Australia's Science Forum

Good morning all,

I'm wondering what recipe you all use for iodine, we tried to do a dialysis filtration prac with iodine, starch and dialysis tubing where the starch solution should turn black. It hasn't worked the last few times for this particular teacher. I have tried different iodine solutions to no avail!
The recipes I have tried are from this site and as follows:

Aqueous: 1g iodine, 5g potassium iodide to 20ml water, dilute to 1 litre.

Lugol: 4g iodine, 6g potassium iodide, to 100ml water, make up to 1 litre.

I would appreciate your suggestions.......

We only use Lugol here, and the above experiment works so well, except we use the little plastic bags from Livingstone instead of the dialysis tubing that is sooooooooooooooooo dear.

We use the Aqueous Iodine as you posted .

I had one teacher complain too . Turns out they were using a 600mL beaker full of water and adding a few drops of Iodine solution .
When I tipped half the water out and poured in the Solution so it was a light brown colour , they gasped .....moments later the Dialysis tube of Starch solution started to go blue ......funny that

Edit: If you suspend it in a gas jar ,you don't need so much Iodine solution . Another hint I picked up at other school was to attach the tubing to Thistle funnes as it makes filling with starch much easier and less messy

Labbie,
The tubing was here when I got here....so I haven't had to buy any, luckily by the sounds!
So I might make up another new batch and see how it goes! Again!
Maybe it's the teacher that is the problem haha!
Thanks for that

My recipe has 15gm Potassium Iodie and 3 grams of Iodine in 1 litre.
The starch solution is just put in freezer bags for the junior school as it is much more cost effective and works just as well.
They need to put about 10ml of iodine solution into 200ml beaker of water making it the colour of unhealty wee.
Most time teachers have failed here is due to not putting enough iodine in

Thanks bigmack and fibreweb,

I'm starting to think it is the teacher in question who is the problem!

Also just out of curiosity, what molarity starch do you guys use?

1% solution

thankyou!

My recipe is the same as Fibreweb's. Always works. Mix 15g potassium iodide in about 50ml or less of water, then add 3g iodine. Because the potassium iodide is concentrated it dissolves the iodine quickly. I just swirl the beaker around to encourage the dissolving. You may need to crush the remaining small bits with a stirring rod. Once dissolved make up to a litre with water. Sure beats waiting 24 hours for the iodine to dissolve. Keeps well in an amber bottle.

Starch solution - 10g soluble starch to 1 litre distilled water.
I mix the starch with a small amount of water until it is just liquid (not paste) then pour in boiling water while stirring. Most of the time I end up with a good solution that doesn't have bits of lumpy starch swirling around in the final product. Just takes a bit of practice.

Thanks Lyn! I think I'll give that one a go!

worth a read