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Hi
I seem to be having trouble with what should be a simple prac. It has worked beautifully every other year.
I Put starch solution in dialysis tubing and tied off the ends. Put this in a beaker of water. Add iodine solution to the water in the beaker. Iodine should diffuse into the bag and make the starch go black.

Didn't work the first time the students tried it. Decided starch solution was too weak. Then made up a really strong solution of starch and put this in a dialysis bag and put the bag into a beaker of water with a few squirts of iodine solution. Also Tested the starch solution and iodine in a beaker by putting a drop of iodine directly into the starch and it turned black. But no turning of starch black inside the dialysis bag.

Tried putting lots of squirts of iodine solution directly into the dialysis bags (students not watching at the time). Turned pinky browny black at the time but by the time the students went to check, the starch that had settled on the bottom was white again.

I just tried putting putting a few drops of iodine solution directly into starch solution in a beaker again. Turned black where the iodine contacted the starch. Left it to sit for awhile. Starch settled to the bottom and showed two pinky smudges that were barely visible.

Where is my problem?
The starch solution?
the dialysis tubing?
the iodine solution?

ps glucose moved through the dialysis tubing very well. That part of the experiment did work well.

Any thoughts would be appreciated. We need to do the prac again with another class next week.

Thanks
Tina

Tina,
I had the same thing happen one year.
I now make fresh 1% starch as per 'The Laboratory' & use
O.1M Iodine solution which is considered strong use a little more than the prac suggests.
Did it today and worked perfectly.
Good Luck!

Was the iodine a freshly made batch? If so what was your recipe? Did you have it stored in the small plastic dropper bottles?
Lyn.

We have just done this prac and we didn't have any problems using 5% glucose and 5% starch solutions.

Kerry

starch will only be any good after you have made it, for THREE 3 DAYS. So it needs to be made fresh.

Did you use soluble starch or potato starch? I think you need to use the soluble starch, but I could be wrong. Also, making it up in a small amount of cold water to make a paste and then add hot water makes it turn like clag glue and the starch doesn't settle out. (but it is rather messy getting it into the tubing!)
Rosalie

Lyn,
see: Iodine solution test starch & cellulose "the Laboratory" p127

My Iodine solution is not a fresh batch but I store it in a brown glass bottle wrapped in paper to keep out the sunlight.

I use soluble starch and Method 1 in the "the Laboratory" p88

good Luck!

We use the soluble starch, so I know that one works. I am not sure about the potato one, but I would imagine, that it too, should work.
Lada

Hi all Chemtalkers
We did this prac. a few weeks ago and it worked very well. I always use potato starch!!
It would probably be best to make all the solutions fresh,that is, if you are given enough notice!!!
Good Luck
Mother