Expt:Electrolysis of potassium iodide solution
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Apparatus and chemicals
- Eye protection
- Disposable gloves
- Aluminium (or other metal) sheet approximately 25 cm x 25 cm but the size is not critical
- DC power supply (0 - 12 V)
- Leads and crocodile clips to connect to the power pack
- Potassium iodide solution (Low hazard) (0.25 mol dm-3, but the concentration is not critical) (100 cm3)
- Starch solution (20 cm3) (see Technical notes below for how to prepare)
- Sodium thiosulfate solution (Low hazard) (approximately 1 mol dm-3, but the concentration is not critical) (a few drops)
- Phenolphthalein solution (20 cm3) (Highly flammable) (see Technical notes below for how to prepare)
- Filter papers, as large as possible to fit on the aluminium sheet
Aqueous potassium iodide (Low hazard) Refer to CLEAPSS® Hazcard 47B
Sodium thiosulfate solution (Low hazard)
Phenolphthalein solution (Highly flammable) Refer to CLEAPSS® Hazcard 32 and Recipe card 36
The starch solution must be prepared shortly before use – it will not keep. It is prepared by mixing 1 g of soluble starch with a little deionised water to form a thin paste then adding to this paste 80 cm3 of boiling water. Stir the mixture, allow it to cool and dilute to 100 cm3.
To make phenolphthalein solution, dissolve 1 g solid phenolphthalein in 600 cm3 of industrial methylated spirits and make up to 1 dm3 with water.
Health & Safety
Both demonstrator and audience should wear eye protection. Wash hands after the experiment. Demonstrators with skin problems or cuts should wear disposable gloves.
A. Mix 40 cm3 of potassium iodide solution with 10 cm3 of starch solution. If the resulting solution has a blue colour (caused by contamination with iodine) add sodium thiosulfate solution dropwise until the solution becomes colourless. Then add 10 cm3 of phenolphthalein solution. If the resulting solution has a pink colour (caused by contamination with alkali) add dilute hydrochloric acid solution dropwise until the solution just becomes colourless.
B. Thoroughly moisten three sheets of filter paper in this prepared mixture and place the papers onto the aluminium sheet one on top of the other. Moistening may be done using a dropping pipette or wash bottle. Connect the aluminium sheet to the negative terminal of the power supply using a lead and crocodile clip. Connect a second lead to the positive terminal and switch on the power pack at between 6 V and 12 V. Now use the end of the positive lead to write or draw something on the top sheet of filter paper. The writing will appear blue/black, as iodine is discharged at the positive electrode and reacts with the starch to produce a blue/black complex. A corresponding pink line will appear on the lower filter paper in contact with the aluminium sheet but this will not be visible. This is caused by the discharge of H+ ions as hydrogen which leaves an excess of OH- ions in the solution. This alkaline solution turns the phenolphthalein pink.
C. Then switch the polarity of the electrodes so that the aluminium sheet becomes positive, and the free lead negative. The writing on the upper sheet of filter paper now becomes pink. (There will also be a corresponding blue/black line on the lower filter paper.) The reason for using three filter papers in a stack is that this blue/black line would be visible through a single moist filter paper and obscure the paler pink line. Some teachers may wish to draw attention to the lines underneath the filter papers, other may wish to ignore them.
If alternating current is used, a dotted line of alternating pink and blue/black is seen provided the lead is drawn over the filter paper quickly enough. This is not as spectacular as might be expected as the pink is much paler than the blue/black. Indicators other than phenolphthalein could be tried.
An aqueous solution of potassium iodide contains the following ions: K+(aq) and I-(aq) from the solute H+(aq) and OH-(aq) from dissociation of the water.
At the positive electrode
I-(aq) and OH-(aq) are attracted to the positive electrode (anode) where iodide ions are converted to iodine: 2I-(aq) – 2e- → I2(aq)
(This occurs in preference to the thermodynamically favoured 4OH-(aq) – 4e- → O2(g) + 2H2O(l) because of the much higher concentration of I-(aq) than OH-(aq)). This iodine then forms a blue/black complex with the starch.
At the negative electrode
H+(aq) and K+(aq) are attracted to the negative electrode (cathode) where hydrogen ions are converted to hydrogen: 2H+(aq) + 2e- → H2(g) (This occurs in preference to K+(aq) + e- → K(s) because the discharge potential is more positive.)
This leaves an excess of OH-(aq) ions around the cathode which turn phenolphthalein pink.
This experiment has been adapted by the RSC.
The RSC wishes to thank Dr Vladimir Volkovich formerly of Manchester University for translating an early draft of this demonstration from Russian. It also thanks Vanessa Byrne and Alison Oliver of North Leamington School for providing laboratory facilities and technical assistance. Dr Lynn Nickerson of Didcot Girls’ School trialled the experiment her Science Club, which made a number of helpful suggestions.