Discussion in 'Supporting Biology' started by Vitreous Humour, Nov 5, 2019.
How do you guys make these... well, cube? I have read ice cube trays. They'll do?
I just let the agar set in a large beaker 2cm depth (smear the beaker with Vaseline to ease release before adding molten agar). I can then cut 2cm cubes 1cm cubes and so on using a razor blade or scalpel.
Mine are happy with ice cube trays now but previously I used to fill a plastic tub to a 2cm mark for the 2cm ones, fill the base of a petri dish for the 1cm and the lid of petri dishes for 0.5cm. A marked out grid of 2, 1 and 0.5cm squares can be used underneath as a cutting guide.
We use an enamel tray (dissecting dish) and then use a cake blade to cut to size
I've got a 4 section gratnell tray insert with depths of 2cm, 1cm, 0.5cm marked, I set the agar in there and tip out.
Then I've nicked the blades from an old broken potato chipper - one of them is 1cm squares already, the other I pulled out every other blade to make 2cm squares. Trim the agar slabs to the size of the chipper blade, push down to cut into squares. I cut the 0.5 ones with the 1cm blade then cut in to quarters by eye with a scalpel.
That's a great idea - I've got a broken potato chipper in the cupboard!
Thanks. How long can these agar cubes last if you put them in the fridge? Will they still be as effective as if made on the day and not start diffusing?
What experiment are you making them for ? I've found that the ones I makes with bromothymol blue do tend to turn yellow (acidic), presumably by diffusion of CO2 into the agar, but only after about a week. If you're making plain agar, they should keep for a couple of weeks in the fridge. Wrap tightly to prevent them from drying out.
If they have phenolphthalein in them - they will start to clear because of CO2 in air acidifying just like JFKtech's bromothymol - you can refresh by squirting the original alkali/base over them and keeping them in a sealed vessel in the fridge. That will increase the shelf like by a week or so. Never kept them any longer but I guess you could repeat the refreshing a few times.
LOL - I am making these as we speak - just waiting for the agar to cool a bit before adding some sodium carbonate solution and phenolphthalein. I will be setting it in ice cube trays, then will cut into cubes tomorrow morning just before the lesson.
I love the idea of mangling an old chipper blade. I was going to use one anyway for the 1cm cubes, but I'l be digging out that old broken chipper too - thanks Snailo
we tried food colouring but it just faded when we cooked the agar.
Ah, I wish I could take the credit for the chipper idea, but I believe that it was JFKTech who planted the seed of this fabulous idea. I should add that I haven't tried this method out yet...
I put the indicator and carbonate in at the start with the agar, then heat it up - they aren't effected by the temperature or the agar
I find the ones with phenolphthalein in are best wrapped in clingfilm and stored in a cool place, not the fridge. I've kept them for weeks like this
Why sodium carbonate over NaOH?
Presumably because it works and is safer (not a strong alkali)
NaOH works too, never had an issue with it.
NaOH does work but I prefer Na2CO3 as the agar can be degraded by strong alkali.
I will give that a try next time I make some
this is OCRs recipe from one of their old practical tasks
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