1.4 (j)
Contents
Class experiment 1
In this demonstration you can see ions travelling as current flows. You should be struck by how slowly they go.
Resources
- power supply, 0–30 V dc e.g. 0 – 24 V supply with large smoothing capacitor (high ripple type) across the output
- 2 demonstration digital multimeters
- 1 M ammonium hydroxide solution and teat pipette
microscope slide covered with filter paper soaked in ammonium hydroxide solution (1 M)
- small crystals of copper sulphate and potassium manganate VII (permanganate)
- 2 pins (large ones are easier to handle)
- 4 mm leads
- 2 Bulldog clips with wire soldered on (and 4mm plugs on other ends)
- stop-watch
- tweezers (plastic ones are best)
Analysis
Charges are made to flow in a strip of filter paper dampened with ammonia solution, using a potential difference connected across two metal electrodes (pins) pressed onto the paper. To see movement of charged ions, put tiny crystals of copper sulphate and potassium permanganate on the wet paper. A blob of dark blue colour slowly spreads from the copper sulphate crystal away from the positive electrode and towards the negative electrode. A blob of purple colour from the potassium permanganate crystal gradually spreads the other way.
There is colour from the copper sulphate because, with the ammonia, it forms positive cuprammonium ions (Cu+2 complexed with ammonia) which colour the solution deep blue. There is colour from the potassium permanganate because it forms negative permanganate ions (MnO4– ), which colour the solution purple. The positive ions move one way and the negative ions move the other way.
The speed of movement is remarkably slow. It is perhaps only one millimetre a minute or less. Using a larger potential difference can increase the speed, but then much more stringent safety precautions have to be taken. Even so, the speed is never great. Compared with the speed of moving charged particles, the speed with which a signal goes round a circuit to set the current going everywhere is enormous – it is near the speed of light. The speed of movement of the ions may be slow, but the current need not be small. In this demonstration it might be about 30 mA for a pd of 30 V. You can calculate the resistance of the damp filter paper:
resistance = potential difference / current:
Outcomes
1. Electric currents are made of moving charged particles. 2. Ions in a current may move very slowly.
Equipment
Class set of: micrometers and circuits.
Safety
- Wear safety spectacles
- Ensure you wear safety spectacles throughout this experiment.
- Take care with hazardous chemicals
- The chemicals in use here should not be brought into contact with the skin or ingested.
- For the high voltage extension:
- special electrophoresis call with interlocking lid
- HT power supply giving at least 150 V dc
- leads with 4mm shrouded plugs
- Wire carefully, no bare conductors, including filter paper, above 40 V
- Applicable if a high pd is used to increase the average speed of the ions
