Australia's Science Forum

I've been asked to get some agar-phenopthalein jelly. I'm guessing they mean agar with phenopthalein in it but i'm just looking for a recipie.

btw, this teacher never says what they mean and when asked about it they never have enough time to explain but still mannage to have the time to complain about how they don't have enough time...lol

If you do a search for Phenolphthalein you will find some very interesting discussions about this topic.

You could also check out the fantastic book "The Laboratory" it has a recipe most people seem to use.

Lorrae

Just add 5-6 drops of phenopthalein to your agar as your making it up. If you want to check it pour a little into a watch glass then add 0.1m NaOH. Good luck

The idea of the experiment is to show differance of volumn to surface area.
What happens is you make up some agar with Phenolpthalien and sodium hydroxide till it's bright red.
It is then cut into 1, 2, & 3 cm cubes and placed in about 0.5 or 1M H Cl and watch it fade from red to clear as the acid penetrates the agar.

I might suggest to try it that way next time to the teachers, we do it different, we don't add the NaOH to the agar. The agar is cut into different size cubes then put into 0.1M to 0.5M NaOh and see how far the pink penetrates. Thanks Ocker

That would work as well, but we do it with the pink phenolphthalein and HCl.
lada

We mix the NaOH into the jelly too, because a layer of clear agar around the pink is easier to see than a layer of pink around clear jelly.

Ricepaper,
Your teachers should be providing you with copies of the practicals that you have to make preparations for as well as notification of the number of groups doing the prac and when the pracs are required. With this information in hand it makes preparation much easier. As Rae suggests you do need a copy of "The Laboratory" by B. Dungey. The best place to get this is probably Southern Biological. It's a must have item for assisting you in your job.
Lyn.

Hi .The recipe I have for this prac is as follows:
To make up 500ml of jelly weigh out about 15gm of agar & 2gm of NaOH. Dissolve the NaOH in some water .Heat about 400ml of water almost to boiling, add the softened agar ( I add some water to the weighed out agar to make a paste that is easier to wash into the hot water than adding the straight powder ) & stir until dissolved.Allow to cool ( below 60 celcius ) & then add dissolved NaOH stirring continuosly.Add phenolpthalein ( either solution or powder )until a nice dark pink colour forms. Allow to cool & set.
With this prac I usually make up three lots of the above & then pour them still warm into a 4l ice cream container & allow it to set in that in the fridge.I find it gives a nice big block of agar to work with.
How much you make up I imagine will depend on the number of senior Bio classes. We only have one so three lots for 8 groups is more than enough.
Hope this is of help.Regards.Andrea

This is the recipe I compiled after experience and is used for surface Area:Volume experiment.
It works well.
---------------------------------
Procedure:
Get the required number of containers (e.g. takeaway containers) for agar jelly to be prepared.

Calculation:

1. Fill the takeaway container (which we want to use for prepararation of jelly) with water 3 cm deep ( to measure the required quantity)
2. check size of cotainer with 3 cm water in it. generally it can be approx. 400-450 ml volume in for takeaway container.
3. if the volume is 450 ml then make it to approx. 500 ml for each container
4. calculate total quantity of water required for the number containers (volume rewuired for each container * total containers to be prepared)
5. calculate quantity of agar powder by considering each litre requires approx. 12 gms of plain agar powder.
6. calculate quantity of 1 M sodium hydroxide for the total quantity of agar solution. Each litre of agar solution requires about 25 ml of 1 M sodium hydroxide solution
--------------------------

Method:

1. Take required quantity of water in a beaker or (two-three beakers as required if you do not have big enough beaker).
2. Allow water of required quantity to boil
3. Add the required quantity of agar powder in the boiling water VERY SLOWLY with constant stirring so as not to form the lumps. Let the powder completely dissolve as far as possible.
4. Allow the agar solution to cool to less than 70 deg C with stirring
5. Filter the solution with Sieve/Strainer (to remove agar lumps if any in the solution).
6.If more than one beaker, mix all beakers with filtering to make all the agar solution in all beakers of uniform concentration.
7. Add into this agar solution very small qunatity of phenolphthalein powder (only 0.1 gm per litre is enough)
(Thanks to suggetion by Science Assistant in One of ACT College) which worked well
8. Add the required quantity of 1 M sodium hydroxide solution slowly with constant stirring and you should get pink colour.
9. stir it well to get uniform pink colour
8. if there are more than one beaker then mix all the beakers with each other to make concentration and colour uniform in all the beakers.
10. when temperature of the pink coloured agar solution in the beaker reaches approx. 60 deg C filter and pour it into the plastic containers for setting them.
11 Allow solution to reach room temperature and keep it in fridge until the actual use time.

