Osmosis and diffusion

[Ian]

Hi All

We did the iodine and starch solution for this prac without measuring side of things. The teacher also asked for sucrose and glucose both 1M. Has anyone used these as well for diffusion prac? Seems a strong concentration....1M sucrose is 34.23%.
Thanks for your thoughts.
Rita

I think sometimes, teachers say "1 molar" when what they mean is just "sort of standard concentration, but not too strong" I once had a teacher ask me for a 1M concentration of Calcium Carbonate!!!! (I stirred that for a long time before twigging that CaCO3 is insoluble! And she wasn't even pranking me!!)

[Ian]

My lot order a saturated sucrose solution, but I've been giving them around 5 - 10% or so and it works just fine.

Saturated Sucrose solution? Is that "Stick jaw" toffee or "Brittle" toffee? Sounds yummy either way!

[Kathryn]

Good morning. My teacher has always asked for litres of iodine solution to pour straight into the beakers containing the starch dialysis tubing. Of course when I make up the iodine as per the recipe it is far too dark so then have to dilute. Should she actually just be adding a few drops of iodine from our usual 0.1M iodine dropper bottles to water filled beakers?
thanks

[bigmack]

Droppers won't do it .....its just not strong enough .

I give them a small stock solution of Iodine that is 10x strength ( I dilute it down for droppers )

They just carefully add it to the water in the beaker , gas jar what ever , until it is a pale brown colour . ....works everytime

[KassH]

Hi, I need some help. I've not prepared this prac before and I have no iodine solution made up. Do you use potassium iodine or straight iodine? And how concentrate?
I started making up a potassium iodine solution but it's clear??

[labbassistant]

We use Aqueous iodine solution, I've attached a photo for the recipe.
And also a 1% or 1.5 % starch solution
Hope this helps

[labbassistant]

Sorry, I should've asked before. Is this with the dialysis tubing diffusion prac?
That's what we use for that diffusion prac

[Ian]

Teachers will often ask for Potassium Iodide solution (KI) when what they really mean is Iodine Solution (I/KI) or sometimes called Lugol's Iodine Solution. Yes, KI is a perfectly clear liquid and will NOT do what they want (ie, turn starch black/purple) but it is necessary to make the iodine dissolve in water. The recipe that Labassistant has given you (from the book, "The Laboratory") is a good one. There are a couple of similar but equally good recipes in that book.
They do suggest that the I/KI solution be made up 24 hours in advance.
Actually, an interesting experiment you can do, if you are not totally run off your feet, like most of us are, most of the time) is to add a few grains of iodine to a beaker of water. No matter how you stir it, it will just sit there and not dissolve. Then add a couple of grains of KI to the beaker. Immediately the I will start dissolving into the water in brown swirls! (maybe I am a nerd, but I found it interesting!)

[KassH]

Thank you labbassistant and Ian. It is for the dialysis tubing prac.
I used the potassium iodine solution I had made up (with 16.6g KI) and added the 3g of iodine. It's now brown so guessing it's good to go. The extra 1.6g of KI shouldn't matter, right? Don't have 24hrs for it to sit but they will have to make do with that.

[labbassistant]

If everything is dissolved then there shouldn't be a problem.
Happy to help!

[RosalieL]

Does anyone have the prac for glucose osmosis? The one where glucose goes in the tubing and distilled water in the beaker, then test the beaker water for glucose at the end with benedicts solution? The only ones I can find online have the starch and glucose combined (which I will suggest we use for next year) but I can't find one that is just glucose. I suggested it to our bio teacher who had only ever done the starch (it only worked for one group this year. I think it just needed more time because I did it in the prep room and it has worked but did take longer than the time the prac method said to leave it.

[bigmack]

We just did that I believe Rosalie .
They put Glucose is one tubing and then thoroughly rinsed it and put in a boiling tube and only just cover it in water . Also needed about 5% Glucose solution to work .

Do the same with the starch in tubing .

Then as soon as they have added the water , they suck some out with a disposable pipette and place each in a well on a spotting tile .
Then they use a Keto-Diastix ( available from Chemist warehouse at a good price ) to test the Glucose well and iodine to test the starch well .

they wait about 20-30 min and repeat with clean pipette and new Keto-Diastix stick .

The diffusion of the glucose seems very low so doing it in a beaker would result in such low concentration of Glucose as to render the test useless , hence we use a boiling tube with minimal water so we can have a high enough concentration to test .....if that makes sense.

I also snip the Keto sticks in half so we get twice as many out of a jar .

Personally I think its a strange way to do the demo as the students never see and starch ( unless they didn't wash the tubing properly)
so as an extra , we pour some iodine into the test tube and they can then see that the starch is all inside the dialysis tube )

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[RosalieL]

Yeah we normally do starch in the dialysis tubing and lugol's iodine in the water. The iodine goes in but the starch doesn't come out. The only one group started to turn black.