Electrolysis experiments provide indirect evidence that charged particles have moved,
but there's real value in closing the loop and actually seeing the ions migrating
through the circuit, especially when it's not as quickly as you might otherwise have expected.
When you turn on a light switch or dip some electrodes into solution, it can seem as if the
charge carriers have arrived instantaneously. The reality, of course, is far from the truth.
One traditional way of showing the ions themselves on the move involves the use of copper chromate
in an ammonia urea electrolyte. This can be fiddly to set up and requires either a lot of
patience for the payoff or higher voltages than might be available in a classroom power supply.
Adolf Cortel's modified version works a little bit faster, is simpler and has the bonus of using
less of the chromium(VI) that we're using for the colored negative ions. Once the solutions
are made up, and as ever instructions are on the EiC website, the demo is ready to go in seconds.
I have some chromatography paper here draped over two Petri dishes into which I've poured some
ammonium chloride in ammonia as the electrolyte. Adolf suggests concentrated ammonia, but CLEAPSS
have revised this to 2 M, which works just as well with fewer associated hazards.
Because some ammonia will vaporise, it's important to keep the room well ventilated.
I have this set-up connected to a classroom power supply via steel paper clips on each side.
I'm going to use a dropping pipette to mark a fine line in the middle of the paper. A capillary tube
will give even nicer results if you have the time to build up a line with that instead.
First check that the paper is moistened with the electrolyte throughout. If you drop any extra electrolyte
directly at the top here, do so sparingly to avoid smudging the line that you're about to draw.
We'll set the power supply to 20 volts and let it do its thing. The first thing you'll notice
is, well, nothing. Except for maybe the odd bubble on an electrode from a bit of water splitting.
So let's leave it for a minute while we talk about what we are expecting to see.
The seemingly green liquid is actually a mixture of yellow and blue substances.
Orange dichromate(VI) ions react in the basic conditions of the concentrated ammonia solvent
to form yellow chromate(VI) ions. These are quite visible already, but the paler blue colour of the
hydrated copper ions gets a boost as water ligands are exchanged by ammonia to form
the tetrammine diaqua copper(II) complex.
So we now have a mixture of the two colours which, of course, appears green. Both ions have the same magnitude, but opposite signs of charge.
The yellow negative ions begin to drift towards the positive terminal and the blue positive ions drift
towards the negative. And as the bands of colour separate the green becomes less prominent.
I've been talking for about a minute now, but already the process is visible, albeit a little
bit slower than students might otherwise have expected.
Speeding up the footage you can see we reach a good separation after around five minutes which leaves enough time to try one more thing.
What do you think is going to happen if we were to switch the polarity of the terminals?
The bands, of course, merge back together and if left long enough will even kick out the opposite side.
I found this demo quite useful to support teaching both in physics
as well as chemistry lessons so give it a go and let us know how you get.