Phenolphthalein Agar

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rae
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Phenolphthalein Agar

Post by rae » 23 Feb 2009, 14:22

Woo Hoo!! Its that time of the year again, making up this agar. What does everyone make it up in to get the various depths required?

Help Lorrae

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Xenon
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Re: Phenolphthalein Agar

Post by Xenon » 23 Feb 2009, 14:46

I use a 10-dropper tray with the divider removed to set the agar. Gives nice sharp corners and a depth of 3cm. I tape some graph paper under a sheet of perspex for cutting the set agar.
I like to make it a really strong colour, so fading isn't a problem. Here's my recipe:

PINK/PHENOLPHTHALEIN AGAR BLOCKS

For 1 block (10 dropper tray, divider removed)

14.4g agar
480ml water
24ml 1M NaOH
4.8 ml phenolphthalein

Put agar in beaker, add water and magnetic stirrer. Full heat, stir to dissolve. Remove stirrer, continue heating till boiled. Remove from heat. Add NaOH and phenolphthalein. Pour into tray, allow to set.
Xe, SD, KOPR

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Robbie
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Re: Phenolphthalein Agar

Post by Robbie » 24 Feb 2009, 08:15

If you have any problem with fading - make sure you cool the agar to below 60 deg C before adding the NaOH and phenolphthalein.

We don't have anything as smart as dropper bottle trays so I have used 3 litre ice cream containers. These also have sharp edges. We invert the block onto a large flat white tray and then the teacher chops it into smaller blocks for the girls to hack away at.

We use 0.1 M H2SO4 for the solution. If you need a faster reaction make the concentration 0.2 M.

Robbie

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noona
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Re: Phenolphthalein Agar

Post by noona » 24 Feb 2009, 08:30

I use dissection trays I find they work well
Make sure you cool the agar a little befor adding sodium hydroxide. :coffee: & Phenolphthalein =D> =D>
Noona

RosalieM
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Re: Phenolphthalein Agar

Post by RosalieM » 25 Feb 2009, 07:58

I use takeaway containers, fill them to a bit more than 3 cm and cut the blocks into thirds. The kids then do the rest to get their 1, 2 and 3cm cubes using rulers and scalpels.

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Robdean
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Re: Phenolphthalein Agar

Post by Robdean » 25 Feb 2009, 11:20

In all the years I've been making this stuff I've found that the colour develops better the longer it's kept. I set it in an enamel tray. I don't refrigerate it as in the past have found it will turn a murky purple grey colour instead of the bright crimson colour; still gives the same reaction though. Has anyone else noticed a colour change in the fridge. We weren't sure if it was due to "old phenolphthalein" but as I said the reaction still worked.

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rae
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Re: Phenolphthalein Agar

Post by rae » 25 Feb 2009, 11:44

Thanks for all the suggestions. I've tried the dropper bottle trays and put in an overhead photo copy of some grid paper on the bottom to make it easier to get the size. I had been using take away food containers but didn't like the dent in the bottom of them. I have made up a 0.5cm, 1cm and 2cm trays. I will keep you posted.

Thanks again
Lorrae

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dime
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Re: Phenolphthalein Agar

Post by dime » 25 Feb 2009, 11:48

Mine always goes the murky purply colour even before I put it in the fridge. I wondered whether it has something to do with puting the NaOH in at the wrong temp, instead of after the temp goes under 60 deg. That's what I am going to try this time, anyway. But as you say, it still works, just not as pretty.

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Robbie
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Re: Phenolphthalein Agar

Post by Robbie » 25 Feb 2009, 13:00

I have definitely found a big difference in the colour depending on when you add the NaOH. Add it after the agar cools below 60 deg and no need to put it in the frig. It should be fine then.

Robbie

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sunray18
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Re: Phenolphthalein Agar

Post by sunray18 » 25 Feb 2009, 13:46

I am a little lazy ...
I make it in ice-cream containers.. then cut it into cubes after it has set... I had so much trouble getting the right size containers, then I found the agar shrunk after cooling... so now I just bung it in above the level I want, when it's set I turn it out onto a copping board and using a sharp knife and metal ruler, I cut it.

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Re: Phenolphthalein Agar

Post by Lyn » 26 Feb 2009, 15:30

I tried leaving the very first batch I made out on the bench overnight to set. Got in the following morning to find an army of ants munching their way through every tray I had made. It now goes in the fridge and teachers just have to live with any colour variation.
Lyn.

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rae
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Re: Phenolphthalein Agar

Post by rae » 27 Feb 2009, 08:02

The agar gel I made was fantastic.The colour was great and the grids in the bottom made it so easy to cut to the correct size. All im all it was very successful. Thankyou all for the great tips. It always makes the job easier.

Lorrae

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dolphinscales
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Re: Phenolphthalein Agar

Post by dolphinscales » 11 Mar 2010, 10:34

just thought i would thank all those who put onto this thread - it has been so long since we did this prac i had forgotten a lot of the hints so did a search found this thread and have got a list of great tips :)


thanks labbies for your help from the past and always the present

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lada
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Re: Phenolphthalein Agar

Post by lada » 11 Mar 2010, 11:17

Hi all,
I would also like to thank you for all the help. I was just asked to do this and I was looking for the recipe and here it is :clap3: , just in time
Lada :coffee:

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Tina
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Re: Phenolphthalein Agar

Post by Tina » 11 Mar 2010, 14:29

Hi Xenon

When you use the 10-dropper-bottle trays to set your agar, do you have a trick to getting the agar to come out nicely? Mine sticks to the plastic sides and refuses to come out, or sometimes half of it comes out and makes a big mess.
:yuck:

Thanks
Tina

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