Agar, diffusion and indicators
There are times when Agar features quite predominantly in the biology curriculum, especially when it is the ‘bacteria time’ of the year: pouring plates, preparing cultures, autoclaving, spreading, incubating – it is a full-on time where all you see, and smell is warm agar. But there is more to life than just nutrient agar!
It was discovered in the 1600s in Japan, supposedly by a Japanese innkeeper who noticed the discarded seaweed soup he threw out had set overnight.
It is a versatile product; first used as a gelling agent in Asian cuisine, to being identified as a good alternative to gelatin as a growing medium for bacteria in microbiology (because of its higher melting point).
It is also the base gel agent in electrophoresis applications (agarose gels) and is used in plant biology as a nutrient-growing base.
Agar can accept nutrients without them affecting its properties and itself won't interfere with what is added to it, it is also virtually tasteless, and you may have seen agar taste balls (agar jelly balls injected with a flavour) popping up in fancy restaurants (usually with Heston Blumenthal’s name behind them!).
With such a versatile product now purified and packaged for different applications, with the right amount of additives in each one to create a consistent working product it can be hard to identify which form of agar you need for the job in hand.
Agar for diffusion experiments.
Agar blocks are a really useful way to show diffusion through a substance. If you colour the blocks with an acid/base indicator that will change colour when mixed with the acid or base, the block will change colour as the acid or base moves (diffuses) through it.
You can adapt this lesson to look at the effect of surface area on diffusion, simply by supplying different sized blocks of coloured agar (1cm3, 2cm3, 3cm3 etc) and timing how long it takes the colour to change.
Equipment:
- Plain agar (technical agar/agar agar) 20 - 25 g
- Distilled or deionised water
- Indicator solution 30 mL (see note below)
- Sodium hydroxide solution, 0.01 moldm-3 30 mL (see note below)
- Balance and weigh boat, spatula, stirring rod, pipette
- Hotplate stirrer, heatproof gloves and goggles
- Container to hold the agar whilst setting
Method:
- Weigh out 20 - 25 g (depending on the firmness of agar you want) of plain agar powder into a large beaker or flask.
- Make up to 1 litre with distilled/deionised water.
- Stir, with heating, to mix and eventually the solution will become clear. This may take some time.
- Allow to cool slightly (to around 50°C), add the sodium hydroxide solution and the indicator solution and mix.
- Pour into the desired container and leave to set. Agar will set at around 40°C.
- Once set, the coloured agar can be cut into blocks ready for the class
Alternative method:
- Make the agar up in water and add an indicator (30 mL) and sodium carbonate solution (30 mL 0.5 moldm-3) when the mix has cooled to around 50°C. you will still need 0.1M hydrochloric acid for the experiment
Diffusion in agar blocks class practical:
Equipment (per group):
- Coloured agar block
- Beaker (100 mL)
- 0.1M HCl (aq)
- Pipette, and blunt forceps
- Timer
- Gloves and goggles
Method
- Add approx. 50 mL HCl to the beaker (enough to cover the block)
- Add one agar block to the beaker (wear gloves if not using forceps)
- Start the timer
- Stop the timer when the block has changed colour (colour change will depend on the indicator used when the blocks were made), and note the time.
- Remove the block, reset the timer, and repeat steps 2-4 for twice more for blocks of the same size, to get an average time for the colour change to occur
- Repeat steps 1-5 if using different sized blocks, to investigate the effect of surface area on diffusion
- Wash hands after the practical
Indicators:
All agar, once made, is best stored in the fridge.
For more information on indicators and diffusion experiments with agar see CLEAPSS/SSERC
