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Ive never had problems with this prac, is your agar old? and do you use a thermometer to check the temp when it boils? I wait til mine gets to around 96 degrees, keeping a lid over it whilst stirring helps otherwise it takes forever to really get hot. Also make sure it is cool enough before adding NaOH I'm always itching to add mine but again use the thermometer so I dont jump the gun. Just a few thoughts. Cheers MariaC

Mel, make sure you added enough agar (10g/500ml) and then bring it to the boil. I have found if I dont boil the agar it wont set properly either. I am a little impatient like Maria , but I have not had a problem setting Phenolphthalein agar, just the plates, but you have to stir it while it is boiling and watch it doesnt boil over.
Lisa

I make mine 3%, so 3g in 100mL or 15g in 500mL, just to be on the safe side. It sets nice and firm, easy to cut.
Also, I would start from scratch.
Lada

I use a microwave for making agar - for plates, for blocks , for electrophoresis gel.
Just make sure you have it in a narrow neck container and place that container in a larger bowl in case it boils over

I have reboiled phenolphthalein agar that hasn't set and it has then set really well. You need to boil it though and I'm not sure if there are any risks with doing that once it has the NaOH and phenolphthalein in it. I did it in the microwave. It goes a bit darker like a (mighty!) maroon sort of colour. But that works well for the prac.

I reheated it, and already it seems to be setting! Big improvement on yesterday's effort. I don't think I heated it enough before adding the agar, but at least that is 1 mistake I won't be making again! I'm too impatient for my own good sometimes...

Thanks everyone for your input, very much appreciated.

have just made ours and made the 1cm ones in the tray? that glue sticks come in. Has nice small round holes. The 2cm ones were made in an ice cube tray and the 3cm ones in a lunch box. I have made models of the sizes with plastic centicubes and use these to check size and depth. Not very scientific but it works well and the students can cut to size easier. cheers bella

I use dropper bottle trays which have removable partitians to get approximately 3 cm sided blocks that the students can easily cut down to size.


Also, I store the agar blocks in sealed tupaware containers to prevent drying out before the prac. They can be stored that way for considerable time.

nickykinz wrote:I have reboiled phenolphthalein agar that hasn't set and it has then set really well. You need to boil it though and I'm not sure if there are any risks with doing that once it has the NaOH and phenolphthalein in it. I did it in the microwave. It goes a bit darker like a (mighty!) maroon sort of colour. But that works well for the prac.

There's no safety risk beyond spills and splashes of a hot corrosive solution. Do handle carefully.

However, as you've noticed, there is some decomposition of the agar agar when boiled while basic. The resulting maroon agar agar can still be used.

The phth agar can even be recycled this way several times, but eventually the colour change will become less and less noticable.

Besides which, the teachers do prefer the nice rose pink of the first iteration.

Yay!! It has set already, next time I won't be so impatient & I will wait for it to boil before adding the agar... thanks again for all your help everyone!

I use boiling water out of the kettle. I put a bit of cold water in with the agar and make a solutio, add boiling and bring it back to the boil all on a magnetic stirrer/hotplate. Works everytime.

Mel, do not add agar powder to boiling water. It will clump. Dissolve it in small amount of warm or cold water and slowly add boiling water from kettle, then boil.
Cool it before adding NaOH and phenolphthalein.I use the same trays as Graham and let kids cut to size . I fill it to 3cm, so that is their largest and they cut the smaller ones.
Lada

We are not doing phth agar at the moment as we do it at the beginning of the year, but as it was being discussed I thought I'd have a bit of a practice as I always have one issue or another. I've got to the stage were it sets lovely and have used the unitrays to get the cubes but I am still struggling with the colour. As it sets it a) gets paler and b) separates into layers so the top of the cube is a lovely pink but the bottom is a brownish colour. Does anyone else have this problem or any solutions to it?

Hi Nicky, you probably not adding enough NaOH & phenolphthalein. If you use my recipe on page 1, it will come out State-of-Origin maroon but teachers here never complain.

Thanks Xenon, my recipe uses about twice as much NaOH as yours so I think it must be the phth that's the problem. I dropped a cube into some and it went bright pink again. The recipe doesn't state an amount of phth; I just squirt until it looks dark, so not sure if I use as much as you or not! So I can either use a lot more or try the powder as some people have suggested.

I haven't had any problems with just putting 1g of powdered phth in my phth agar. Colour has been the least of my problems... my problems always stem around operator error!

I always use a extra strong phth brew. Just add some to a small amount of metho just before adding it to the agar. I also flood the top of the agar with phth once it is set. I leave this over night and then tip off any excess in the morning if there is any left. It does tend to absorb into the gel.

Thanks everybody. I am now a happy bunny and feel all ready for the next time we do phth agar. I used the phenolphthalein powder and it mixed in really well (I was worried it may clump). Fantastic dark colour. If it did fade it wasn't enough to stop it looking great and there were no layers. Will be doing that from now on. I used the equivalent of 1g per litre and had already increased my agar to 3%.

Seriously dude - time for some photos! If for no other reason, then to entertain those of us with cabin fever stuck at home, with no life at the moment!

The one on the left is the one made a few days ago now. The colour has really evened out a lot but is still patchier than it looks in the photo. The one on the right is with the phth powder, made yesterday. They have been together in a box overnight in the fridge with all their friends from the various attempts over the last few days!

I've had batches in both of those colours. And seriously, what does it matter as long as the little cherubs have the same start point?

Thanks for the photos, btw.