The Biochemical Oxygen Demand (BOD) is a chemical procedure for determining the amount of dissolved oxygen needed by aerobic organisms in a water body to break the organic materials present in gives water samples at certain temp. Over a specific period of time.

BOD is the amount of O2 needed for the biological decomposition of dissolved organic matter to occur under standard conditions at standard time and temperature.

Temp = 20°C, Time = 5 days

Apparatus:- BOD bottle, Beaker (250ml), Measuring cylinder, Dropper, Stirrer

Reagents:- Manganous sulphate soln, Alkaline potassium iodide soln, 0.025μ Sodium thiosulphate, Starch soln (indicator), conc H₂SO₄ 

BOD is the principle test to give an idea of the biodegradability of any sample & strength of waste. The determination of BOD of water is useful in the design of treatment facilities.

BOD test is among the most imp process in sanitary analysis to determine the polluting power or strength of sewage, polluted water.

Drinking water std for BOD is 0.2mg/L at 20°C

Procedure:-

1. Add 1ml of Manganous sulphate soln to BOD bottle by means of pipette dipping in end of pipette just below surface of water.
2. Add 1 ml of Alkaline potassium iodide soln to BOD bottle in a same way.
3. Insert stopper and mix by inverting the bottle several times.
4. Precipitaes settle & mix again halfway.
5. Add 1 ml of H₂SO₄ . Immediately insert stopper and mix as before.
6. Allow soln to stand at least 5 min.
7. Withdraw 100ml of soln into flask.
8. Add about 1 ml of Starch soln & continue until the blue color just disappears.
9. Record the ml of thiosulphate used.