NOTES FOR LAB STAFF:
0.5 M or 3% v/v sulphuric acid + spill tray
Hoffman voltameter + electrodes
2 x stopcocks to fit voltameter
power pack + leads
retort or voltameter stand
dissecting tray (not board) + bench roll piece
2 small & 2 large test tubes + stoppers
test tube rack or 600mL beaker
wooden splints (short AND long)
ceramic tile for hot splints
Can use 0.2 M or 0.5M sodium sulfate instead of acid. It works well and is not corrosive. Add a few drops of bromocresol green indicator to it. When current is passed through, blue colour develops at cathode (hydrogen ) and yellow at the anode (oxygen). Need to use fresh solution for each electrolysis
Check all parts (BE CAREFUL – VERY FRAGILE!), remove stopcocks and lubricate if necessary (do not re-insert yet). Carefully insert electrodes. slowly add 3%v/v sulfuric acid (or 0.5 M ) almost to top of graduations – AVOID TRAPPING AIR IN TUBING or acid will overflow. Replace stopcocks, matching colour coding if present. Close stopcocks and test operation. Use the 12V setting for best results. Include power pack, leads, lighter, splints and two or more test tubes with stoppers in a rack or beaker.
Reset between sessions:
SLOWLY open the oxygen stopcock (the one with the least gas), being careful not to let the liquid rise too quickly. Close it when the fluid has nearly reached the top, repeat for other stopcock, adjust until tubes are equal. Top up with acid if needed, close stopcocks. If stopcocks stick, remove and grease with a little petroleum jelly. If holes are blocked, clear with cotton bud.
Open stopcocks, pour fluid down sink. Remove stopcocks and electrodes. Wash all pieces carefully in hot water, using detergent if required to remove petroleum jelly. Rinse well and drain.
NOTES FOR TEACHERS:
The voltameter will be set up ready to use. DO NOT add more water or acid as the fluid in the reservoir should be level with the top of the graduated tubes before switching on. The reservoir is there to hold the fluid displaced by the gas – if you add more liquid, the reservoir will overflow!
Check that the power pack is set to 12V, then switch on. Allow 10min or so for sufficient gases to be produced â€“ check periodically as it may need longer. It is best to wait until one tube is nearly full of gas (the other one will be half-full). Small leaks may occur so the volume ratio might not be precisely 2:1, but it should be close. Turn off the power before collecting the gases.
To collect a gas, hold a test tube over the top end of one of the voltameter tubes, then slowly open the stopcock to release the gas. Be ready to close it again before the liquid spurts out the top! H2 is less dense than air, so keep the test tube open side down while capturing the gas and stopper it before turning upright. O2 is slightly denser than air – insert the flexible tubing attached to the top of the voltameter tube into an upright test tube, then the O2 will sink to the bottom. There might not be enough O2 to fill a large test tube so you may wish to use a smaller one (both sizes provided).
Use wooden splints for both gas tests. Leave hot splints on the ceramic tile until cool.
To test for H2 (from the voltameter tube that was full of gas): pop test
Light a splint and hold the flame just inside the mouth of the upended test tube (keep the stopper in until you upend it or the H2 will escape). The gas will combust with oxygen in the air beneath the tube, making a loud popping sound.
To test for oxygen (from the half-full voltameter tube): glowing splint test
Hold the test tube upright and remove the stopper. Light a long splint, then blow it out so the tip is glowing but not alight. Dip the splint deep into the tube and the O2 should reignite the flame.
To reset the voltameter:
Carefully open the stopcocks and let the fluid level in both tubes – dont let it spurt out the top! Then close the stopcocks.