The determination of dissolved oxygen content in water serves as the basis of BOD test.All the gases of the atmosphere are soluble in water to some degree.The amount of oxygen found dissolved in given water at a given temperature and pressure is known as the dissolved oxygen.
Water being the liquid in greatest abundance and importance, the environmental engineers must be concerned with this parameter, namely dissolved oxygen. Oxygen is poorly soluble in water; its solubility is proportional to its partial pressure.
The solubility of atmospheric oxygen in fresh water ranges from14.6mg/L at zero degree Celsius to about 7 mg/L at 35-degrees Celcius under 1 atmospheric pressure.
The Winkler method of experiment and its modifications are the standard volumetric procedures for determining dissolved oxygen at the present time.
These methods depend upon reactions that release an amount of iodine equivalent to the amount of oxygen originally present, with subsequent measurement of the amount of iodine released by means of a standard solution of a reducing agent.
Dissolved Oxygen (DO) can oxidize divalent manganese ions (Mn2+) to tetravalent manganese ions (Mn4+) in the presence of a strong alkali.
This tetravalent Mn is again reduced to divalent Mn in the presence of a strong acid, liberating an equal amount of free iodine from the iodide.
The quantity of iodine liberated will be proportional to the quantity of DO present. The iodine can be estimated by titrating with a standard solution of sodium thiosulphate using starch as indicator.
When interference from nitrites is present it is impossible to obtain a permanent endpoint. Nitrite interference may be easily overcome by the use of sodium azide (NaN3) with the alkali-KI reagent.
- 300mL BOD bottles
- Volumetric flasks
- Standard manganese sulphate Solution (MnSO4)
- Alkali – Iodide – Azide reagent (NaOH + KI + NaN3)
- Concentrated Sulphuric Acid
- Standard Sodium Thiosulphate solution (0.025N)
- Starch indicator
Procedure for determination of dissolved oxygen in water.
For the determination of dissolved oxygen content in water by Winkler method experiment.First we needed to
- Fill the given water sample in a glass-stoppered 300mL BOD bottle. Be careful to avoid contact of the sample with air. The bottle should be completely filled.
- Immediately after filling the BOD bottle add 2 mL of manganese sulphate solution by means of a pipette, dipping the end of the pipette just below the surface of the water.
- Add 2 mL of Azide Alkali potassium iodide in a similar manner. (Since the bottle was already full, 4mL of water will be spilt out of the bottle by the addition of the above two reagents, for which correction will have to be made while taking out known volume water for testing).[wpforms id=”523″ title=”true” description=”true”][wpforms id=”523″][wpforms id=”523″ description=”true”]
- Insert the stopper with care to exclude air bubbles and mix by repeatedly inverting and shaking the bottle vigorously.
- A red precipitate will form if D.O is present in water. Allow the precipitate to settle
- Add 2 mL of a con: sulphuric acid to the bottle.
- Insert the stopper at once and mix thoroughly as before.
- Allow the solution to stand for at least five minutes to ensure the formation of iodine, which is to be titrated against sodium thiosulphate.
- Withdraw 200 mL of this solution and titrate with standard sodium thiosulphate solution to a pale yellow colour. 10. Add 1-2 mL of starch solution.This will give a blue colour.
- Now, continue the titration to the first disappearance of blue colour.
- Record the volume of thiosulphate added (A mL)
The DO of the given sample will get in terms of mg/L
The video below shows the Winkler method experiment to find dissolved oxygen content in water.