Supporting Practical Science, D&T and Art
- in schools and colleges
A Micro-scale Hoffman-type voltameter
3-way taps
Two 5 cm3 syringes
50 mm Petri dish in
a plastic container
1M sodium (or magnesium)
sulfate(VI)
Platinum electrodes
Terminal blocks supported on
Corriflute and glued to the base
Copper wire leads
Procedure
1. Wear eye protection.
2. Set up the apparatus as shown above. Support the Petri dish on a plastic container or platform in
which there are holes so that the terminal blocks/wires can be inserted. Use Blu-Tack® (or similar)
to secure the Petri dish in the platform.
3. Place ~ 0.8 –1 M sodium sulfate(VI) solution in the Petri dish.
(Also add bromothymol blue indicator if you wish.)
4. Attach a 20 cm3 syringe to one of the the 3-way taps.
Adjust the tap and draw up sodium sulfate(VI) solution to fill the 5 cm3 syringe.
Rotate the tap so that the solution remains in the syringe.
You may need to add a
further small volume of
the sodium sulfate(VI)
solution to the Petri dish.
5. Repeat to fill the other vertical syringe.
6. Place Luer-lock caps on the taps connected to the 5 cm3 syringes.
7. Connect the copper wires to the power pack/battery and note which syringe covers (i) the positive
electrode (anode) and (ii) the negative electrode (cathode).
8. Switch on the power pack/battery and observe the relative volume ratio of the two gases produced.
ASE 01/14_DRAFT
Page 1 of 2
©CLEAPSS®, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk
Extension 1
·
Once electrolysis is completed (i.e. the syringes are each full of gas), attach a 20 cm3 syringe to the
‘anode syringe’ and, by manipulating the 3-way tap correctly, transfer the collected oxygen to the
20 cm3 syringe.
·
Repeat the above process at the ‘cathode syringe’, transferring the collected hydrogen into the
same 20 cm3 syringe. Place a Luer-lock cap on the syringe.
·
Fill a small Petri dish or bottle top with bubble mixture.
·
Wear eye and ear protection! Light a Bunsen burner placed at least 1 metre away.
·
Warn the audience (all standing at least 3 m away) to place their hands over their ears.
·
Light a splint, bubble a small volume of the hydrogen/oxygen gas mixture into the soap solution and
then quickly ignite the bubbles with the splint.
Extension 2
Place an ammeter in series to read the current and time how it takes to collect 10 cm3 of hydrogen gas.
Various calculations can be made from this depending on the topic being studied, e.g., volume of a
mole of gas.
Disposal
All the liquids can be washed down the sink into the foul water drain.
Notes
A full-size Hoffman voltameter with Pt electrodes can cost around £100 to £150. The small-scale
equipment described here uses only about 5 mm of Pt wire (costing about £10). The experimental
procedure can be projected onto a large screen.
Hoffman voltameters are usually filled with dilute sulfuric(VI) acid but the sodium sulfate(VI) solution
used here is a low hazard material and safer to use as fingers may get contaminated.
This practical activity shows that it is water that is being electrolysed at the electrodes with the
surrounding solutions turning acidic or alkaline (with the use of an indicator such as bromothymol blue).
Reaction at the 𝐜𝐚𝐭𝐡𝐨𝐝𝐞:
Reaction at the 𝐚𝐧𝐨𝐝𝐞:
4H2 O(l) + 4e− → 2H2 (g) + 4OH − (aq)
4H2 O(l) → O2 (g) + 4H + (aq) + 4e−
Sodium ions and sulfate(VI) ions are solvated by water molecules (remain in solution).
An earlier version of the small-scale Hoffman
voltameter in action.
ASE 01/14_DRAFT
Page 2 of 2
©CLEAPSS®, The Gardiner Building, Brunel Science Park, Kingston Lane, Uxbridge UB8 3PQ
Tel: 01895 251496; Fax: 01895 814372; E-mail: science@cleapss.org.uk; Web site: www.cleapss.org.uk