[alicia_tan2113]

Describe a simple test, other than displacement, that can be used to show that sea water contains iodide ions.
Test:
Result:______________________________________________________________________________________________________________________________________________

I got stuck at this last question in my exam paper so I just need someone to go through this with me:)

[Evaldas]

Starch? Cos for example to prove that a potato contains starch you take tincture of iodine and put a few drops and the starch particles change color into a dark blue.

[Fluorine]

Silver nitrate test will give a precipitate...though if you have other halides it may cause a bit confusion. I did a lab on this though I recall very little of the procedure.

[Evaldas]

Silver nitrate test will give a precipitate...though if you have other halides it may cause a bit confusion. I did a lab on this though I recall very little of the procedure.

But "other than displacement".

[alicia_tan2113]

Thats why i couldn't think of anything xD

[Fluorine]

I read over that part somehow - oops.

[alicia_tan2113]

I learnt chromatography,decanting, filtration, evaporating to dryness, crystallization, sublimation... I wonder if that helps.

[Twigg]

Ooooh I used to know this. There's a soluble compound that changes color in the presence of different halogen ions but I can't remember it! I *think* it's AgNO3 or some other silver-based salt.

@Evaldas, that's what I was thinking. I don't think seawater would behave the same way as the tincture though. If it does... I am throwing out my Lugol's solution 

[Evaldas]

Ooooh I used to know this. There's a soluble compound that changes color in the presence of different halogen ions but I can't remember it! I *think* it's AgNO3 or some other silver-based salt.

@Evaldas, that's what I was thinking. I don't think seawater would behave the same way as the tincture though. If it does... I am throwing out my Lugol's solution 

But we said that it's not AgNO3, because "other than displacement".

[vmelkon]

Sea water doesn't contain much iodide ions. How could a visual chemical test show it? Most of it is chloride and to some minor extent, sulfate, some bromide, some nitrate, some borate.
It probably requires mass spectrometry which is much more sensitive and precise.

[Twigg]

Vmelkon: point taken. I think you're on to the right answer. Mass spectrometry would give the identity of iodide ions by their unique mass-to-charge ratio.

[sci994]

After adding HNO₃ and Pb(No3)₃ a yellow ptt is formed (PbI₂)
Reaction:
Pb(NO3)₂ (aq) + 2KI (aq)  PbI₂ (s) + 2KNO₃

Pb²⁺+2I^- PbI₂

[cliverlong]

Iodine / thiosulphate clock reaction, Hydrogen peroxide variation?

http://en.wikipedia.org/wiki/Iodine_clock_reaction

[Evaldas]

After adding HNO₃ and Pb(No3)₃ a yellow ptt is formed (PbI₂)
Reaction:
Pb(NO3)₂ (aq) + 2KI (aq)  PbI₂ (s) + 2KNO₃

Pb²⁺+2I^- PbI₂

Other than displacement!
Plus there are other halogen compounds in sea water, like NaCl which would also react with the nitrate and form a precipitate.

[alicia_tan2113]

  you guys get my pain xD