[L3ul]

This month, I have to practice redox and acid-base titrations for the Chemistry Olympiad, using reagents like: Sodium Thiosulfate, Potassium Permanganate, Potassium Dichromate etc. (also using at least 5 uncommon indicators for the acid-base titrations.)

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Today, I worked on titrating Iodine with a solution of Sodium Thiosulfate, using starch as an indicator (adding it after the solution took a pale yellow colour) for noticing the end point.

My problem is determining when I had reached the end point of the titration.
Everything went well until I got really close to the end-point. As I read on my materials and as my teacher said, the solution should be totally be colourless at that point; but as I've seen on some Youtube videos showcasing this titration, their solution wasn't perfectly colourless, but it had a certain 'darker' aspect.

Getting from that aspect to completely colourless took me a significant quantity, so I can't just choose blindly which is the correct variant.

What should I be looking for when doing this titration?

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Along with this one, I worked on determining the concentration of Cu2+ with KI using the procedure shown above.

This one was more troublesome because of the CuI (a whitish yellow precipitate) that really makes me unable to notice the end point.

How should I proceed in this case, so that I can notice the end point?

[Borek]

The change in color is typically quite fast - one or two drops of the titrant should change the color completely. It is this sudden change that matters more than the exact colors before and after the endpoint.

Note the volume after the change, than add few drops one by one, observing what is happening. If the color doesn't change anymore, you are set.

[L3ul]

Oh, makes sense!

But why is there still a shade of colour in the flask after we've reached the end-point (the moment where all Iodine molecules got reduced to Iodide)?
Is it because of the solubility of the Iodine-Starch complex, making those very few molecules to not get reduced and give the solution the colour I'm talking about?

[Borek]

If you add iodine too early during the titration, the iodine/starch complex produced is too stable and the desorption becomes too slow, so the color stays. I believe it may sometimes also depend on the starch (which is not something well defined and standardized).

[L3ul]

I'll try and add it a little bit later, then. Thanks!

But I'm still unsure about when I have reached the endpoint for the 2nd titration; I haven't seen a sudden change in colour like in the 1st one, but more like a gradual one, getting really close to a white aspect.

[Borek]

But I'm still unsure about when I have reached the endpoint for the 2nd titration; I haven't seen a sudden change in colour like in the 1st one, but more like a gradual one, getting really close to a white aspect.

I am afraid there is not much that I can suggest. What can help is preparation of what we call in Polish "witness" - slightly overtitrated sample, that you can compare with the one you are titrating, such a reference point often makes the titration easier.

[L3ul]

That's actually a great idea! I was thinking of filming the titration and analysing the colour change with more care, but the 'witness' idea with the reference flask is a lot better. Thank you!