2.11 Winkler Method for Biological Oxygen Demand | Electrochemistry | Chemistry 12

The Biological Oxygen Demand (BOD) is a crucial environmental parameter defined as:

The amount of oxygen used to decompose the organic matter in a sample of water over a specified time period (usually 5 days) at a specified temperature (typically $2 0^{\circ} C$ ).

Significance of BOD:

A high BOD indicates a greater quantity of organic waste in the water, which leads to a lower level of dissolved oxygen (DO).
Dissolved oxygen (DO) is a key indicator of the health of an aquatic ecosystem. Higher DO concentrations are generally correlated with high productivity and minimal pollution, supporting aquatic life.

Oxidation Reduction Concepts→

The Winkler Method

The Winkler Method, also known as the Iodometric Method, is a widely used redox technique to measure dissolved oxygen concentrations in freshwater systems.

Principle of the Winkler Method

The core principle involves a series of redox reactions:

Oxygen Fixation: Oxygen dissolved in the water sample is fixed by reacting with manganese(II) ions ( $Mn^{2 +}$ ) in an alkaline environment to form a manganese(IV) oxide ( $MnO_{2}$ ) precipitate.
Iodine Liberation: The manganese(IV) oxide precipitate is then reacted with iodide ions ( $I^{-}$ ) in an acidic medium, which reduces $Mn (IV)$ back to $Mn (II)$ and oxidizes iodide ions to molecular iodine ( $I_{2}$ ).
Titration: The amount of iodine produced, which is stoichiometrically equivalent to the original dissolved oxygen, is determined by titrating with a standard solution of sodium thiosulphate ( $N a_{2} S_{2} O_{3}$ ).
The amount of oxygen initially present in the water sample is calculated from the volume of thiosulphate solution used in the titration.

Activity 2.2: Determining Oxygen Present in a Water Sample (Winkler Method Procedure)

Materials Required

$300 cm^{3}$ BOD bottle
Manganese(II) sulphate solution ( $2 cm^{3}$ )
Alkali iodide azide solution ( $2 cm^{3}$ - an aqueous solution of $32.7% NaOH + 15% KI + 1% Na N_{3}$ )
Concentrated $H_{2} S O_{4}$ ( $2 cm^{3}$ )
Potassium iodide solution ( $2 cm^{3}$ )
Sodium thiosulphate solution ( $0.1 M$ )
Starch solution ( $2 cm^{3}$ )

Procedure Steps

Sample Collection: Collect the water sample in a $300 cm^{3}$ BOD bottle. This is done by immersing the bottle in the water, removing the cap underwater, filling the bottle completely, and then recapping it underwater to prevent air bubbles (and thus, additional oxygen) from entering.
Oxygen Fixation (Precipitation):
- Add $2 cm^{3}$ of alkaline iodide azide solution.
- Immediately after, add $2 cm^{3}$ of manganese(II) sulphate solution.
- Close the bottle with the cap and swirl it a few times.
- Observation: Oxygen dissolved in the alkaline solution oxidizes the manganese(II) ions to manganese(IV) oxide, which appears as a brown precipitate.
- Reaction 1 (Oxygen Fixation): $2 Mn_{(aq)}^{2 +} + 4 OH_{(aq)}^{-} + O_{2 (aq)} \to 2 MnO_{2 (s)} + 2 H_{2} O_{(l)}$
Iodine Liberation:
- Add $2 cm^{3}$ of concentrated $H_{2} S O_{4}$ .
- Then, add $2 cm^{3}$ of potassium iodide ( $KI$ ) solution.
- Observation: The brown precipitate will dissolve into a solution. In this acidic environment, $Mn (IV)$ is reduced back to $Mn (II)$ , liberating iodine ( $I_{2}$ ) in the process.
- Reaction 2 (Iodine Liberation): $MnO_{2 (s)} + 2 I_{(aq)}^{-} + 4 H_{(aq)}^{+} \to Mn_{(aq)}^{2 +} + I_{2 (aq)} + 2 H_{2} O_{(l)}$
Titration:
- Titrate $201 cm^{3}$ of the above prepared water sample against a standard sodium thiosulphate solution.
- Continue the titration until the solution turns pale yellow.
- At this point, add $2 cm^{3}$ of starch solution as an indicator. The solution will immediately turn blue (due to the starch-iodine complex).
- Continue adding sodium thiosulphate solution dropwise until the blue colour just disappears, indicating the endpoint of the titration.
- Reaction 3 (Iodine Titration): $2 S_{2} O_{3}^{2 -}_{(a q)} + I_{2 (a q)} \to S_{4} O_{6}^{2 -}_{(a q)} + 2 I^{-}_{(a q)}$
(Here, thiosulphate ions are oxidized to tetrathionate ions, and iodine is reduced back to iodide ions.)

