Phenolphthalein/agar re-revisited!

[mtg]

I am about to do "body shape vs diffusion" and in the past my NaOH jelly loses its colour as it cools. I follow the recipe, wait til it cools to whatever etc but as far as I can recall it still fades. What strenghth phenolphthalein and how much should I use to ensure a good pink? And to get 5mm, 10mm,and 20mm thick what do others use? And do you think it's better to make clear phenolphthalein jelly and put it in NaOH or pink jelly into acid????????? Also it has to be keep for a few days....

[RosalieM]

The only advantage of starting pink and going clear is that the kids can see it as it happens whereas when you put it in clear and turn it pink they don't see it till they cut it open. I use a plastic dropper tray thanks to someone else on this site. I leave the divisions in and the kids get on square each. They then cut their own squares to size. I usually have enough for spares in case some kids mess up and don't cut the biggest one first. I had it losing it's colour too, so now I just throw a heaped spatula of the powdered phenolphthalein into a little bit of metho to dissolve and add that. I also use a fairly strong NaOH and pour a whole dropper in (50ml) sometimes more. Forget the drop by drop thing!

[AnnNos]

For a class of 10 groups
1 1/2 litres distilled water. Remove from heat and add 45g Agar Agar powder slowly stirring until dissolved. Allow to cool slightly, then add 30 mLs phenolphthalein in ethanol, running down the stirring rod, stirring well. Finally add 37 mLs 1M sodium hydroxide, running it down the stirring rod. (Liquid should be a deep pink) Pour into a 4 litre ice cream container and allow to set overnight. Should be made the day before use, any longer and the phenolphthalein colour starts to fade. The depth of container will be enough to cut all the squares required for 10 groups.

[rae]

I have kept some for over 12 months and it doesn't fade. When I make up fresh once it is set I flood the surface with extra phenolphthalein and leave it overnight. I know longer have any problems.

I use the recipe in "the Laboratory" and it works well. I set the gel in the dropper trays.

We use 0.1M H2SO4 as the acid it works quicker. This was the method in the old Heinemann prac manual.

[rae]

Oh I also use fresh phenolphthalein made up by 1/2 a spatula of powder with about 20 ml metho and just dribble it in until I have a good strong colour.

[nickykinz]

I have always struggled a bit with this. Mine fades as it sets and ends up in layers of colour. Once it is set it doesn't change even if I keep it for a few days. I have also had trouble with it being mushy. It looks lovely and set on the top but then when I tip it out it is mushy underneath. This year I remelted the mushy stuff by bringing it to the boil in the microwave. It set beautifully and was a really dark maroon type colour. I think from now on I will do that! I had made it in takeaway containers so that each group could have a chunk each and cut different sized squares. It was easy to just pop each container in the microwave for a couple of minutes and leave to set again. There are probably some implications with boiling it with the NaOH and phenolphthalein already in it though!

[mtg]

WOW THANK YOU ALL HEAPS!!!!!!
It's just so helpful. I feel so much better knowing I'm not the only one having issues with this pracs "fade-out" and now I know how to remedy it. EXCELLENT. Thanks again to you all.

[Graham Kemp]

There are probably some implications with boiling it with the NaOH and phenolphthalein already in it though!

Well, Phenolphthalein is a laxative and suspected carcinogen, and you don't really want NaOH in food either, so make sure that the containers remain sealed! Or use a dedecated science microwave.

Experiment wise, you're agar is accumulating NaSO4--as each time you soak in H2SO4, then add more NaOH to recolour--adversely affecting diffusion rates. Also the agar itself will start to decompose when heated in acid or basic conditions. I found that after too many redos, the phth-agar begins to reset into a mahogany-brown colour that won't fade well.

Otherwise, it should recycle quite a few times.

[nickykinz]

I'm not recycling it. I was just trying to get it to set as it had stayed mushy. And yes it is a dedicated science microwave.