Phenolphthalein Agar

and any other non-chem subjects.
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Labbie
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Re: Phenolphthalein Agar

Post by 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?
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nickykinz
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Re: Phenolphthalein Agar

Post by 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.
Nicky
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emma.hunt
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Re: Phenolphthalein Agar

Post by 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
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Re: Phenolphthalein Agar

Post by 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
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Re: Phenolphthalein Agar

Post by 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?
Marama T
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Re: Phenolphthalein Agar

Post by Marama T »

People have been wondering how long the phenolphthalein agar lasts for - I had some in the fridge when I started at this school in 2016 and didn't have to make more until this year. It worked perfectly and the only reason I had to make more was because it went mouldy.
bigmack
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Re: Phenolphthalein Agar

Post by bigmack »

Marama T wrote: 27 Aug 2019, 08:17 People have been wondering how long the phenolphthalein agar lasts for - I had some in the fridge when I started at this school in 2016 and didn't have to make more until this year. It worked perfectly and the only reason I had to make more was because it went mouldy.
That's funny Marama , I just ditched a couple of icecream containers full of moldy stuff myself a few days ago . It sure does last a long time in the fridge but yup....it goes moldy after a while :whistling2:
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kimmy
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Re: Phenolphthalein Agar

Post by kimmy »

i know this is a old old post but i have just came across a great idea of how to prepare the squares for the diffusion experiment.
K mart have silicone ice-cube molds that are perfect for the 3cm square cubes then students can cut another one down to get a 2cm square and 1cm square.
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Labbie
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Re: Phenolphthalein Agar

Post by Labbie »

That's a great idea Kimmy
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kimmy
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Re: Phenolphthalein Agar

Post by kimmy »

Labbie wrote: 28 Jul 2020, 14:01 That's a great idea Kimmy
yes when i read it i thought, wow what a great idea.
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melsid
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Re: Phenolphthalein Agar

Post by melsid »

There are also silicon ice cube trays perfect for 2cm cubes and 1 cm cubes - so much less waste!
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Milo
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Re: Phenolphthalein Agar

Post by Milo »

It's my time of year to do this again. Last year I saved the leftovers for the first time, and it seems to have kept well in the fridge. For any others that do this, what method do you use to re-melt the agar for pouring? I'm reluctant to use the microwave method since we don't have our own science microwave; I would have to use the staff room ones people use for their lunch, and I have concerns about using science equipment in there, spillage, etc.
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Labbie
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Re: Phenolphthalein Agar

Post by Labbie »

Good point Milo, we got a new one for the staff room, so I was able to take the old one just for the prep room. One of the best things we ever did.
Regards Labbie

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J
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Re: Phenolphthalein Agar

Post by J »

Short of using the microwave, perhaps heating in a water bath?

I couldn't manage without my cranky old microwave :thumbup:
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remlap12
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Re: Phenolphthalein Agar

Post by remlap12 »

It takes an age without a microwave......just sayin'!!
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Milo
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Re: Phenolphthalein Agar

Post by Milo »

Indeed it does. I ended up buying a new microwave for us last week! Now I just need to make sure nobody uses it for their lunch...
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Labbie
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Re: Phenolphthalein Agar

Post by Labbie »

Good one Milo, is it in the prep room? We had a sign on the door of microwave, Needs deep cleaning if used for food.
Regards Labbie

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Now retired :wub:
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Milo
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Re: Phenolphthalein Agar

Post by Milo »

Yes it's in the prep room, which directly adjoins the Science office. For now I've just written on it in texta, need to get around to making a sign.
emma.hunt
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Re: Phenolphthalein Agar

Post by emma.hunt »

Hey,

So I've got this agar from 3-4 weeks ago. It's been stored in the fridge uncovered and has cracked and come away from the edges as you can see.
How do you all store yours for the next time someone wants to do this?
Is it past saving now?

Thanks
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Merilyn1
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Re: Phenolphthalein Agar

Post by Merilyn1 »

Wrap it up in a few layers of cling wrap. This will help although I've never had to keep it longer than a few weeks.
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