The migration of ions during electrolysis of potassium manganate(VII)

In this experiment, students observe what happens during the electrolysis of potassium manganate(VII), with a focus on how (+) and (–) ions migrate towards electrodes of opposite charge.

Students use a glass microscope slide to support a wet strip of filter paper on which a crystal of potassium manganate(VII) is placed, producing a small amount of an ionic solution. They then apply a DC voltage across the filter paper, which causes the (+) and (–) ions in solution to move in opposite directions. Students can then observe the migration of an intensely coloured purple plume of manganate(VII) ions moving towards the positive terminal.

This experiment can be carried out in small groups of two or three students, or as a demonstration – with the aid of a camera link, if available, to project the movement that occurs on the microscope slide.

The experiment takes about ten minutes to set up and about 20–30 minutes to give observable results.

Equipment

Apparatus

• Eye protection
• Glass microscope slide
• Filter or chromatography paper, capable of being cut the same size as the slide
• Scissors
• Tweezers
• DC power supply, capable of delivering up to 40 V (see note 4 below)
• Connecting leads, fitted with a crocodile clip at one end, x2

Chemicals

• Potassium manganate(VII) (permanganate) (HARMFUL, OXIDISING, DANGEROUS FOR THE ENVIRONMENT), 1 small crystal

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Potassium manganate(VII) (HARMFUL, OXIDISING, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC081.
• A power pack delivering a maximum of 20 V will suffice, but the mobility of the ions is reduced and it takes longer for a convincing observable result to be obtained.

Procedure

1. Cut the piece of filter paper to the width of the glass slide, but slightly longer than its length.
2. Draw a pencil line down the middle of the paper to divide it into two sections lengthwise.
3. Thoroughly moisten the paper with tap water, hold it vertically to remove any drips of excess water before placing it on the slide, then fold the excess at each end over the end of the slide.
4. Using tweezers, transfer a small crystal of potassium manganate(VII) to the centre of the pencil line on the filter paper.
5. Attach the leads from 40 V DC power supply by clamping the crocodile clips over the ends of the slide and the paper, noting which end is positive.
6. Switch on the power supply and leave the apparatus for about 20–30 minutes. Write down any observations that you make.

Teaching notes

This experiment normally gives observable results within a few minutes, and after about half an hour a purple plume should be seen to be slowly spreading towards the positive terminal.

In solution potassium manganate(VII) releases mobile, negatively charged manganate(VII) ions, MnO₄^–(aq), which are attracted towards the oppositely charged positive terminal.

This experiment works particularly well because manganate(VII) ions are so intensely coloured. If potassium manganate is replaced with another coloured salt, such iron(III) chloride, the Fe³⁺(aq) ions are seen travelling towards the negative terminal, but the observations are less spectacular as the intensity of the yellow-brown colour is relatively low compared with that of the purple manganate(VII) ions.

Other resources

ChemToddler provides a video of a similar, more complicated electrolysis experiment, showing ion migration of both coloured (+) and (–) ions in a copper compound, [Cu(NH₃)₄]CrO₄.