Australia's Science Forum

H All
This prac has been giving me grief ever since I started working here.
I wrap the end of a thistle funnel with dialysis tubing and fill with a coloured sucrose solution. It's not supposed to come out but it drips out like there is no dialysis tubing. The teacher concerned keeps saying " the lab techs at my last school never had a problem with this, I don't know why you can't get it to work"...... *#@* I rang them and I don't appear to do anything differently to them.
AAAAAAAAAAAHHHHHHHHHHH I'm going nuts.
Krysia

I suppose replying "I'm not a qualified Biology Teacher. How about you teach me how to do it properly" would not get you anywhere?

If it is any consolation, I just supply the materials to my teacher. She then explained to her class how to set it all up. This year she had one satisfactory result out of 6.

Ian

I don't know this prac. What is is and how is it supposed to work? I think you should put it back on your teacher. You can't be expected to know absolutely everything!! And if it is that big a deal why hasn't he or she just done it themselves anyway? If it is so 'easy' they should be able to show you how to do it this first time at least.

Krysia,
Try,
-soak the dialysis tubing in some water so it can be easily opened
-tie one end off with cotton (sewing type)
- fill with sucrose solution
- then tie the top off with cotton.
Good luck, our students do all this themselves!

Krysia,
I am not sure what you mean, but if you have the tubing tied to thisle funnel, it is possible that you are creating a vacuum and the solution is not flowing in.
Follow Bindi's suggestion and put the sucrose soln. in with a pipette or a very small beaker and steady hands.
Rinse the outside with water to remove residue of sucrose.
Good luck
Lada

I have used an apparatus made with dialsys tubing and a cut-off plastic pipette, with lots of success, and less cost than thistle funnels. See my attached diagram.

If you fill the dialysis tubing pretty much to the top, you can squeeze it to get the liquid up to the top and then tie it off with cotton. Then it can stand in a salt solution and get limp, or a distilled water solution and get more rigid, depending on what you're experimenting with. In an experiment where you are trying to prove that fluid goes into the tubing, the excess fluid goes up the stem of the pipette and is easy to see. It is way easier to tie off, using the plastic pipette, than trying to get that tight seal on the glass thistle funnel.

The other thing I do, which is just wonderful, is cut lengths of tubing, wet them and tie them off at one end, and then dry them upside down on the pegs of a wooden test tube rack. Means I have a supply of ready-made already open dialysis tubes.

Good luck!

Hi all,
For the last few years, we've just knotted the dialysis tubing top and bottom with the solution inside (looks like a sausage) , and then this goes in a measuring cylinder. You can tie a string around the end of the 'sausage', and attach to measuring cylinder, then it is easily pulled up to show any volume change. Seems to work OK.
Ellice

We have a new teacher here and she did osmosis (starch & iodine), in a way I hadnt seen before, can someone explain the procedure for me I have looked in texts and I cant find the way she did it. She had starch in dialysis tubing in beakers of water, for about 4 days, then cut ends off tubing pour into test tubes and add iodine. Other times I always clean up tubing ballons that have turned blue.
Lisa

Our prac suggests to put iodine into water in beakers which will turn starch in tubing blue. This shows, that starch is a large molecule and wont go into beaker, while iodine /water solution will go into starch soln.

Your way can only show, that starch stayed in tubing.
Me thinks.
Lada

Lis,

That'd work IF you tested the water that was outside the tubing with the iodine, to show that no starch went into the water. THEN testing the solution that was inside the tubing would prove the presence of starch. Otherwise it's half a test.

So, Lada, you right girl!

Thanks Smiley & Lada,

I thought I was going mad

This is my 8th year in this job, and there is always something new to learn, but on this occassion I think 1 for Lisa 0 for teacher

Lisa

Good Morning,

For those of you who use iodine and starch, what strength iodine do you use please? 0.1M?

Thanks in Anticipation,
Rita

We used full strength Iodine,

I usually just give them the 0.1M - haven't had any complaints.

An old topic but we use small Rubber bands to hold the tube to the funnel ...you know where you twist and go over about 6-8 times Kids set up and fill the funnel with starch and the gas jar with water Teacher then adds our concentrated Stock Iodine solution to "colour" the water a light brown colour . the Starch in the Dialysis tube will start to go blue after a bout 10 minutes .This is about 40 minutes later I have no idea what molarity our Iodine solution is . We just make it up from the recipe book out of Potassium Iodide , Iodine and water and store it in a dark bottle to be diluted down when filling dropper bottles .

But basically if the water around the tube is brownish , it will work .

And last time , the Bio teacher asked for 5% starch solution . I told her that sounded really thick but she insisted . When I tried to make it , it was so thick , it turned to porridge . Gave her 0.5% . Think her book might have had a typo in it .

Do a search for the little plastic bags that are ade just for the experiment. They really are very very good.
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. taken from fibreweb post.

Also see thread of Iodine for a good read.

We did this one last week, with 1% starch, just as you have illustrated. One set up worked, in that the starch went black, and the other 6 or so did not work!!! (go figure!) In all of them, the level of water in the thistle funnel went DOWN, Not UP. I am beginning to believe that Science is NOT "Cause and Effect", and reproducible experiments after all, but actually voodoo, and magic with a mind of its own!!! (and sent to torment me!)

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

Ian wrote:We did this one last week, with 1% starch, just as you have illustrated. One set up worked, in that the starch went black, and the other 6 or so did not work!!! (go figure!) In all of them, the level of water in the thistle funnel went DOWN, Not UP. I am beginning to believe that Science is NOT "Cause and Effect", and reproducible experiments after all, but actually voodoo, and magic with a mind of its own!!! (and sent to torment me!)

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