Australia's Science Forum

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!

I keep no living things in my labs....

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.

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!

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

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.

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

I put mine in ice cube trays, then the kids cut up.

So far, so good - it went a beautiful colour!

Now to wait and see if it sets...

Bugger!! It didn't set. Will it set if I heat & add more agar? Or should I cut my losses & start from scratch??

Mel

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

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 , 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.
Lisa

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

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

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.

I reheated it, and already it seems to be setting! Big improvement on yesterday's effort. I don't think I heated it enough before adding the agar, but at least that is 1 mistake I won't be making again! I'm too impatient for my own good sometimes...

Thanks everyone for your input, very much appreciated.

have just made ours and made the 1cm ones in the tray? that glue sticks come in. Has nice small round holes. The 2cm ones were made in an ice cube tray and the 3cm ones in a lunch box. I have made models of the sizes with plastic centicubes and use these to check size and depth. Not very scientific but it works well and the students can cut to size easier. cheers bella

I use dropper bottle trays which have removable partitians to get approximately 3 cm sided blocks that the students can easily cut down to size.


Also, I store the agar blocks in sealed tupaware containers to prevent drying out before the prac. They can be stored that way for considerable time.

nickykinz wrote: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.

There's no safety risk beyond spills and splashes of a hot corrosive solution. Do handle carefully.

However, as you've noticed, there is some decomposition of the agar agar when boiled while basic. The resulting maroon agar agar can still be used.

The phth agar can even be recycled this way several times, but eventually the colour change will become less and less noticable.

Besides which, the teachers do prefer the nice rose pink of the first iteration.

Yay!! It has set already, next time I won't be so impatient & I will wait for it to boil before adding the agar... thanks again for all your help everyone!

I use boiling water out of the kettle. I put a bit of cold water in with the agar and make a solutio, add boiling and bring it back to the boil all on a magnetic stirrer/hotplate. Works everytime.