Why Is Starch Added at the End of an Iodometric Titration

In Iodometric Titration, starch gives a water-insoluble complex with iodine. This water-insoluble complex creates problems in detecting the actual endpoint. This means that the endpoint appears before the actual endpoint. That is why the starch indicator is added just before the end point when the colour is pale-straw yellow.

In analytical chemistry, iodometric titration stands out as a reliable method for determining the concentration of oxidising agents. Whether you’re titrating copper(II) ions or testing for the presence of chlorine, one small yet critical component often appears in the final stages of this process: starch. But why do chemists wait until the end of the reaction to add it? Let’s discuss in detail.

What Is Iodometric Titration?

Iodometric titration involves the use of iodine (I₂) and thiosulfate (S₂O₃²⁻). Typically, an oxidising agent is introduced into an acidic solution containing excess iodide (I⁻). This oxidising agent reacts with iodide to produce iodine:

Oxidizing agent + I⁻ → I₂

Then, this iodine is titrated with sodium thiosulfate:

I₂ + 2 S₂O₃²⁻ → 2 I⁻ + S₄O₆²⁻

The iodine has a deep yellow-brown colour, which fades as it reacts with thiosulfate, giving a visual cue. But to make the endpoint unmistakably clear, chemists rely on starch as an indicator.

Enter Starch: The Visual Cue Master

Starch forms a highly visible blue-black complex with iodine (I₂), making it much easier to detect the exact moment when all the iodine has reacted.

This complex is extremely sensitive and can detect even trace amounts of free iodine. However, it’s not added at the beginning of the titration. Why not?

The Problem with Early Addition

If starch is added too early, several issues can arise:

1. Formation of a Stable Complex: The starch-iodine complex is very stable. When iodine binds to starch early on, it becomes “locked in,” and the iodine becomes less available to react with thiosulfate.
2. Sluggish Reaction Kinetics: The tightly bound complex can slow down the reaction with thiosulfate, making the titration less efficient and accurate.
3. Potential Decomposition: Under acidic and warm conditions, starch can degrade, potentially interfering with the results or causing cloudiness in the solution.

The Smart Strategy: Add Starch at the End

To avoid these pitfalls, starch is added near the endpoint, when most of the iodine has already reacted with thiosulfate. At this point:

• Only a small amount of iodine remains.
• The starch immediately reacts with the trace iodine, forming a sharp blue-black color.
• As titration continues and all iodine is consumed, the color suddenly disappears, clearly signaling the endpoint.

This makes starch the perfect late-stage visual indicator.

Conclusion

Why add starch at the end of iodometric titration?

• To avoid premature complex formation.
• To ensure accurate, rapid titration.
• To achieve a sharp, unmistakable endpoint.

So next time you perform an iodometric titration, remember: starch is your silent assistant—just make sure to call on it at the right time!

Related

• Relative Response Factor (RRF) in Pharmaceutical Analysis

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Further

• Iodometry