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Re: Phenolphthalein Agar

Posted: 19 May 2011, 14:24
by Narelle01
i will also not be sad to see the end of this prac!!!

Re: Phenolphthalein Agar

Posted: 23 May 2012, 11:37
by curie
Made some yesterday, should have checked this thread first. It was originally quite pink but has faded considerably. I will try the layer of phenolphthalein on the top trick, if that doesn't work its back to the drawing board.

Re: Phenolphthalein Agar

Posted: 23 May 2012, 11:39
by Narelle01
paint it with Na Oh - that should turn it pink quick!

Re: Phenolphthalein Agar

Posted: 23 May 2012, 12:27
by DavidPeterson
I love this prac - one of the few Biology pracs that actually works (probably because it relies on Chemistry - which always works!)...spoken by a true chemist........

Re: Phenolphthalein Agar

Posted: 23 May 2012, 13:34
by bindibadgi
Oh David, I say that just means Chemistry's not as much fun as Biology coz it's too predictable :giggle:

I reckon the more organic & less predictable biology pracs are cool....even if they don't always work as expected :crazy: .....& do Chem classes get to play with anything living? Nup.....but we biologists get to have lessons with stick insects, bearded dragons, bunnies, all sorts of plant life, gorgeous microscopic critters....& we get to make capsicum soup & blow lungs up with a bicycle pump! :w00t: ....spoken by a true biologist :wink2:

Re: Phenolphthalein Agar

Posted: 23 May 2012, 13:50
by sunray18
and we get to wash up all those goey messes that students make and teachers put out of sight so they go right 'off' before we find them!

Re: Phenolphthalein Agar

Posted: 23 May 2012, 14:15
by DavidPeterson
I keep no living things in my labs....

Re: Phenolphthalein Agar

Posted: 23 May 2012, 14:56
by Graham Kemp
Narelle01 wrote:paint it with Na Oh - that should turn it pink quick!
Well, unless the indicator is fading from being in a pH too far above 9. "Add more" won't cure "too much"!

I always prepare by adding NaOH slowly in small portions, until just passed the color change; rather like a crude titration. The excess in the fridge hasn't faded in the months since the last time.

Re: Phenolphthalein Agar

Posted: 24 May 2012, 08:41
by Narelle01
are you a chemist too Graham? ;)
I'm a bucket chemist taught by a master bucket chemist, Noona :p

I'm more a biologist - i love 'my guys' love my LIVING Euglena!

Re: Phenolphthalein Agar

Posted: 24 May 2012, 09:23
by lada
I have adopted a recipe from this website a long time ago and it is fool proof.
20g of agar, boil in 1L of water. Cool to about 50oC, add 4g NaOH and enough phenolphthalein powder to colout the solution. I use a tip of spatula.Always works,
Happy days
Lada

Re: Phenolphthalein Agar

Posted: 04 Sep 2012, 15:20
by carolmay
You all have some wonderful ideas of making Phenolphthalein Agar, will have to give it a try, I use a large tray at 3cm depth, but the kids have to do the rest.

Re: Phenolphthalein Agar

Posted: 03 Jun 2013, 11:56
by melsid
Hi all,

wish me luck, I'm about to make my 1st batch... thanks for all the hints & tips, I'm sure it will be fine :D

Re: Phenolphthalein Agar

Posted: 03 Jun 2013, 13:06
by macca
I put mine in ice cube trays, then the kids cut up. :crazy:

Re: Phenolphthalein Agar

Posted: 03 Jun 2013, 14:56
by melsid
So far, so good - it went a beautiful colour! :clap3:

Now to wait and see if it sets...

Re: Phenolphthalein Agar

Posted: 04 Jun 2013, 09:25
by melsid
Bugger!! It didn't set. Will it set if I heat & add more agar? Or should I cut my losses & start from scratch??

Mel

Re: Phenolphthalein Agar

Posted: 04 Jun 2013, 09:38
by MariaC
Ive never had problems with this prac, is your agar old? and do you use a thermometer to check the temp when it boils? I wait til mine gets to around 96 degrees, keeping a lid over it whilst stirring helps otherwise it takes forever to really get hot. Also make sure it is cool enough before adding NaOH I'm always itching to add mine but again use the thermometer so I dont jump the gun. Just a few thoughts. Cheers MariaC

Re: Phenolphthalein Agar

Posted: 04 Jun 2013, 09:46
by Lis
Mel, make sure you added enough agar (10g/500ml) and then bring it to the boil. I have found if I dont boil the agar it wont set properly either. I am a little impatient like Maria :giggle: , but I have not had a problem setting Phenolphthalein agar, just the plates, but you have to stir it while it is boiling and watch it doesnt boil over.
:wink2: Lisa :thumbup:

Re: Phenolphthalein Agar

Posted: 04 Jun 2013, 09:50
by lada
I make mine 3%, so 3g in 100mL or 15g in 500mL, just to be on the safe side. It sets nice and firm, easy to cut.
Also, I would start from scratch.
Lada

Re: Phenolphthalein Agar

Posted: 04 Jun 2013, 10:38
by sunray18
I use a microwave for making agar - for plates, for blocks , for electrophoresis gel.
Just make sure you have it in a narrow neck container and place that container in a larger bowl in case it boils over

Re: Phenolphthalein Agar

Posted: 04 Jun 2013, 10:40
by nickykinz
I have reboiled phenolphthalein agar that hasn't set and it has then set really well. You need to boil it though and I'm not sure if there are any risks with doing that once it has the NaOH and phenolphthalein in it. I did it in the microwave. It goes a bit darker like a (mighty!) maroon sort of colour. But that works well for the prac.