[benji-t]

I'm doing a report on how the concentration of iodate in iodised salt changes with respect to temperature. I raised the salt to a range of temperatures (18° - 95°C) then titrated against sodium thiosulfate for iodate content. I found that as the temperature increases, iodate concentration gradually decreases.

What i need help with is that I don't know why this happens, and I need to explain this in my report. Can someone please help me! 

[Borek]

Please describe procedure used, with reaction equations.

[benji-t]

I dissolved the salt in distilled water, then heated it in a water bath for 5 minutes. I then determined the amount of iodate by first reacting the salt solution with potassium iodide, under acidic conditions, to produce iodine:

IO₃^- + 5I^- + 6H⁺ --> 3I₂ + 3H₂O

Then i titrated the resulting iodine with thiosulfate:

I₂ + 2S₂O₃^2- --> 2I^- + S₄ O₆^2-

I basically followed the method outlined on this page: http://www.chemteach.ac.nz/investigations/documents/salt_iodate.pdf

Help would be much appreciated.

[Borek]

Is the only difference between samples this 5 minutes heating in a water bath? Were your samples cooled to ambient temperature before titration? Were your samples heated after iodide and HCl were added, or before?

[benji-t]

Is the only difference between samples this 5 minutes heating in a water bath?

Yes, at different temperatures.

Were your samples cooled to ambient temperature before titration?

No, i titrated immediately after the 5 minutes was up.

Were your samples heated after iodide and HCl were added, or before?

Before. After the 5 minutes was up I added iodide and HCl, then titrated.

[Borek]

Still your samples were at different temperatures when iodide and HCl were added?

What do you know about elemental iodine?

[benji-t]

Still your samples were at different temperatures when iodide and HCl were added?

Yes.

What do you know about elemental iodine?

I know that it sublimes, is it being lost after I add the iodide and HCl? If that's the case, i can write about that when I discuss sources of error and suggestions for improvement.

What do you think could potentially cause the concentration of iodate to decrease when it is heated? Is there some sort of equilibrium system or could the iodate be carried away with steam etc?

[AWK]

Iodate but not iodine content loss.
Iodate can thermally disproportionate to iodate(VII) and iodide, then to iodide, iodine and oxygen, finally after prolonged heating some loss of iodine is possible.


http://www.springerlink.com/content/1r43751l83267325/
Iodine loss from iodised salt on heating. Indian Journal of Pediatrics
64(6) 1997,883-885

corrected

[Borek]

Iodate but not iodine content loss.

If you have hot solution of iodine (after I^-/IO₃^- reaction) some iodine may escape.

Iodate can thermally disproportionate to iodate(VII) and iodine, then to iodide, iodine and oxygen, finally after prolonged heating some loss of iodine is possible.

That's interesting. Unfortunately, at least in the abstract, there is no information about iodate decomposition in temperatures below 100 deg C (and full text is $32, I am not THAT interested).

Besides, I have never cooked anything in 350 deg C, I though it happens only to blondes making double pizza

[benji-t]

Iodate can thermally disproportionate to iodate(VII) and iodine,

Can you explain this for me, equations etc? I assume this reaction is endothermic?

[AWK]

Iodate can thermally disproportionate to iodate(VII) and iodine,

Can you explain this for me, equations etc? I assume this reaction is endothermic?

sorry - should be iodide
4IO3- = = 3IO4- + I- sorry - [fcolor=red]this reaction is wrong - only chrorates(V) react in this way[/fcolor]
Yes, this reaction is endothermic, but temperature of boiling water is sufficient.

[benji-t]

Iodate can thermally disproportionate to iodate(VII) and iodine,

Can you explain this for me, equations etc? I assume this reaction is endothermic?

sorry - should be iodide
4IO3- = = 3IO4- + I-
Yes, this reaction is endothermic, but temperature of boiling water is sufficient.

Thanks for the equation AWK. Do you have a reference for it? I can't find anything about that on the internet. If you dont have one, would i be able to have your detals so I can cite you in my report?

[AWK]

Sorry, look at my correction in a previous post. Perchlorates can be obtained in this way but not perbromates (unknown) or periodates.

Iodates(V) when warming decompose to iodides and oxygen