Iodine solution that doesn't show starch...

[Meldream]

Hi, as you can probably tell with one of those handy post counts (or the like), I am new here. Be nice. =)

 

I am also a high school student (senior year :[ ) so forgive my ignorance regarding anything you say. Erm. I just have a quick question; I've performed an experiment to test for starch and sugar with amylase, starch, and glucose solution.

 

And theory says there must be starch somewhere (it is starch solution. How can there not be starch? Unless there was a problem with starch, but I'll get to that later).

 

I use iodine, and there is no starch. For any of my trials.

 

And I really think the starch was fine, because I put amylase with the starch and it gave me sugar (tested using Benedict's reagent--it turned orange). Thus...there was starch, in my reasoning. As an added note, when I tested what should have had starch for sugar, no sugar was found either.

 

This is a mystery to me. =/

[insane_alien]

the solution is simple. the amylase converted all the starch available. or at least to the extent that it is no longer visually detectable by iodine solution.

[ChemSiddiqui]

amylase converted starch into maltose which gave you the positive result with benedict solution, however since starch has been broken down iodine test will be negative

[Meldream]

Err...perhaps I was not as clear as I could have been.

 

I meant that I had one beaker with both amylase and starch, which might or might not have starch in it still. It did, however, have sugar.

 

Now, I had another beaker with only starch. Sorry if this was not clear. I hope you will agree with me that there is supposed to be starch in a beaker that contains only starch. However, the iodine did not show any starch. There was no blue-black coloring; it remained brown. I am also fairly certain I put enough iodine into the solution. Oh, and I believe the solution's bottle said it was 1% starch.

Edited September 7, 2008 by Meldream
Must add more info..

[big314mp]

http://www.elmhurst.edu/~chm/vchembook/548starchiodine.html

 

Did you have any iodide salt in the iodine solution?

[ChemSiddiqui]

Err...perhaps I was not as clear as I could have been.

 

I meant that I had one beaker with both amylase and starch, which might or might not have starch in it still. It did, however, have sugar.

 

Now, I had another beaker with only starch. Sorry if this was not clear. I hope you will agree with me that there is supposed to be starch in a beaker that contains only starch. However, the iodine did not show any starch. There was no blue-black coloring; it remained brown. I am also fairly certain I put enough iodine into the solution. Oh, and I believe the solution's bottle said it was 1% starch.

 

i am not sure about what you mean. You say that the starch and amylase are in a single beaker then you say that the beaker had only starch. Well, if it did have only starch then i imagine that the solution you are using may be bad ones[its just a guess]. Try to repeat the test and i hope this time things clear up for you;)!