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Topic: Volumetric Analysis
Level: OL/HL
Exam Focus: Section A Practical Skills
1. Experiment Snapshot
Feature
Details
Experiment
Iodine-Thiosulfate Titration
Exam Importance
⭐⭐⭐⭐ Practical exam standard
Typical Marks
5–15 marks
2. Aim
To determine iodine concentration using sodium thiosulfate solution.
3. Quick Theory
Iodine is reduced by thiosulfate. Starch forms a blue-black complex with iodine and is added near the endpoint.
Examiner Tip: Always connect the observation or reading to the purpose of the experiment.
4. Apparatus and Chemicals
Apparatus
Chemicals
Burette, pipette, conical flask, white tile.
Iodine solution, sodium thiosulfate solution and starch indicator.
Safety: Wear goggles and follow teacher instructions, especially with acids, alkalis, hot apparatus and oxidising agents.
5. Step-by-Step Procedure
Pipette iodine solution into a conical flask.
Fill the burette with sodium thiosulfate.
Titrate until the iodine colour becomes pale yellow.
Add starch indicator.
Continue until the blue-black colour just disappears.
Repeat for concordant titres.
6. Observations / Results
Situation
Expected Result
Before starch
Brown/yellow iodine colour.
After starch
Blue-black.
Endpoint
Blue-black disappears.
7. Student Common Mistakes
Adding starch at the beginning instead of near the endpoint.
Continuing the titration after the blue-black colour disappears.
Forgetting the colour sequence.
Not swirling the flask.
8. Examiner Traps
Trap: Students often describe the apparatus but forget the exact observation, reading, or endpoint. Use precise words.
8A. Labelled Diagram
This titration is really about watching the iodine colour disappear in stages. Starch is added near the end so the final colour change is sharp and easy to see.
Iodine - Thiosulfate Titration
Near the endpoint, starch gives a strong blue-black colour that disappears at the end.
Teacher Voice: Students should remember the sequence: brown iodine, then starch near the end, then colourless at the endpoint.
9. 🎮 Interactive Simulator
Try It Yourself
When should starch be added?
What is the endpoint?
Result Window
Ready
Choose the correct actions to complete the practical.
Score:0/0
10. Past Paper Style Questions
State the aim of this experiment. [2]
Describe the procedure in order. [4]
State one safety precaution. [2]
State one common error and one improvement. [4]
Explain the key observation/result. [3]
11. Mark Scheme
Aim correct [2]. Procedure in correct order [4]. Safety precaution [2]. Error and improvement [4]. Observation/result explained [3].