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Topic: Iodine indicator  (Read 252 times)

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Offline albertog

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Iodine indicator
« on: September 10, 2021, 05:53:07 AM »
Dear all,
i have a terrible doubt i cannot explain.
Why in some titration do i have to use iodine indicator titrating with thiosulphate?
For example, a iodometric method for copper in brass. Bu also in other method for tin it is advised to use iodine indicator for the same purpose.
I produce iodine from copper reduction in excess of potassium iodide. Then i titrate the stechiometric quantity of iodine with thiosulphate.
The method advice me to use iodine indicator close to the endpoint to observe clearly. Not startch.
I can't unterstand why. I have to be sure that iodine is stechiometrically related to copper. So why do i have to add it as indicator?
Thanks in advance.

Offline Borek

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Re: Iodine indicator
« Reply #1 on: September 10, 2021, 06:23:09 AM »
No idea what you mean by "iodine indicator", can you elaborate?

Is there a chance you are misreading the procedure?
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Offline albertog

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Re: Iodine indicator
« Reply #2 on: September 10, 2021, 06:35:20 AM »
Looking on web iodine indicator means a solution of iodide and iodine.
I think.
I've seen that Sigma has the product in the catalogue.

Offline Borek

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Re: Iodine indicator
« Reply #3 on: September 10, 2021, 08:43:14 AM »
Definitely something doesn't sound right, but not seeing the whole recipe itself it is hard to comment.

Any chances they call the starch "iodine indicator"? That's what it does, after all, it indicates the presence of the iodine, and it is better to add it near the endpoint.
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Offline albertog

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Re: Iodine indicator
« Reply #4 on: September 10, 2021, 09:59:38 AM »
I red on web that it is a powder of startch....non the same i know solved in water...but something very similar... that goes blu in a lack of iodine....

Offline Orcio_Dojek

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Re: Iodine indicator
« Reply #5 on: September 10, 2021, 11:09:57 AM »
@albertog Starch becomes dark blue in presence of iodine (or rather poliiodide anion).

Offline Borek

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Re: Iodine indicator
« Reply #6 on: September 10, 2021, 11:41:44 AM »
I red on web that it is a powder of startch....non the same i know solved in water...but something very similar... that goes blu in a lack of iodine....

Looks like you were misreading the procedure.

Compare

http://www.titrations.info/iodometric-titration-end-point-detection

and

http://www.titrations.info/iodometric-titration-solutions
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