13 – Standardising a Sodium Thiosulfate Solution (LC 2027) Revision Notes for Leaving Cert Chemistry

13 – Standardising a Sodium Thiosulfate Solution

Introduction and aim

Standardisation is the process of finding the exact concentration of a solution by titrating it against another solution of known concentration. In this experiment, you will prepare an approximate 0.1 M solution of sodium thiosulfate and then find its precise concentration by titrating it against a solution of iodine.

infoNote

Standardisation is crucial in analytical chemistry because solutions prepared by dissolving solids often don't have precisely the expected concentration due to factors like water content in crystals, impurities, or measurement errors. By standardising against a known standard, we can determine the true concentration for accurate quantitative analysis.

The aim is to determine the exact concentration of sodium thiosulfate solution through iodine-thiosulfate titration, which is a classic redox titration used in analytical chemistry.

Chemical reactions involved

This experiment involves two key chemical reactions that work together:

Step 1: Generating iodine from potassium permanganate
When acidified potassium permanganate (KMnO₄) reacts with excess potassium iodide (KI), iodine is produced:

$2\text{MnO}_4^- + 10\text{I}^- + 16\text{H}^+ \rightarrow 2\text{Mn}^{2+} + 5\text{I}_2 + 8\text{H}_2\text{O}$

Step 2: Titrating iodine with sodium thiosulfate
The iodine produced is then titrated with sodium thiosulfate solution:

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

The half-equation for the permanganate reduction is:

$\text{MnO}_4^- + 8\text{H}^+ + 5e^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}$

chatImportant

Understanding both reactions is essential — the first reaction generates a known amount of iodine from the standard KMnO₄ solution, while the second reaction allows us to determine the thiosulfate concentration by measuring how much is needed to react with that iodine.

Equipment and materials

The experimental setup includes:

Burette containing the sodium thiosulfate solution of unknown concentration
Conical flask for the reaction mixture
White tile placed under the flask to observe colour changes clearly
Pipette for accurate measurement of solutions
Measuring cylinder for approximate measurements

infoNote

The white tile is essential for clearly observing the subtle colour changes, especially near the end point when the solution becomes very pale yellow. This seemingly simple piece of equipment significantly improves the accuracy of end point detection.

Chemicals needed:

Sodium thiosulfate (Na₂S₂O₃) solution
Potassium permanganate (KMnO₄) solution
Dilute sulphuric acid (H₂SO₄)
Potassium iodide (KI) solution
Starch solution (indicator)
Distilled water

Procedure

Preparation stage

Weigh some sodium thiosulfate crystals using a digital balance and transfer them to a beaker containing distilled water
Stir the mixture until the crystals fully dissolve, then transfer the solution to a clean volumetric flask
Make up to the mark with distilled water and mix thoroughly
Wash the burette with some sodium thiosulfate solution, then fill it with the thiosulfate solution

Setting up the titration

Pipette a standardised KMnO₄ solution into a clean, dry beaker and wash with distilled water
Add the required amount of KMnO₄ solution to the conical flask using a pipette filler
Add dilute sulphuric acid to the conical flask to make the solution acidic
Add excess potassium iodide solution to the conical flask

chatImportant

Always add the sulphuric acid before the potassium iodide to ensure the solution is properly acidified. The permanganate reaction requires acidic conditions to proceed correctly.

