Phenolphthalein Agar

[Labbie]

I always found Agar 1000 was just awful, it just never never set. I like agar 750 but find it so hard to purchase. There never use to be different agar's around, so perhaps there is another one?

[nickykinz]

I usually use 25g per L but just because it gives a firmer agar, not because it didn't set. I do also use powdered phenolphthalein as it seems to give a better colour. I do 1g per litre. It sounds like you did everything else right though. I make sure it boils but not for a particular amount of time and I add the NaOH as a solution once it has cooled to 60. I don't see why putting it in the fridge would stop it from setting. It should have just speeded it up. I wouldn't have been surprised if you got some cracking or something from cooling it quickly but not stopping it set at all.
I would suggest increasing the amount of agar in case it is old, maybe boiling it for an extra 30 seconds, using powdered phenolphthalein and crossing your fingers.
If you have time though you could boil it up again and see if it sets. You should be able to do that quite quickly and may save you redoing the whole lot. I've reused ones when they haven't set properly, but I think they were from not being brought to boil properly in the first place.

[emma.hunt]

Heya.
If using the dropper trays (or any trays), does the agar come out easily enough or does it stick to the edges?

Thanks

[Merilyn1]

Comes out fine. It's been ages since I made it, but I don't ever remember a problem. Might have to run a blade around the edge but can't remember doing that.

[KylaW]

Hi Lyn, my phenolpthalein agar didn't work this year either. Two failed batches. The third time I boiled it for a few minutes (I've always just added boiling water), and used bromothymol blue instead. It set great, plus the agar turns from blue to yellow (rather than clear) which makes the diffusion easier for the students to see. The teacher asked to stick to this recipe in future years.
Still not sure what went wrong, perhaps the age of my agar meant it needed the extra boiling?