4.7 - An Iodine and Sodium Thiosulfate Titration

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Experiment Summary

In this experiment, a standard iodine solution is generated in situ by reacting potassium iodate ( $KIO₃$ ) with potassium iodide (KI) in acidic conditions.

The iodine formed is titrated against sodium thiosulfate ( $Na₂S₂O₃$ ) to determine its concentration.
Sodium thiosulfate reduces iodine ( $I₂$ ) to iodide ( $I⁻$ ), and the endpoint is detected using starch as an indicator, which produces a blue-black complex with iodine that turns colourless at the endpoint.

Materials and Apparatus Required

Chemicals

0.02 M potassium iodate solution
0.5 M potassium iodide solution
1 M sulfuric acid
Sodium thiosulfate solution
Starch indicator solution
Deionised water

Apparatus

Pipette (25 cm³) and pipette filler
Burette (50 cm³) with retort stand and clamp
Conical flask (250 cm³)
Graduated cylinder (100 cm³)
Wash bottle
Filter funnel
White tile or card
Beakers (250 cm³)
Safety glasses

Safety Precautions

Wear safety glasses throughout the experiment.
Sodium thiosulfate is an eye irritant; handle with care.
Sulfuric acid is corrosive. Always add acid to water when diluting, not the other way around.
Iodine is harmful by inhalation and should not contact the skin or eyes.

Method

Rinse the pipette, burette, and conical flask with deionised water.
Rinse the pipette with potassium iodate solution and the burette with sodium thiosulfate solution.
Pipette 25 cm³ of potassium iodate solution into a clean conical flask.
Add 20 cm³ of dilute sulfuric acid and 10 cm³ of 0.5 M potassium iodide solution to the conical flask using graduated cylinders. Iodine is liberated, forming a golden-brown solution.
Fill the burette with sodium thiosulfate solution, ensuring that the portion below the tap is filled.
Titrate the iodine solution by adding sodium thiosulfate while swirling the conical flask.
When the solution fades to pale yellow, add a few drops of freshly prepared starch solution. The solution will turn blue-black.
Continue adding sodium thiosulfate dropwise until the solution turns colourless.
Record the volume used.
Repeat the titration until two titres agree within 0.1 cm³.
Calculate the concentration of sodium thiosulfate based on the titration results.

Results

Measurement	Value
Rough titre	22.8 cm³
Second titre	22.4 cm³
Third titre	22.5 cm³
Average of accurate titres	22.45 cm³
Volume of iodine solution used	25.0 cm³
Concentration of iodine solution	0.06 M

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Sample Calculation

Using the balanced equation:

\text{I}_2 + 2\text{S}_2\text{O}_3^{2-} \rightarrow 2\text{I}^- + \text{S}_4\text{O}_6^{2-}

Moles of $I₂$ used:

\frac{25.0 \times 0.06}{1000} = 0.0015 \, \text{moles}

From the equation, 2 moles of $Na₂S₂O₃$ react with 1 mole of $I₂$ .

Moles of sodium thiosulfate reacting:

0.0015 \times 2 = 0.003 \, \text{moles}

Concentration of sodium thiosulfate:

\frac{0.003}{22.45 / 1000} = 0.1336 \, \text{M}

Example Questions with Answers

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Q1: Why is sodium thiosulfate not suitable as a primary standard?

Sodium thiosulfate is efflorescent and its water content can vary, making it unsuitable as a primary standard.

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Q2: Why is starch added near the endpoint of the titration?

Starch forms a blue-black complex with iodine, providing a sharp colour change from blue-black to colourless near the endpoint.

Adding starch too early can cause a slow or unclear endpoint.

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Q3: What happens at the endpoint of the titration?

At the endpoint, all the iodine has reacted, and the blue-black colour turns colourless, indicating that no iodine remains in the solution.

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Q4: Why is potassium iodide added to the solution?

Potassium iodide keeps iodine in solution as I₃⁻ ions, which are more soluble than I₂ in water.

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Q5: Why must the starch solution be freshly prepared?

Starch solution deteriorates quickly and can give false endpoints if not freshly prepared.

- An Iodine and Sodium Thiosulfate Titration Simplified Revision Notes for Leaving Cert Chemistry

4.7 - An Iodine and Sodium Thiosulfate Titration

Experiment Summary

Materials and Apparatus Required

Chemicals

Apparatus

Safety Precautions

Method

Results

Sample Calculation

Example Questions with Answers

Q1: Why is sodium thiosulfate not suitable as a primary standard?

Q2: Why is starch added near the endpoint of the titration?

Q3: What happens at the endpoint of the titration?

Q4: Why is potassium iodide added to the solution?

Q5: Why must the starch solution be freshly prepared?

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