Observations and colour changes

This titration involves a fascinating sequence of colour changes that help you identify the end point:

Initial observations

Purple colour: The solution initially appears purple due to the KMnO₄ present
Reddish-brown formation: When KI is added in excess, the solution turns reddish-brown due to iodine formation according to the equation:
$2\text{MnO}_4^- + 10\text{I}^- + 16\text{H}^+ \rightarrow 2\text{Mn}^{2+} + 5\text{I}_2 + 8\text{H}_2\text{O}$

During titration

Beginning of titration: The solution appears dark brown/red due to high iodine concentration
Progress: As sodium thiosulfate is added, the dark brown colour becomes less intense and gradually changes to a yellow colour
Near end point: When the solution becomes very pale yellow (very dilute iodine), this indicates you are close to the end point

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Colour Change Sequence During Titration

Step 1: Purple (KMnO₄) → Reddish-brown (I₂ formed)
Step 2: Dark brown → Lighter brown (I₂ being consumed)
Step 3: Yellow → Pale yellow (very little I₂ remains)
Step 4: Add starch → Blue-black (starch-I₂ complex)
Step 5: Blue-black → Colourless (end point reached)

End point determination

Add starch indicator: A few drops of starch solution are added when the solution becomes pale yellow
Blue-black colour: The solution immediately turns blue-black due to the starch-iodine complex
End point reached: Continue adding thiosulfate drop by drop until the blue-black colour completely disappears, leaving a colourless solution

Understanding the chemistry

Why does the colour change occur?

The reddish-brown colour appears because iodine (I₂) has been formed when the potassium permanganate oxidises the iodide ions. The potassium iodide is added in excess to ensure complete reduction of the permanganate ions.

Role of starch indicator

Starch forms a distinctive blue-black complex with iodine, making it easier to spot the exact end point. However, starch should only be added near the end point (when the solution is pale yellow) because:

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Why Timing Matters for Starch Addition

Adding starch too early makes the colour change difficult to observe due to the intense dark colour
The starch-iodine complex is more stable at lower iodine concentrations
Adding starch when the solution is pale yellow ensures a sharp, clear end point

Why use excess reagents?

Excess sulphuric acid ensures the solution remains acidic, which is essential for the permanganate reaction
Excess potassium iodide ensures all permanganate ions are completely reduced, producing the maximum amount of iodine

chatImportant

Using excess reagents is crucial for ensuring complete reactions. Insufficient acid would prevent the permanganate from reacting completely, while insufficient iodide would mean not all the permanganate gets reduced, leading to inaccurate results.

End point determination

The end point is reached when the blue-black colour completely disappears, indicating that all the iodine has reacted with the thiosulfate ions. At this point:

There is no more iodine left in solution
The starch-iodine complex breaks down
The solution becomes colourless

Titration technique: The titration should be continued drop by drop near the end point to ensure accuracy. Two or three accurate titration readings should agree within 0.1 cm³.

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Recognising the True End Point
The end point is NOT when the blue-black colour becomes very faint — it must completely disappear. A persistent pale blue tinge means there is still iodine present. Keep adding thiosulfate drop by drop until the solution is completely clear and colourless.

Calculations

Once you have your titration results, you can calculate the exact concentration of the sodium thiosulfate solution using the stoicheiometry of the reactions involved. The method involves:

Recording accurate titration volumes
Using the molar ratios from the balanced equations
Calculating the concentration based on the volume relationships

infoNote

The calculation process uses the fact that you know the exact amount of KMnO₄ used, which determines the amount of I₂ produced, which in turn tells you how much Na₂S₂O₃ was needed. By working backwards through the stoicheiometry, you can determine the precise concentration of your thiosulfate solution.

The calculation method is detailed in examples throughout your textbook and should be practised thoroughly.

Remember!

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Key Points to Remember:

Standardisation determines the exact concentration of a solution through titration against a known standard
Two reactions occur: permanganate + iodide → iodine, then iodine + thiosulfate → products
Colour sequence follows: purple → reddish-brown → yellow → blue-black → colourless
Starch indicator is added only when the solution becomes pale yellow, not at the beginning
End point is when the blue-black colour completely disappears, leaving a clear, colourless solution
Excess reagents (H₂SO₄ and KI) ensure complete reactions and accurate results
Drop-by-drop addition near the end point is essential for accuracy

13 – Standardising a Sodium Thiosulfate Solution (LC 2027) (Leaving Cert Chemistry): Revision Notes