Dissolved oxygen determination methods and calculations

Dissolved oxygen determination methods and calculations

Dissolved oxygen parameter indicates the biological activity in water. It is one of the spec measurement used for knowing the condition of waste water. Two familiar methods are used for determining the DO in waste water.

• Winkler’s method (chemical analysis)
• Polarographic method

Sample for testing Dissolved Oxygen (DO) should be minimum 250ml. Water sample should be collected in bottle with narrow neck and a rubber or glass stopper to prevent exposer to atmospheric oxygen.

Winkler’s method for Dissolved oxygen determination (chemical analysis):

Reagents:

• Concentrated sulfuric acid
• KMnO4, 6.32g/liter distilled water prepare in 1 liter volumetric flask
• Potassium oxalate
• Manganous sulphate
• Manganous hydroxide, dissolve 400g of MnSO4.2H2O in 1 liter distilled water
• KOH, potassium hydroxide
• KI, potassium iodide
• sodium thiosulphate

Procedure:
Sample preparation: For the water in the sample bottle, 0.7 ml concentrated sulfuric acid is added and continued with 1 ml KMnO4 solution of 0.2 Normality. These reagents oxidize all the organic matter and metallic and non metallic compounds ( sulphides, ferrous etc.). After addition of the reagents throughly mix the bottle so that a precipitate formation occurs. Allow the bottle to settle the precipitate to the bottom if turbidity is visible add few more KMnO4 the sample in the bottle. When all the precipitate is settled out add potassium oxalate (2.17 Normality) to neutralize excess KMnO4. Now the sample is ready for determining the DO.
Alkaline KI solution is prepared by adding 150g of KI and 700g of KOH to 1L distilled water. 5 ml of the alkaline KI solution is added along with 1 ml manganous sulphate solution to the sample bottle. Finally to react the dissolved oxygen in the sample added 1 ml concentrated sulfuric acid, iodine will be liberated by the dissolved oxygen reaction with KI.
Titration: 0.025 Normality sodium thiosulphate is filled in the burette and sample is taken in 500 ml conical flask. 2 to 3 drops of starch solution is added in the conical flask as colour indicator. The sample turns to blue colour when starch is added. Gradually add the sodium thiosulphate from the burette to the conical flask till the blue colour disappears.
Calculation:
Dissolved oxygen ppm = volume of sodium thiosulphate consumed in ml X Its Normality X 0.008 X 10^6 / volume of sample in ml.