Australia's Science Forum

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.

I have enjoyed re-reading this thread. Well done to you all.

Here it is the whole 8 page post on Our favourite topic with lots of tips on what to do and what not to do including tried and true recipes!!

worth bumping this up for the new members.

Well, it's that time of the year again here. Last year one of my teachers bought some of the silicon ice cube trays - they come in 3 sizes and work perfectly to give 3cm, 2 cm and 1 cm cubes (plus a bit in case they shrink!)
So easy to pour into the moulds, then turn out and give one of each size to the students to cut to exact sizes required. So much less waste too!!

Can you post some photos of the trays? The bit I am intrigued by is "one of my teachers bought..."
In the past I have used the unitrays, sometimes with the divider and sometimes without (that's a whole other post). This year, a new teacher wanted trays of 1cm, 2cm and 3cm depths - in the past I've just given the large ones and they have cut it up themselves. As I was time poor, I just used three trays, but there was a lot of waste. I'm interested in these ice cube trays. Any ideas where they came from?

Hi Merilyn1, I don't have any pics yet but I'll get some soon!

THe trays came from KMart and cost a couple of dollars each. We have 2 large grey ones with 6 holes in each, 1 of the medium blue one with 15 holes and 1 of the small red one with 14 holes - but it measures 2cm x 1cm, so the students use half a lump.

Here we go, finally remembered to take a pic of the trays we now use.

Each of the cubes is slightly larger than the size we actually want, to allow for shrinkage. We cut the red ones in half to make two 1cm cubes.

It took one 800mL batch to fill what you can see here.

Sweet Melsid . we had sooo much wastage last time . Love it .

Great thank you for sharing

That is so clever! I wasted heaps this year, this looks a great idea. I will be stopping in at the local K-Mart to find these!