Stoichiometry and Calculation

The stoichiometric relationships derived from the above reactions are crucial for calculating the dissolved oxygen:

$1 mole of O_{2} \to 2 moles of MnO_{2} \to 2 moles of I_{2} \to 4 moles of S_{2} O_{3}^{2 -}$

Therefore:

The moles of $O_{2}$ in the original sample are equal to $1/4$ the moles of $S_{2} O_{3}^{2 -}$ used in the titration.
After determining the number of moles of iodine produced from the titration, you can work out the number of moles of oxygen molecules present in the original water sample using this ratio.
The oxygen content is usually presented as $mg/d m^{3}$ (which is equivalent to ppm, parts per million) for practical reporting.

Possible Questions and Answers

Q: What is the primary purpose of the Winkler method?
A: To measure the concentration of dissolved oxygen in a water sample.
Q: Why is the Winkler method considered a redox technique?
A: It involves a series of reactions where oxidation and reduction occur, specifically the oxidation of $Mn^{2 +}$ by $O_{2}$ , followed by the oxidation of $I^{-}$ by $MnO_{2}$ , and finally the reduction of $I_{2}$ by $S_{2} O_{3}^{2 -}$ .
Q: What is the role of starch solution in the Winkler titration?
A: Starch acts as an indicator, forming a distinct blue complex with iodine ( $I_{2}$ ). Its disappearance signals the endpoint of the titration when all $I_{2}$ has reacted with thiosulphate.
Q: Why is it important to prevent air bubbles from entering the BOD bottle during sample collection?
A: Air contains oxygen, which would artificially increase the measured dissolved oxygen concentration, leading to an inaccurate result.
Q: If $0.0125$ moles of sodium thiosulphate were used in the titration, how many moles of $O_{2}$ were in the original sample aliquot?
A: From the stoichiometry, $4 moles of S_{2} O_{3}^{2 -}$ react with $1 mole of O_{2}$ .
Moles of $O_{2} = \frac{1}{4} \times Moles of S_{2} O_{3}^{2 -} = \frac{1}{4} \times 0.0125 mol = 0.003125 mol$ .

The Winkler Method

The Winkler Method, also known as the Iodometric Method, is a widely used redox technique to measure dissolved oxygen concentrations in freshwater systems.

The core principle involves a series of redox reactions:

Oxygen Fixation: Oxygen dissolved in the water sample is fixed by reacting with manganese(II) ions (

Mn^{2 +}

) in an alkaline environment to form a manganese(IV) oxide (

MnO_{2}

) precipitate.

Iodine Liberation: The manganese(IV) oxide precipitate is then reacted with iodide ions (

I^{-}

) in an acidic medium, which reduces

Mn (IV)

back to

Mn (II)

and oxidizes iodide ions to molecular iodine (

I_{2}

Titration: The amount of iodine produced, which is stoichiometrically equivalent to the original dissolved oxygen, is determined by titrating with a standard solution of sodium thiosulphate (

N a_{2} S_{2} O_{3}

The amount of oxygen initially present in the water sample is calculated from the volume of thiosulphate solution used in the titration.

Activity 2.2: Determining Oxygen Present in a Water Sample (Winkler Method Procedure)

300 cm^{3}

BOD bottle

Manganese(II) sulphate solution (

2 cm^{3}

)

Alkali iodide azide solution (

2 cm^{3}

- an aqueous solution of

32.7% NaOH + 15% KI + 1% Na N_{3}

)

Concentrated

H_{2} S O_{4}

(

2 cm^{3}

)

Potassium iodide solution (

2 cm^{3}

)

Sodium thiosulphate solution (

0.1 M

)

Starch solution (

2 cm^{3}

)

Sample Collection: Collect the water sample in a $300 cm^{3}$ BOD bottle. This is done by immersing the bottle in the water, removing the cap underwater, filling the bottle completely, and then recapping it underwater to prevent air bubbles (and thus, additional oxygen) from entering.

