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Is it possible to use Iodine Solution as a colour indicator for modified starch?

  1. Sep 8, 2011 #1
    Hello there. I was reading something on the affect sodium hydroxide has on ordinary starch from plain flour. I read that sodium hydroxide can change the physical and chemical properties of starch. I conducted an experiment where I mixed 2mol and 8mol sodium hydorxide with plain flour in a test tube. When I added the colour indicator (Iodine solution) to this mixture, there was no colour change. I was wondering if this is due to the sodium hydroxide changing the chemical properties of the starch or if I made a mistake somehow.

    Thanks guys, any help or response will be appreciated :biggrin:
     
  2. jcsd
  3. Sep 8, 2011 #2

    Borek

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    Staff: Mentor

    Have you tried to neutralize the solution before adding iodine?

    There are several possibilities here. One is - starch was changed and it won't ever react. Other is - starch was not changed, but the iodine reacted with hydroxide creating hypoiodide (which further disproportionated forming iodide and iodate). Another one - starch surface was changed in the hydroxide solution, but it will get to its initial state after pH gets back close to neutral.
     
  4. Sep 8, 2011 #3
    I see, I'm not entirely sure wheather or not the production of hypoiodite existed during the investigation because I didn't observe anything which suggested a chemical reaction. Although I don't know what is meant by the change of surface?
     
  5. Sep 8, 2011 #4

    Borek

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    Staff: Mentor

    When you mix diluted acid and diluted base you will not observe anything as well, it doesn't mean there was no neutralization taking place. So lack of simple observations doesn't tell whole story.

    It is assumed that reaction between iodine and starch takes place on the starch surface, where iodine adsorbs and changes its color in effect (as far as I known mechanism is not 100% known/sure). It is enough that that in the high pH OH- are already adsorbed on the surface in the places where iodine could attach itself, to block the reaction (I am not saying that's what happening, I am just signaling kind of a mechanism). It is also possible that in high pH structure of the molecules is changing - they get twisted/straightened or something - and this new structure has no "pockets" for iodine adsorption (again: I am not saying that's the way it is, but similar mechanisms are known to be present in other molecules). Plenty of interesting possibilities.
     
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