> MnO2 ↓ + H2O
MnO2 + 2 I– + 4H+ ———–> Mn+2 + I2 + 2H2O
I2 + 2S2O3-2 ————–> 2I– + S4O6-2
From above, 2mol of S2O3-2= 1 mol I2 = ½ mol O2