Oxygen Fixation (Precipitation):

Add $2 cm^{3}$ of alkaline iodide azide solution.
Immediately after, add $2 cm^{3}$ of manganese(II) sulphate solution.
Close the bottle with the cap and swirl it a few times.
Observation: Oxygen dissolved in the alkaline solution oxidizes the manganese(II) ions to manganese(IV) oxide, which appears as a brown precipitate.
Reaction 1 (Oxygen Fixation): $2 Mn_{(aq)}^{2 +} + 4 OH_{(aq)}^{-} + O_{2 (aq)} \to 2 MnO_{2 (s)} + 2 H_{2} O_{(l)}$

Iodine Liberation:

Add $2 cm^{3}$ of concentrated $H_{2} S O_{4}$ .
Then, add $2 cm^{3}$ of potassium iodide ( $KI$ ) solution.
Observation: The brown precipitate will dissolve into a solution. In this acidic environment, $Mn (IV)$ is reduced back to $Mn (II)$ , liberating iodine ( $I_{2}$ ) in the process.
Reaction 2 (Iodine Liberation): $MnO_{2 (s)} + 2 I_{(aq)}^{-} + 4 H_{(aq)}^{+} \to Mn_{(aq)}^{2 +} + I_{2 (aq)} + 2 H_{2} O_{(l)}$

Titration:

Titrate $201 cm^{3}$ of the above prepared water sample against a standard sodium thiosulphate solution.
Continue the titration until the solution turns pale yellow.
At this point, add $2 cm^{3}$ of starch solution as an indicator. The solution will immediately turn blue (due to the starch-iodine complex).
Continue adding sodium thiosulphate solution dropwise until the blue colour just disappears, indicating the endpoint of the titration.
Reaction 3 (Iodine Titration): $2 S_{2} O_{3}^{2 -}_{(a q)} + I_{2 (a q)} \to S_{4} O_{6}^{2 -}_{(a q)} + 2 I^{-}_{(a q)}$

(Here, thiosulphate ions are oxidized to tetrathionate ions, and iodine is reduced back to iodide ions.)

The stoichiometric relationships derived from the above reactions are crucial for calculating the dissolved oxygen:

1 mole of O_{2} \to 2 moles of MnO_{2} \to 2 moles of I_{2} \to 4 moles of S_{2} O_{3}^{2 -}

Therefore:

The moles of

O_{2}

in the original sample are equal to

1/4

the moles of

S_{2} O_{3}^{2 -}

used in the titration.

After determining the number of moles of iodine produced from the titration, you can work out the number of moles of oxygen molecules present in the original water sample using this ratio.

The oxygen content is usually presented as $mg/d m^{3}$ (which is equivalent to ppm, parts per million) for practical reporting.

Possible Questions and Answers

Q: What is the primary purpose of the Winkler method?
A: To measure the concentration of dissolved oxygen in a water sample.

Q: Why is the Winkler method considered a redox technique?
A: It involves a series of reactions where oxidation and reduction occur, specifically the oxidation of $Mn^{2 +}$ by $O_{2}$ , followed by the oxidation of $I^{-}$ by $MnO_{2}$ , and finally the reduction of $I_{2}$ by $S_{2} O_{3}^{2 -}$ .

Q: What is the role of starch solution in the Winkler titration?
A: Starch acts as an indicator, forming a distinct blue complex with iodine ( $I_{2}$ ). Its disappearance signals the endpoint of the titration when all $I_{2}$ has reacted with thiosulphate.

Q: Why is it important to prevent air bubbles from entering the BOD bottle during sample collection?
A: Air contains oxygen, which would artificially increase the measured dissolved oxygen concentration, leading to an inaccurate result.

Q: If $0.0125$ moles of sodium thiosulphate were used in the titration, how many moles of $O_{2}$ were in the original sample aliquot?
A: From the stoichiometry, $4 moles of S_{2} O_{3}^{2 -}$ react with $1 mole of O_{2}$ .
Moles of $O_{2} = \frac{1}{4} \times Moles of S_{2} O_{3}^{2 -} = \frac{1}{4} \times 0.0125 mol = 0.003125 mol$ .