Please note that the prepared Jelly contains 1 M sodium hydroxide solution that means jelly should be handled carefully by students during experiment.

I hope it helps.

Cheers

NT

I've always made my cubes from agar, but I'm wondering if I can make them from Gelatine instead.

Thanks

Vicki

No it has to be agar. I tried it many years ago, as agar is expensive and had lots of gelatine. Didn't work.

I make up 2L icecream tubs of the stuff. I cut each tub into 6 pieces & give a piece to each prac group.

To make 4L of phelphthalein NaOH agar (2 x 2L icream containers)

In a conical flask, pour 250mL of water, add 10 g of NaOH granules & stir till dissolved. Leave aside to cool.

Pour 1600mL into a 2000mL beaker.
Place on hotplate/magnetic stirrer & add large stirring bar.
Heat on high till water just begins to gently simmer, turn stirrer on high & gradually add 100g plain agar till emulsified.
Continue heating & stirring till smooth consistency.

Set two clean 2L icecream tubs on bench.
Pour half of the agar into each icecream tub.
(should be approx 800mL each)
Add 400mL cold water to each tub & stir till thoroughly mixed.
(helps agar set faster & produces firm blocks)

Leave to cool for 15 minutes, then add 125mL NaOH solution to each tub.
Stir till thoroughly mixed.

Add 80mL of 0.5% phenolphthalein (in ethanol) to each tub.
Stir till completely mixed in & agar is a deep pink colour.

Leave tubs to cool, preferably overnight.

Agar will keep well in tubs for several weeks (seal lids on well).

Hello people with lots of knowledge!
On this topic slightly different question. Today I happen to take note on the cost of agar from Qstores ($200+ for 100grams).
Shocked is not the word for it.......
Anyway i immediately went and discussed this with HOD, he was amazed to,My dept all these years have been able to order as many plates as they want as we have never had to order agar as we had a good supply-till now.(down to 1kg)
Anyway was thinking can we substitute gelatine or something for plates for junior classes?????
For cost saving.
Or if anyone knows how to buy agar powder cheap.Let us know your secret
Trish.
ps. just searched google and some mention guar gum or xanthan gum any one tried these substitutes??????

Lyn, you live in an ideal world. i cannot remember, when was the last time a teacher gave me copy of practical, usually is
The thing with pink cubes, or agar cubes with acid or base???
We must have a ESP. Yesterday a teacher asked for a prac that did not order, insisting she MENT to order it. I did say, my magic power does not extend to ESP
Lada

Trish, I know agar is expensive, for this phenolphthalein exp you need normal agar not nutrient. It might be a bit cheaper.
lada

Hi Trish

No you cannot substitue gelatine for agar. Gelatine has a melting point at around room temperature. Trust me the plates end up a sloppy stinking mess. Go with agar every time.
You can get it from Silform for $78.00 or crown scientific for $73.70. I guess thats why its so expensive because there is no substitute.

oh my goodness petal.
You can buy agar powder from science supply australia for $72 for 500g and $29 for 100g.If you need nutreint agar we just add vegimite to the normal agar powder
M.

Hi Trish - I'm wondering if you looked up the wrong Agar? The nutrient agar is about $250 for 500g and there is an agar malt extract one for about $85 for 100g. The "agar agar gel (powder)" (which I don't know what is different about it to the others) is the expensive one at over $200 for 100g. We use the nutrient agar which only needs 26g to make up 1L which is enough to make about 40 plates I think so I can't imagine how you could go through 1kg quickly - you must go through a lot of plates!! I'm glad it's not me making them all up! We use the cheap agar (which we got in 500g bottle from southern bio - can't remember the price, but check them out. I think they were cheaper than Q-Stores for the nutrient one too but I'm not sure) for the phenolphthalein agar blocks as it doesn't contain the nutrients to grow anything. Hope this helps

I had ordered the agar from Q stores (the $ 200 odd dollars is actually for a carton of 6 bottles, if you look under purchasing unit it has carton and we have got it from there before) HOWEVER we got and note back from them that they no longer carry that item anyway. I last purchased it in 2005 when the 6 jars cost $172. I'm now onto the last bottle so needed to restock

I had been going to write and ask if anyone knew where else to get Agar at a reasonable price but my questions have now been answered. Think I'll go the 500gm for $72 as that works out half the price if the 100gm jars. I go through a carton of 500 petri dishes each year making up my plates so that is a lot of agar!

Wendy