Topic 3 · page 4 of 15

Migration of ions

Coloured ions move towards the electrode of opposite charge. Two practicals show it, potassium manganate(VII) on filter paper and copper(II) chromate(VI) in a U-tube. Learn the observation and the reason for each.

10 min read3.1

Read first: Evidence for ions

Why a colour can show a charge 3.1

In a d.c. electric field, a positive ion is attracted to the negative electrode and a negative ion is attracted to the positive electrode. If the ion has a colour, we can watch it move. Where the colour goes tells us the sign of the charge on the ion.

Movement of coloured particles towards charged electrodes is evidence that the particles are charged. This is the third piece of evidence for ions in 3.1. Ions must be free to move for this to work, so the compound has to be dissolved in water. This is why the crystal in the first practical sits on damp filter paper.

Practical 1: potassium manganate(VII) on filter paper 3.1

PracticalPotassium manganate(VII) on filter paper

Method

  1. Lay a piece of filter paper on a microscope slide and moisten it with an electrolyte.
  2. Place a small crystal of potassium manganate(VII) in the middle of the paper.
  3. Connect a d.c. supply across the two ends of the paper, using a low voltage.
  4. Switch on and watch the purple colour for a few minutes.

Safety. Potassium manganate(VII) is an oxidiser and it stains the skin. Use a low-voltage supply and wear eye protection.

A microscope slide with filter paper, a purple crystal at the centre, and electrodes at each end. After a few minutes the purple spreads towards the positive electrode.
Migration on a slide. The purple MnO₄⁻ ions move towards the positive electrode.

Observation: the purple colour spreads towards the positive electrode.

Explanation: the purple colour is due to manganate(VII) ions, MnO₄⁻. They are negative, so they are attracted to the positive electrode. The K⁺ ions move the other way, but they are colourless, so we cannot see them.

Practical 2: copper(II) chromate(VI) in a U-tube 3.1

This version has a mixture of two coloured ions in one solution, so it shows a positive ion and a negative ion moving in the same experiment.

PracticalCopper(II) chromate(VI) in a U-tube

Method

  1. Fill a U-tube with green copper(II) chromate(VI) solution, in both arms, with nothing else in the tube.
  2. Put an electrode in each arm and connect a d.c. supply, using a low voltage.
  3. Switch on and leave it for some time.

Safety. Chromate(VI) compounds are toxic and a health hazard. Wear gloves and eye protection and use a low-voltage supply.

Two U-tubes. At the start the whole tube is green. After some time the arm at the negative electrode is blue, the arm at the positive electrode is yellow, and the middle is still green.
Migration in a U-tube. Cu²⁺ ions (blue) move to the negative electrode and CrO₄²⁻ ions (yellow) move to the positive electrode.
Where Colour Ion responsible Charge on ion
Near the negative electrode Blue Cu²⁺ positive
Near the positive electrode Yellow CrO₄²⁻ negative
Between Green, as at the start both ions still mixed

The green colour is the combined colour of the blue and yellow ions. It separates because the two ions travel in opposite directions.

Check yourself

  1. A purple crystal of potassium manganate(VII) is placed on damp filter paper with a d.c. supply across it. State what is observed and explain it.
  2. Why is the filter paper damp, and why is the colour not seen to move if the same crystal is on dry paper?
  3. In the U-tube experiment, state the colour seen near each electrode at the end and name the ion responsible.
  4. A student says: "the green separates into blue and yellow because copper atoms and chromate atoms move to different electrodes". Give two corrections.
  5. In the slide experiment the potassium ions also move. In which direction, and why can we not see them?
  6. Why does migration of coloured ions count as evidence that ionic compounds contain ions?

Answers cover these until you have tried the questions

  1. The purple colour spreads towards the positive electrode. The purple colour is due to MnO₄⁻ ions. These are negatively charged, so they are attracted to the positive electrode.
  2. The water dissolves the crystal, so that the ions are free to move through the paper. On dry paper the ions are held in fixed positions in the lattice and cannot move, so no movement is seen.
  3. Near the negative electrode the colour is blue, from Cu²⁺ ions. Near the positive electrode the colour is yellow, from CrO₄²⁻ ions. The middle is still green.
  4. First, the particles that move are ions (Cu²⁺ and CrO₄²⁻), not atoms. Second, they move to different electrodes because they have opposite charges: Cu²⁺ is positive and goes to the negative electrode, and CrO₄²⁻ is negative and goes to the positive electrode. (A third point: the green does not vanish, it remains in the middle.)
  5. K⁺ ions are positive, so they move towards the negative electrode, the opposite way to the purple ions. They are colourless, so there is nothing to see.
  6. Particles that are attracted to an electrode of the opposite charge must be charged. The compound therefore contains charged particles, which are free to move when dissolved, and this is the model of an ionic compound.