March 04, 2021, 01:32:35 AM
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Topic: Unknown precipitate formed in Titration.  (Read 176 times)

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Offline Jack.Sweeney

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Unknown precipitate formed in Titration.
« on: February 17, 2021, 03:06:54 AM »
Currently performing a summative assessment on an indirect titration using a standard solution of sodium thiosulfate against a mixture of diluted bleach with sulfuric acid and potassium iodide, using starch as an indicator.

When adding the potassium iodide into the conical flask with the diluted bleach and sulfuric acid, an unknow precipitate is formed, before the starch is added. This has left myself, my peers and my Chemistry teacher confused as no-one recognises what it is. And I'd appreciate it if anyone who knew what it is could respond, as I'd like to mention this as a systematic/random error, as it has occurs in every trial, but it also makes it difficult to determine the end point of the reaction as the mixture changes colours from a black/dark brown to a foggy colourless, then after mixing the solution, the precipitate bleeds more colour into the mixture.

More details on the Titration.
Aim: To determine the %w/v of the bleach solution.
-Using a diluted 20mL aliquot of the bleach.
-Using 1.0M Sulfuric Acid solution
-Using 0.1M Sodium thiosulfate solution
-Using 0.1M Potassium Iodide solution
-All equipment was thoroughly washed before use to ensure that there were no contaminants.

If anyone knows what the precipitate could possibly be, let me know.

Offline chenbeier

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Re: Unknown precipitate formed in Titration.
« Reply #1 on: February 17, 2021, 06:22:56 AM »
What does the bleach contain. Some bleach is made by calciumhypochlorite. So if you add sulfuric, Calciumsulfate will precipitate. Check for calcium first. Maybe change for methansulfonic acid.

Offline Borek

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Re: Unknown precipitate formed in Titration.
« Reply #2 on: February 17, 2021, 07:41:13 AM »
Can you describe the precipitate?
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Offline Jack.Sweeney

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Re: Unknown precipitate formed in Titration.
« Reply #3 on: February 17, 2021, 05:49:07 PM »
It was primarily a black precipitate, kind of brownish, and there they were in a spherical shape, if I had to guess how much was produced I'd have to say around 1-1.5 grams. When the solution went colourless and was mixed again, it would turn the solution back into the blackish/brown solution.

Offline Jack.Sweeney

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Re: Unknown precipitate formed in Titration.
« Reply #4 on: February 18, 2021, 01:24:38 AM »
Calcium hypochlorite was not a listed ingredient of the bleach used. But it was determined that the potassium iodide used was not thoroughly mixed causing some aliquots to be more concentrated than others, allowing this precipitate to form. After conducting additional trials, the precipitate was no longer forming. Thanks to everyone who left suggestions on this thread, but I no longer need any more help.

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