BOD is a measure of the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material in water over a specific time period, usually 5 days. It is determined by measuring the dissolved oxygen concentration in a water sample before and after incubation. The Winkler method is used, in which manganese is oxidized to a brown precipitate by dissolved oxygen, then dissolved and titrated against a standard solution. COD measures the amount of oxygen required to chemically oxidize all organic compounds in water, determined by refluxing a sample with potassium dichromate and titrating the remaining dichromate with ferrous ammonium sulfate.

1. S.Keerthanan
Bs-1121
EUSL

2. What is BOD?
 Is the amount of dissolved oxygen required to oxidize
the organic compound by living organism in water
sample.
 Which occurs at certain temperature over a specific
time period.
 Here the organic compound oxidize by CO2 and H2O.

3. Method to determination of BOD
 Dissolved oxygen concentration in a sample must be
measured before and after incubation.
 Incubation occurs at dark place.

4. Winkler method
 Mn2+ and KI are added to the water sample.
 In alkaline solution, the dissolved oxygen will oxidize
Mn2+ as Mn4+.
 2 MnSO4 + O2 → 2 MnO2
 Which appears as brown color precipitate.
 In acid medium, the brown color precipitate will
dissolve and back to solution.

5. Cont……
 The Mn4+ reduces as Mn2+ while the I- oxidize as I2.
 Mn(SO4)2 + 2 I− → Mn2+ + I2 + 2 SO4
2−
 Next, the liberated I2 is titrated against standard
S2O3
2− solution.
 2 S2O3
2−(aq) + I2 → S4O6
2−(aq) + 2 I−(aq)

6. Net balanced equation
 O2(aq) + 2H2O (l)+ 4 S2O3
2−
(aq) → 2S4O6
2−
(aq) + 4OH-
(aq)
Equation for BOD
 BOD= [Disolved oxygen]before incubation – [Disolved oxygen]After incubation

8. What is COD?
 The COD is a test.
 To determine the amount of oxygen that needs to
oxidize all organic compound in water sample
chemically.
 COD is normally expressed in ppm OR mg/l.

9. Method to determine the COD
Reflux Method.
‐ Initially add HgSO4.
‐ Add H2SO4 very slowly.
‐ Add Excess amount of K2Cr2O7(aq).

10. Cont….
 titrate remaining unreduced K2Cr2O7 with FAS(Ferrous
Ammonium Sulfate), using 2 to 3 drops ferroin
indicator.
 At end point, the color change from blue-green to
reddish brown.

12.  a = volume of FAS used for blank solution.
 b = volume of FAS for used for sample.
 m = molarity of FAS.
 vs = volume of sample.

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