Phenolphthalein agar: size and diffusion:

The original prac is in Biological Science, The Web of Life Teacher’s Guide Part 2, Exercise 10b.4
1. Approx 25g agar (increase from 18g)
4g NaOH
Dissolve NaOH in 100ml water.
Heat 900ml water in a beaker almost to boiling
Add agar and continue boiling while stirring till dissolved. Allow to cool until temperature drops below 60C then add NaOH, still stirring.
Add phenolphthalein until very deep pink – 6 droppers full.
Allow to set then cut unto 1cm, 2cm and 3cm cubes, leaving some for cylinders.
Do NOT refrigerate (it fades). Can be made 2-3 days in advance if you take it out of container and leave loosely covered with plastic wrap on glass Petri dish and store in chemical store shelf.
2. Calculate amount of agar required to make gel at 1 cm, 2 cm & 3 cm deep
Dissolve 18 g agar per litre in distilled water. Microwave in 1L beakers, with caution
Add 4 g NaOH per litre after agar solution has cooled below 60C then add – phenolphthalein and pour into containers. Can last for 2 weeks in the fridge
3. We provided kits for students:-
Kit set up in large plastic tray containing – 500 ml 0.1M Sulphuric Acid
2 pairs gloves
plastic ruler
plastic spoon
paper towel
large crystallizing dishes. These make removing the agar blocks easier than using beakers which are much deeper.
small white plastic dish for cubes of phenolphthalein agar -1 cm3, 2 cm3 & 3 cm3
To make up 500mL of jelly for the cubes, weigh out 9g of agar and 2g solid NaOH. Dissolve the NaOH in 50mL of water. Heat 400mL of water in a beaker almost to boiling, add the agar and stir until dissolved. Allow to cool then, when the temperature falls below 60C, add the NaOH solution, stirring continuously, until the jelly is deep pink. Allow to set. The agar can then be cut into blocks of the appropriate sizes.
4. 36gm agar
1600ml water, enough for 24 groups with 1 x 3cm,1 x2 cm,1 x 1cm block each
9gm NaOH in 100mls water
Phenolphthalein in dropper bottles
Make agar with water & boil till dissolved until clear solution
Dissolve NaOH in the water stirring constantly as this is exothermic
Cool agar to 60 deg C then add NaOH soln – this is important
Add phenolphthalein – a lot at first then drop by drop till agar is a deep pink colour
Pour into containers to set. You can use the rectangular “Unitray” which has square corners.
5. 1800mL boiling water + 40g Agar, stir until dissolved
Turn hot plate off, but leave stirrer on and wait until it drops to less than 60oC.
Mix 6g NaOH in 150mL water and add to above
Squirt in 200mL Phenolphthalein. Yes, 200mLs!
Pour it into plastic dropper bottle trays, the “Unitray”. Leave overnight to set and the students then cut them to sizes
6. We changed the prac because the agar tends to fade. Make the cubes ahead of time. Students place them in 0.5% potassium permanganate solution for 5 minutes then slice as per the Student Prac book. It works really well.

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