Want to keep learning?

This content is taken from the National STEM Learning Centre's online course, Teaching Practical Science: Chemistry. Join the course to learn more.

Skip to 0 minutes and 10 seconds Another small way of doing electrolysis is to use a single battery. In this case, a PP3 battery, which is 9 volts. And we’re using the prepared U tube which has been made by bending glass in hot flame. To make your mini U tube, use 0.5 millimetre thick, 6 millimetres external diameter borosilicate glass, which is readily available. Mark out your tubing at 16 centimetres and cut using glass tubing cutters. Flame polish the ends if you want to avoid possibly sharp edges. Here, we have a very clean cut. Using a pen, mark your tube at 7.5 and 8.5 centimetres. First, heat at 7.5 centimetres. Rotate whilst heating. When soft, bend at 90 degrees. Then heat your mark at 8.5 centimetres.

Skip to 1 minute and 1 second This won’t take as long as it is hot already. And carefully bend. Place on a heat proof mat to cool. We’ve strapped this to our battery and we’ve used a little bit of adhesive to hold it in place on the bench. We’ve wrapped the battery in some white tape so it gives a bigger contrast for us to be able to see what goes on in the U tube. To make electrodes for this, we’ve used a propelling pencil and used some of the thin, carbon propelling pencil lead. What we’re going to do now is make a solution up to undergo electrolysis. And this time we’re going to electrolyze potassium iodide solution. We’re using a 0.1 molar solution.

Skip to 1 minute and 38 seconds To enable us to see what’s going on we’re going to add some starch indicator, which is a 5% solution of soluble starch, which is potato starch dissolved in hot distilled water. We’re also going to use phenolphalene as well as an indicator. And that solution is usually made up in ethanol. So what we’re going to do now is we’re going to take some of our potassium iodide and pour that into our beaker.

Skip to 2 minutes and 4 seconds We’re going to add a few drops of phenolphalene indicator. And then we get to add some starch indicator to the solution. Give that little swirl. And then we’re going to use a different pipette to suck that up and very gently fill our U tube until it’s almost filled to the top.

Skip to 2 minutes and 30 seconds What we’re now going to do is take our carbon electrodes and insert them into the U tube. It’s sometimes easier to actually connect the battery first, but you must make sure that the two electrodes don’t meet to each other, otherwise they will short out. So we will attach that to our battery. And then we’ll insert negative electrode into that one, and the positive electrode into that side.

Skip to 3 minutes and 1 second And then we can look at the reaction that’s happening inside the container. Very quickly we can see there’s two colours being produced. At the positive electrode, at the anode, we’ve got a blue colour appearing. And this is because the iodine is being produced from the iodide as it loses an electron. And that is then reacting instantly with the starch solution to give this blue black precipitate. At the negative electrode, much like in the brine solution, we have hydrogen being produced. And because that’s coming out of solution it’s leaving hydroxide ions behind. And that then reacts with the phenolphalene indicator, which shows us there’s alkaline around it and we get the fuchsia pink colour. So a very simple reaction.

Skip to 3 minutes and 45 seconds The reagents can be discharged down the drain. And we’re using very little chemicals so it reduces the risk immensely.

Alternative electrolysis: U-tube small scale

This experiment is another version of electrolysis, which is very self-contained. It can help students work with smaller amounts of solutions, and also allows for individual practical work. The U-tubes should be made prior to the practical lesson, but once a class set is made they can be reused after careful rinsing.

Equipment list

Note that making the U-tubes needs to be done by a teacher or technician, not by the students. Borosilicate tubing is used here as it is easier to work with than soda glass tubing. We used 0.5mm thick walls, 6mm external diameter tubing to make ours.

The solutions used here are 0.1moldm-3 potassium iodide solution, as well as phenolphalein indicator solution (a few drops) and a few drops of starch indicator solution (made with 10g soluble starch in 100ml hot distilled water).


What are your thoughts about this practical? Share the advantages and disadvantages for you below. Consider also how you can involve your technician colleagues in preparation. You’ll look at one of the three electrolysis practicals in more depth in step 1.13.

Share this video:

This video is from the free online course:

Teaching Practical Science: Chemistry

National STEM Learning Centre