9.4 - Estimation of Dissolved Oxygen by Redox Titration Revision Notes for Leaving Cert Chemistry

9.4 - Estimation of Dissolved Oxygen by Redox Titration

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Experiment Summary

In this experiment, dissolved oxygen (D.O.) in a water sample is estimated using Winkler's method, a redox titration.

The dissolved oxygen in the water sample reacts with manganese (II) sulphate to form manganese (IV) hydroxide, which is then acidified to release iodine from potassium iodide. The iodine produced is titrated with sodium thiosulfate to estimate the amount of oxygen in the sample. The dissolved oxygen concentration is calculated in parts per million (ppm).

Materials and Apparatus Required

Chemicals

Manganese (II) sulphate solution
Alkaline potassium iodide solution
Concentrated sulfuric acid ( $H₂SO₄$ )
0.005 M sodium thiosulfate solution
Starch indicator solution
Water sample

Apparatus

Burette (50 cm³)
Pipette (25 cm³)
Pipette filler
Droppers
Philtre funnel
Beaker (250 cm³)
Conical flask (250 cm³)
Reagent bottles (250 cm³) with stoppers
Graduated cylinder (10 cm³)
Retort stand, boss head, and clamp
Wash bottle
White tile
Safety glasses

Safety Precautions

Wear safety glasses throughout the experiment.
Manganese (II) sulfate is harmful; avoid inhalation and skin contact.
Concentrated sulfuric acid is highly corrosive and can cause severe burns.
Sodium thiosulfate can irritate the eyes.
Handle all chemicals with care and follow proper disposal methods.

Method

Sample Preparation: Rinse a 250 cm³ reagent bottle with deionised water and fill it completely with the water sample, ensuring no air bubbles are trapped.
Addition of Reagents: Using a dropper, add approximately 1 cm³ each of manganese (II) sulphate solution and alkaline potassium iodide solution to the water sample under the surface of the liquid. Stopper the bottle immediately.
Mixing and Precipitation: Invert the bottle repeatedly for one minute to mix thoroughly.
A white precipitate of manganese (II) hydroxide will form, followed by a brown precipitate as the dissolved oxygen reacts.
Dissolving the Precipitate: Carefully add 1 cm³ of concentrated sulfuric acid by running it down the side of the bottle.
Stopper the bottle and invert repeatedly until the brown precipitate dissolves, releasing iodine, resulting in a golden-brown solution.
Titration Setup: Rinse the burette with sodium thiosulfate solution and the pipette with the iodine solution.
Fill the burette with 0.005 M sodium thiosulfate solution.
Titration: Pipette 50 cm³ of the iodine solution into a conical flask.
Titrate with sodium thiosulfate until the solution turns pale yellow.
Add 1 cm³ of starch indicator, causing the solution to turn blue.
Continue titrating until the blue colour disappears.
Repeat Titrations: Perform at least two more titrations and calculate the average titre.

Results

Measurement	Value
Rough titre	13.3 cm³
Second titre	13.0 cm³
Third titre	13.0 cm³
Average of accurate titres	13.0 cm³
Volume of water sample	50.0 cm³
Molarity of thiosulfate	0.005 M

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Sample Calculations

Moles of thiosulfate used:

\text{Moles of thiosulfate} = \frac{\text{Volume of thiosulfate (cm³)} \times \text{Molarity of thiosulfate}}{1000}

= \frac{13.0 \times 0.005}{1000} = :highlight[0.000065 \, \text{moles}]

Moles of dissolved oxygen (O₂):

\text{Moles of O₂} = \frac{\text{Moles of thiosulfate}}{4}

= \frac{0.000065}{4} = :highlight[0.00001625 \, \text{moles}]

Dissolved oxygen in ppm:

\text{Concentration of O₂} = \frac{0.00001625 \, \text{moles}}{0.050 \, \text{litres}} \times 32 \, \text{g/mol}

= :highlight[10.4 \, \text{ppm}]

Example Questions with Answers

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Q1: Why is manganese (II) sulphate added to the water sample?

Manganese (II) sulphate reacts with dissolved oxygen to form a brown precipitate of manganese (IV) hydroxide, which is necessary to indirectly measure the dissolved oxygen.

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Q2: Why is concentrated sulfuric acid added during the experiment?

The sulfuric acid dissolves the manganese (IV) hydroxide, releasing iodine from potassium iodide, which can then be titrated with sodium thiosulfate.

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Q3: What indicates the endpoint of the titration?

The endpoint is reached when the blue colour from the starch-iodine complex disappears, indicating that all iodine has reacted with thiosulfate.

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Q4: Why is it important to fill the reagent bottle completely with water?

To ensure no air is trapped, which could introduce additional oxygen and give inaccurate results.

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Q5: What does a low dissolved oxygen level indicate about water quality?

Low dissolved oxygen levels may indicate pollution or high levels of organic matter, leading to oxygen depletion, which can harm aquatic life.

9.4 - Estimation of Dissolved Oxygen by Redox Titration (Leaving Cert Chemistry): Revision Notes