5 minutes reading time (1062 words)

    Movement of Ions During Electrolysis


    A petri dish containing sodium sulfate solution and bromocresolpurple indicator (yellow in acid, blue in alkali) is placed on an overhead projector. The solution is electrolysed and a blue colour streaming from the cathode shows the movement of hydroxide ions. These can be deflected by a magnetic field and shown to obey the left hand motor rule.


    • Petri dish
    • Power pack, 0-12V
    • Two leads with crocodile clips.
    • A flat "magnadur" type magnet.
    • A ring-shaped ceramic magnet such as one recovered from an old loudspeaker (optional). Ideally the ring magnet should be the same external diameter as the petri dish, but smaller ones can be used.
    • An overhead projector and screen.


    Anhydrous sodium sulfate, 7g
    A few drops of bromocresol purple indicator solution (approximately 1% solution in ethanol, note that this is more concentrated than the normal indicator solution).
    A few cm2 of aluminium foil.
    A little dilute acid.

    Before the demonstration

    Dissolve about 7 g of sodium sulfate in 100 mL of water. Add several drops of bromocresol purple solution to give a deep blue colour. Add the minimum amount (no more than a drop or two) of dilute acid just to turn the indicator yellow. Make sure you know the polarities of the faces of the magnets.

    The demonstration

    Stand the petri dish on the centre of the stage and focus the OHP. Clip the crocodile clips to the rim of the petri dish on opposite sides. They should dip into the solution forming electrodes. Connect the crocodile clips to the 12V DC terminals of the power pack and switch on. Bubbles of hydrogen will be seen at the cathode and a blue colour will spread from the cathode. This is caused by OH-(aq) ions which remain in the solution and are repelled from the cathode after the discharge of H+(aq).

    Now hold one of the flat pole faces of the magnet above the petri dish, pole face horizontal, so that the magnetic field passes vertically through the petri dish. Take care not to obscure the audience's view - tweezers are useful. Observe the movement of the coloured stream to the left or right (depending on the direction of the magnetic field). Turn the magnet over and show that the stream now moves in the opposite direction (Fig. 2). Confirm that the deflection is in the direction predicted by the left hand motor rule remembering that the ions are negatively charged so that conventionally, current flows in the opposite direction to that in which the ions are moving.

    Movement of ions in a magnetic field
    Alternatively, stand the petri dish over a ring magnet so that the cathode can be seen through the centre of the ring (Fig. 3). The deflection can be seen more easily without being obscured by the magnet or the demonstrator's hand. The stream of moving negative ions is in fact deflected into a circular path, as theory predicts.
    Movement of ions in a ring magnet

     A second possibility is to make a ring-shaped anode from a strip of aluminium foil placed around the inside of the rim of the petri dish. Stand the dish on the OHP above the ring magnet so that the dish and the magnet are concentric. Connect the aluminium foil anode to the positive terminal of the power pack with a lead and crocodile clip.

    Connect a second lead and crocodile clip to the negative terminal to act as the cathode and dip this crocodile clip into the solution in the centre of the petri dish (Fig. 4). The stream of indicator will travel outwards from the cathode in a spiral path in the direction predicted by the left hand motor rule.
    Aluminium foil ring electrode

     Visual tips

    Adjust the amount of indicator so that the colour is seen easily when projected onto the screen. Use as little acid as possible to turn the indicator yellow. This will result in a darker blue colour when OH-(aq) ions are formed. Clean the OHP thoroughly and focus it carefully. Make sure any currents in the water caused by stirring have stopped before staging the demonstration. Method two is much better for visibility and should be used if a ring magnet is available.

    Teaching tips

    Students, and perhaps some teachers, will need to be reminded of the left hand motor rule (Fig. 5). Stress that the deflection of the ions is to the left or right i.e. at right angles to the magnetic field. The ions are not being attracted towards the pole of the magnet as a piece of iron would be, so the ions are not 'magnetic'.
    Students may not be familiar with bromocresol purple and its colour changes. This indicator has been selected because it gives a good contrast on the OHP but others can be used if desired.

    If appropriate, point out to students that this experiment is similar to the situation in a mass spectrometer except that there the ions are positive (and in the gas phase).

    The left hand motor rule
    Students could be asked to predict the direction of deflection of the ions before it is demonstrated.

    In some trials teachers reported that they used the demonstration simply to show the movement of ions during electrolysis and omitted the magnetic deflection.


    A stream of moving anions is equivalent to a conventional (positively charged) current in the opposite direction. A charged particle moving in a uniform magnetic field is deflected in a circle as the force on it is always at right angles to its direction of motion.
    At the cathode, H+(aq) ions are discharged:

    2H2O(l) + 2e- → H2(g) + 2OH- (g)

    leaving behind OH-(aq) ions so the solution around the cathode becomes alkaline. The OH-(aq) ions are repelled from the cathode.


    Other indicators could be used eg phenolphthalein. Other electrolytes can be used and the nature and shape of the electrodes can be changed. The voltage can be varied as can the concentration of the solution. All of these extensions could be done as student projects.


    Wear eye protection.
    Take care not to spill water onto the OHP's electrics. The solution can be rinsed down the sink after use.
    It is the responsibility of teachers doing this demonstration to carry out an appropriate risk assessment.


    This demonstration is based on ideas from Colin Johnson at Techniquest, Cardiff, and Brian Gray at the University of the Western Cape, South Africa.

    The "Breathalyser" Reaction


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    Wednesday, 21 April 2021

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