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Dissolved Oxygen Measurement by Winkler Method

In this dissolved oxygen measurement by winkler method post we have briefly explained about biological oxygen demand of water, winkler method principle, requirements, procedure, result, and calculation.

Biological Oxygen Demand of Water

All aquatic species rely on dissolved oxygen to survive. Organic debris released into the water serves as a food supply for aquatic microbes. The use of dissolved oxygen by bacteria will rise when waste products are disposed of more often. As a result, the oxygen content of the water decreases.

Microorganisms will develop offensive compounds in this anaerobic environment, which may cause undesired outcomes such as fish asphyxiation. As a result, the amount of dissolved oxygen in the water is a good predictor of its quality.

Biological oxygen demand of water method is extensively used to express the amount of organic matter in waste water samples. Biological oxygen demand of water test a measurement of how much dissolved oxygen in use.

Principle

Manganese sulphate generates a white precipitate of manganese hydroxide when exposed to alkaline conditions by adding Alkaline-iodide-azide. This forms a brown precipitate when it reacts with the dissolved oxygen in the sample. Manganese shifts to its divalent state in acidic conditions, releasing iodine. With starch as an indication, the liberated iodine is titrated against sodium thiosulphate.

Mn(OH)2 + ½.O2 —–> MnO(OH)2

Requirements

Chemicals

1. Manganese Sulfate

2. Alkali-Iodide-Azide

3. Concentrated sulfuric Acid

4. Starch Solution

5. Sodium Thiosulfate

Materials

1. OD bottle

2. Water bottle

3. Pipettes

4. Measuring cylinders

5. BOD Incubator

6. Burette and burette stand

7. Standard flask

Procedure

1. Fill a 300-mL glass biological oxygen demand of water stoppered bottle with sample water to the brim.

2. By putting the calibrated pipette just below the surface of the liquid, immediately add 2mL of manganese sulphate to the collection container. In the same way, add 2 mL of alkali-iodide-azide reagent.

3. A brownish-orange cloud of precipitate or floc will form if oxygen is present. When the floc has settled to the bottom, turn the sample upside down several times and let it settle again.

4. Using a pipette placed just over the surface of the sample, add 2 mL of concentrated sulfuric acid. The sample is now “fixed,” and it can be stored in a cool, dark place for up to 8 hours.

5. Titrate 200 mL of the sample with sodium thiosulfate to a pale straw colour in a glass flask. Titrate by slowly pouring titrant solution into the flask using a calibrated pipette while stirring or swirling the sample water. To make a blue colour, add 2 mL of starch solution.

6. Continue titrating carefully until the sample becomes clear. The number of millilitres of titrant used is equal to the concentration of dissolved oxygen in the sample.

Results

The concentration of dissolved oxygen in the sample is equivalent to the number of millilitres of titrant used.

Temperature (C)

Oxygen Solubility (mg/L)

        0

          14.6

        5

          12.8

       10

          11.3

       15

          10.2

       20

          9.2

       25

          8.6

       100

          0

Further Readings

Reference