Phenolphthalein Agar

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

Post by Labbie »

Good buy Milo, it's oh so handy.
Regards Labbie

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Now retired :wub:
labbassistant
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Re: Phenolphthalein Agar

Post by labbassistant »

Hey,
So in the past when I've made the agar, it looks set but the bottom layer seems to crumble and not set properly.
We usually let it set overnight out of the fridge, then the next day in the fridge. (2 days for colour to develop well and set)

Should I put it straight in the fridge or keep it out?

Thanks!
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superbug
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Re: Phenolphthalein Agar

Post by superbug »

Hi
Did you add too much agar? was it boiling enough to dissolve all the gelatine and mixed well when you added in the cold water to the final volume.
i also made some 2 days ago myself.
I place in unitrays and let it sit until it is cool enough to place in fridge. i wrap in gladwrap first and place them all in fridge ready for use.
it has kept in the fridge for a few months with no loss of colour.
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Re: Phenolphthalein Agar

Post by Merilyn1 »

labbassistant wrote: 25 Feb 2022, 10:37 Hey,
So in the past when I've made the agar, it looks set but the bottom layer seems to crumble and not set properly.
We usually let it set overnight out of the fridge, then the next day in the fridge. (2 days for colour to develop well and set)

Should I put it straight in the fridge or keep it out?

Thanks!
I would guess the agar hadn't dissolved properly. Could be that you have had the heat too high to start with which can make it go lumpy (bit like making white sauce :D )
It will set out of the fridge and in fact can lose its colour a bit if it is cold. Usually best to bring it to room temperature.
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Anna Z
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Re: Phenolphthalein Agar

Post by Anna Z »

We made up a huge batch this week, cooled on the bench then in the fridge. Love how the pink fades and then as it comes to room temp again it goes a nice pink. Fun stuff to play with, beakers are crap to wash :yuck: . Oh and I found out, don't be tempted to add the hydroxide before you have a beautiful smooth agar..... it goes black :cheesy:
Merilyn1
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Re: Phenolphthalein Agar

Post by Merilyn1 »

Yeah, let it cool to below 60 deg before adding anything. I had to chuck my NaOH in at about 70 - went a bright yellow, so was just heavy handed with the phenolphthalein. All good :)
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Re: Phenolphthalein Agar

Post by bigmack »

Anna Z wrote: 12 Mar 2022, 08:29 Oh and I found out, don't be tempted to add the hydroxide before you have a beautiful smooth agar..... it goes black :cheesy:
Haa , thanks Anna . First time I made it that happened to me .Wasn't sure why .
It certainly is an art making this stuff .
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KassH
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Re: Phenolphthalein Agar

Post by KassH »

First time in charge of this one. How do you dispose of the agar once the students have finished?
bigmack
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Re: Phenolphthalein Agar

Post by bigmack »

Stuff they haven't used can be remelted and reused .

Stuff they have used ,I just poor it down the drain ( finger in the plug hole ). It has nothing growing on it so doesn't need to be Autoclaved
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KassH
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Re: Phenolphthalein Agar

Post by KassH »

Thanks Bigmack.
I was more worried about the phenolphthalein in it. Does anyone have an other way of doing this prac without using it?
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Anna Z
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Re: Phenolphthalein Agar

Post by Anna Z »

if it's still solid, we throw it in the bin.
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cactus155
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Re: Phenolphthalein Agar

Post by cactus155 »

If you are worried place it in either NaOH (if its the clear one) or an acid (if its the pink one) as I believe the pH should be neutral or close to when the colour has changed to the opposite so pink to clear or clear to pink, drain and then throw in bin or just throw straight into bin
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KassH
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Re: Phenolphthalein Agar

Post by KassH »

Has anyone tried doing this one with a different indicator? I worry about giving the students the phenolphthalein and also the younger female teachers. They wear gloves but if we can eliminate the risk all together that would be better. Right?
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Re: Phenolphthalein Agar

Post by labbassistant »

Yes!
We made some with Bromothymol Blue, it was great! Southern Biological have a video showing how they do it, we ended up needing to add a little more sodium hydroxide than they did, but it came out great!
We gave 1M acetic acid but found 0.1M sulfuric was better
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KassH
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Re: Phenolphthalein Agar

Post by KassH »

Thank you labbassistant!!!
I knew there'd have to be a better option.
I found one video that used zucchini with liquid food dye but it just didn't cut it.
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Re: Phenolphthalein Agar

Post by Merilyn1 »

I have one teacher who prefers to use potato and iodine.
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