[research123]

I am trying to find the peroxide value in cottonseed oil.
First I added methane chloride so the solution became yellow.
Then I titrated it with potassium iodide until the yellow was almost gone.
Then I added 2% starch indicator and the solution was supposed to turn blue so I could titrate again until the blue disappeared, but it never turned blue.

Why wouldn't it turn blue?

Thank you!

[Borek]

Your description of the procedure doesn't make sense to me.

You were titrating with KI?

[marquis]

It sounds like you are trying to do an iodometric test. Usually, this involves KI, sodium thiosulfate, starch and iodine. Haven't seen it used for peroxide in cottonseed oil.  But it is often used for chlorine level, reducing agents, etc.

One other test often done on these oils is oxirane oxygen.  This isn't the same as peroxide.  Can you give anymore background information on how the peroxide is in the cottonseed oil?

Thanks.

[research123]

Oh my mistake, my description is wrong.
I used dichloromethane as a solvent for cottonseed oil.  Then I added potassium iodide and the solution turned yellow, so I titrated with sodium thiosulfate until the yellow almost disappeared.
Then I added the starch solution, which was supposed to make the entire solution blue, but there was no color change.

If it did turn blue, I would titrate with sodium thiosulfate again until the blue color disappeared, so I could determine the peroxide value in cottonseed oil.

I'm wondering if the original peroxide value is too low, so the solution didn't turn blue.

[Borek]

Perhaps you added too much thiosulfate before adding starch.

You can always try to add a very tiny amount of starch at the beginning of the titration. While it can make end point detection difficult (starch colored when the concentration of iodine is high doesn't react fast with thiosulfate, so the color change is not easy to spot), at least you will be sure iodine was in teh solution.

[research123]

I used chloroform instead of dichloromethane, added less sodium thiosulfate for the first titration, and used more starch than before, and it worked!

Thanks for your *delete me*

[JGK]

Most official methods have abandoned the use of chloroform as it use in labs is being phased out. the AOCS method has replaced it with an acetic acid/isosoctane mix