Migration of coloured ions
Demonstrations designed to capture the student's imagination, by Colin Baker of Bedford School.
In this issue: migration of coloured ions
There are many examples in the literature of ion migration, such as copper(II) sulfate or potassium manganate(vii), either in solution or solid phase. However, to find an example where both cation and anion movement can be observed in a single compound is relatively rare.
Copper chromate is a dark green solid, which can be bought or made by mixing copper sulfate with potassium chromate. Passing an electric current through a specially prepared copper chromate solution results in the migration of two coloured ions. The green solution produces two bands of colour: blue copper cations and yellow chromate anions.
U-tube; two graphite rods with slit corks to fit the U-tube
100 V dc supply; electric cables
Retort stand, boss and clamp
Ammonia solution, 2 mol dm-3
Make the copper(II) chromate(VI) solution by dissolving solid CuCrO4 in the minimum amount of ammonia solution and then saturating with urea to increase its density. Alternatively, you can mix 100 cm3 of molar solutions of copper sulfate and potassium chromate. An orange-brown precipitate will form by double decomposition. Filter the solution through a Buchner flask and scrape the solid into a beaker containing 200 cm3 ammonia solution
(2 mol dm-3). Stir the solution using a magnetic stirrer until the solid is completely dissolved. Add urea until the solution is saturated. Clamp the U-tube to the retort stand and half fill with the ammonia solution. Using a pipette carefully add the copper chromate down the side of the U-tube beneath the ammonia. Place the graphite rods in the U-tube so that they dip into the ammonia solution and connect to the dc supply. Immediately, bubbles of gas are evolved at both electrodes (H2 at the cathode and O2 at the anode) caused by the electrolysis of water in the solution. After a few minutes a blue band forms near the cathode and a yellow band forms near the anode.
Copper and chromate salts are toxic and may be fatal if swallowed. Potassium chromate is a potential carcinogen. Contact with the eyes can cause long-term damage. Ammonia solution is corrosive and skin contact may cause burns. Concentrated solutions release dangerous amounts of ammonia vapour into the air, a hazard if inhaled. The copper chromate solution in the U-tube should be disposed of as toxic solid waste.
The trick is to ensure the copper chromate is pipetted to the bottom of the U-tube without any mixing taking place so that the ammonia remains clear and colourless.
This is an ideal demonstration to introduce electrolysis because the movement of ions towards the electrodes is clearly visible. The secondary reaction of the electrolysis of the water which also occurs can be used to discuss electrode reactions in more detail.
CrO42-(aq) + 4H2O(l) + 3e- Cr(OH)3(s) + 5OH-(aq)
Cu2+(aq) + 2e- Cu(s)
2H+(aq) + 2e- H2(g)
O2(g) + 2H2O(l) + 4e- 4OH-(aq)
The difference between electrolysis of molten or aqueous salts can be discussed and used as an introduction to industrial electrolysis, egmolten sodium chloride or brine electrolysis.
During electrolysis, either a metal or hydrogen is produced at the negative electrode as positive ions gain electrons (reduction), eg in molten sodium chloride electrolysis. At the positive electrode, non-metals such as oxygen or chlorine are evolved as negative ions lose electrons (oxidation), eg in molten sodium chloride.
During electrolysis of aqueous sodium chloride, sodium and hydrogen ions are attracted to the negative electrode. The positive hydrogen ions are reduced by electron gain to form hydrogen molecules . The positive electrode attracts hydroxide and chloride ions. The chloride ions are oxidised by electron loss to give chlorine molecules . The overall reaction is:
2NaCl(aq) + 2H2O(l) 2NaOH(aq) + H2(g) + Cl2(g)
With sixthformers, a discussion of electrode potentials and the Nernst equation could lead to an under-standing of why a particular substance is produced by electrolysis and how concentration has a part to play.