Phenolphthalein Agar Revisited!!

[rae]

Help!!!

I've now managed to make lovely pink agar but the reaction time with 0.1M H2SO4 was really slow like about 40 mins. Any suggestions?? Stonger H2SO4??

Thanks Lorrae

[Jazz]

Is this for diffusion in agar cell, if yes then it can be done two ways

Set-up -Mix 1 packet of unflavored gelatin with 1 cup of water, then add a few drops of phenolphthalein, and a few drops of NaOH.
Continue to add enough NaOH and phenolphthalein so that the gelatin mixture remains bright pink after swirling (1%phenolphthalein and 0.2N NaOH work fine). Soak gelatin in acid

OR

Use 20 g of agar per one L of distilled water (tap water will do) .
Heat almost to a boil, (if you have hot plate with stirrer use it), add small amounts of agar constantly stirring (you do not want agar to clump).
Remove from heat and while agar mixture cools add 10 mL of 1% phenolphthalein solution (1 g phenolphthalein in 100 mL 95% ethanol) and stir.
If mixture is pink add a few drops of dilute hydrochloric acid until pink colour disappears. Pour into a shallow tray to a depth of 3 cm and allow it to set .A tray measuring 12 cm x25 cm; 3 cm deep will accommodate one litre of agar.
Pour 200 mL of 0.1 M sodium hydroxide solution into the 400-mL beaker. Note the time and immerse the three blocks in the sodium hydroxide solution. Let them soak for few minutes with periodic gentle stirring and turning. (stronger NaOH less time).Use a spoon or tongs to remove the blocks and blot dry with a paper towel. Cut each block in half and measure the depth to which the pink colour has penetrated.

[rae]

But how long would you expect it to take to diffuse through the 5mm gel??

[Ocean Breeze]

Rae

I noticed that you said you had been using H2 SO4
I use H Cl, same conc 0.1 M/L, but its a lot quicker than that.
try using the H Cl

[rae]

Hi

Thanks for the tip I'll give it a go!!

Lorrae

[Tina]

I was just going to ask if anyone in Tamworth wanted some made up agar because I made up another batch and now it is not needed.

I used to have a lot of trouble with this but now, thanks to Liz B., it works every time .
I use:
900 ml of water boiled in the kettle, placed in a 2L beaker
add 18g of agar to the top and stir
Allow to cool to 60 degrees
While it is cooling,
add 4g NaOH to 100 ml of water and stir to dissolve
and add a small amount (1g?) of phenophthalein powder to about 10-20ml of ethanol

when it is at 60 degrees
add NaOH and phenophthalein solutions to the agar solution and stir
Pour into a plastic dish or dishes, depending on how you want to get cubes of the right depth
Allow to set for at least 6 hours in the fridge.
Will keep for up to 2 weeks in fridge

Use disposable spoons to scoop cubes of agar (we use 1cm, and 2cm cubes) into 0.1M HCl
Takes about 15 minutes for the colour to disappear.

Did the 0.1 M HCl work for you?
Good luck
Tina

[RosalieM]

We did it the other way around. I made the agar in the usual way, and added Phenolphthalein to it. After it set, it was cut into 1, 2, 3cm cubes and put in 0.1M NaOH. Changes purple instantly. It was left for 15-20 minutes and then the cubes were cut in half to see how far it had penetrated. The kids thought it was great! Beautiful clear square on the inside with about 5mm purple on the outside.

[rae]

Hi All,
Tried the HCl yesterday and was OK.
I then made up new gel as per "The Laboratory" but when it was set I added more phenolphthalein on top. This absorbed into the gel and turned it dark pink. I then left it at room temp overnight. The excess phenolphthalein was tipped off. We used the 0.1M H2SO4 and it took 15mins for the 5mm gel to diffuse totally. So all in all very successful!!

Lorrae

[Mother]

I have just realised a booboo with making the agar with Nutrient Agar instead of Agar gel.In all the years I have been making it, this is the first time I messed up. What I wanted to know is will the prac.still work? Surface Area to Volume Ratio is the experiment.
Cheers
Mother

[dime]

Can't imagine why it wouldn't work. Just be a bit smelly - beefy I suppose.

[lada]

As long as you don't have it sitting outside fridge and grow bacteria on it.
If you use it soon, acid will kill everything anyway, ha ha .
Lada

[Graham Kemp]

It should work if you can avoid having anything grow on it.

A disinfectant or antiseptic wash or spray might help dissuade any growth.

Keep it cool, use it soon, dispose of it quickly afterwards.

[Mother]

Hi all.
Just wanted to let you know that my mistake of using Nutrient Agar for Phenophthalein Jelly did work the same as Agar Agar. Expensive way to do it though, especially if the teachers don't use it all and particulary if you buy the Nutrient Agar Powder in !!!!!!!
Have a good day
Mother

[superbug]

Hi All,

I was wondering how you cut the agar 2cmx2cm, 1cmx1cm and .5cmx.5cm cubed. Is there a special technique as mine seems too crack or is never the right size when I cut them with a scalpel.
Any help would be greatly appreciated.

Thanks Superbug

[Labbie]

I have merged post hope this helps????????????

[Narelle01]

I send mine out uncut....teacher or students cut it themselves - less worry for me

[lada]

Same as Narelle. Let teachers blame kids, if not cut exactly like they want it.
L

[JudyM]

Try using a single sided razor blade to trim to size.

[mtg]

I make it in trays of the required thicknesses and then just a sharp knife or razor to cut from there. Most of the time the students do that cutting.

[Graham Kemp]

I use dropping bottle trays. They divide into approximate 3cm x 3cm squares which makes easy to handle blocks. Pour in to the required depth, set, then cut down to size.