1. Weigh aspirin (0.25 g) and add to 100 cm3 of distilled water in a beaker. Warm on a hotplate to > 80?C and stir until the aspirin has dissolved.
2. Add agar (2.0 g) gradually, trying to avoid forming lumps as much as possible. Heat the mixture until the agar has dispersed and you have an almost clear solution, if the agar has not fully dispersed you will not get a gel on cooling (BE CAREFUL NOT TO OVERHEAT AS AGAR SOLUTION HAS A TENDENCY TO BOIL OVER RAPIDLY).
3. Leave to cool using an ice bath. You should be able to invert the beaker without the agar gel falling out. Whilst waiting do step 4.
4. Set up the conductivity cell. Place the electrode in distilled water and record the value of conductance (see the explanation * below). The meter will probably give a reading in ?S but when you come to do your measurements it may change to mS. (1mS = 103 ?S). This is how you will take measurements later on. Note that the electrode also records the temperature of the solution so record this as well.
5. Using a hollow steel cylinder (actually a cork borer) provided, make a cylinder of gel and place it on a filter paper. Cut the cylinder so that it weighs approximately 4.0g (but record the exact weight) and place it in a dry 250 cm3 beaker and add distilled water (100 cm3) using a measuring cylinder. Start the clock
6. Leave to stand and after about 45 seconds, gently swirl the beaker and contents for 10 seconds. Place the conductivity electrode in the water, keeping it away from the agar gel (to avoid damaging the latter) and leave it there until the reading on the meter stabilises. Note the reading and the time (on the stopclock) at which the reading was taken. Take the electrode out and repeat this procedure about once every two minutes. The times do not have to be perfectly spaced. The most important thing is that you are consistent and do a 10 s swirl just before putting the electrode in. If the agar breaks then you might get a sudden burst in conductivity…