Electrolysis Made Easy(ish)

Some STEM Learning trainee teachers, looking very keen!

Back in November last year (was it really that long ago??) I wrote a blog post about water, in which I described a simple at-home version of electrolysis. I didn’t think much of it at the time, beyond the fact that it was oddly exciting to do this experiment—that usually involves power-packs and wires and all sorts of other laboratory stuff—with just a 9V battery, a tic tac box and some drawing pins.

Then, hey, what do you know, someone actually read my ramblings! Not only that, read them and thought: let’s try this. And so it was that Louise Herbert, from STEM Learning (that’s their Twitter, here’s their website), contacted me last month and asked if I’d mind if they used the Chronicle Flask as a source for a STEM learning course on practical work.

Of course not, I said, and please send me some pictures!

And they did, and you can see them scattered through this post. But let’s have a quick look at the chemistry…

Electrolysis is the process of splitting up compounds with electricity. Specifically, ionic compounds: the positively-charged ion in the compound travels to the negative electrode, and the negatively-charged ion moves to the positive electrode.

Water is a covalent compound with the formula H2O, but it does split into ions.

Only… wait a minute… water isn’t ionic, is it? So… why does it work on water? Er. Well. Water does split up into ions, a bit. Not very much under standard conditions, but a bit, so that water does contain very small amounts of OH and H+ ions. (In fact, I can tell you exactly how many H+ ions there are at room temperature, it’s 1×10-7 mol dm-3, and, in an astonishing chemistry plot twist, that 7 you see there is why pure water has a pH of, yep, 7.)

So, in theory you can electrolyse water, because it contains ions. And I’ve more than once waved my hands and left it at that, particularly up to GCSE level (age 16 in the U.K.) because, although it’s a bit of a questionable explanation, (more in a minute), electrolysis is tricky and sometimes there’s something to be said for not pushing students so far that their brains start to dribble out of their ears. (As the saying goes, “all models are wrong, but some are useful.”)

Chemists write half equations to show what the electrons are doing in these sorts of reactions and, in very simple terms, we can imagine that at the positive electrode (also called the anode) the OH ions lose electrons to form oxygen and water, like so:

4OH —> 2H2O + O2 + 4e

And conversely, at the negative electrode (also called the cathode), the H+ ions gain electrons to form hydrogen gas, like so:

2H+ + 2e —> H2

These equations balance in terms of species and charges. They make the point that negative ions move to the anode and positive ions move to the cathode. They match our observation that oxygen and hydrogen gases form. Fine.

Except that the experiment, like this, doesn’t work very well (not with simple equipment, anyway), because pure water is a poor electrical conductor. Yes, popular media holds that a toaster in the bath is certain death due to electrocution, but this is because bathwater isn’t pure water. It’s all the salts in the water, from sweat or bath products or… whatever… that do the conducting.

My original experiment, using water containing a small amount of sodium hydrogen carbonate.

To make the process work, we can throw in a bit of acid (source of H+ ions) or alkali (source of OH ions), which improves the conductivity, and et voilà, hydrogen gas forms at the cathode and oxygen gas forms at the anode. Lovely. When I set up my original 9V battery experiment, I added baking soda (sodium hydrogencarbonate), and it worked beautifully.

But now, we start to run into trouble with those equations. Because if you, say, throw an excess of H+ ions into water, they “mop up” most of the available OH ions:

H+ + OH —> H2O

…so where are we going to get 4OH from for the anode half equation? It’s a similar, if slightly less extreme, problem if you add excess alkali: now there’s very little H+.

Um. So. The simple half equations are… a bit of a fib (even, very probably, if you use a pH neutral source of ions such as sodium sulfate, as the STEM Learning team did — see below).

What’s the truth? When there’s plenty of H+ present, what’s almost certainly happening at the anode is water splitting into oxygen and more hydrogen ions:
2H2O —>  + O2 + 4H+ + 4e

while the cathode reaction is the same as before:
2H+ + 2e —> H2

Simple enough, really, but means we use the “negative ions are going to the positive electrode” thing, which is tricky for GCSE students, who haven’t yet encountered standard electrode potentials, to get their heads around, and this is why (I think) textbooks often go with the OH-reacts-at-the-anode explanation.

Likewise, in the presence of excess alkali, the half equations are probably:

Anode: 4OH —> 2H2O + O2 + 4e
Cathode: 2H2O + 2e —> H2 + OH

This time there is plenty of OH, but very little H+, so it’s the cathode half equation that’s different.

Taking a break from equations for a moment, there are some practical issues with this experiment. One is the drawing pins. Chemists usually use graphite or platinum electrodes in electrolysis experiments because they’re inert. But good quality samples of both are also (a) more difficult and more expensive to get hold of and (b) trickier to push through a tic tac box. (There are examples of people doing electrolysis with pencil “leads” online, such as this one — but the graphite in pencils is mixed with other compounds, notably clay, and it’s prone to cracks, so I imagine this works less often and less well than these photos suggest.)

A different version of the experiment…

Drawing pins, on the other hand, are made of metal, and will contain at least one of zinc, copper or iron, all of which could get involved in chemical reactions during the experiment.

When I did mine, I thought I was probably seeing iron(III) hydroxide forming, based, mainly, on the brownish precipitate which looked fairly typical of that compound. One of Louise’s team suggested there might be a zinc displacement reaction occurring, which would make sense if the drawing pins are galvanized. Zinc hydroxide is quite insoluble, so you’d expect a white precipitate. Either way, the formation of a solid around the anode quickly starts to interfere with the production of oxygen gas, so you want to make your observations quickly and you probably won’t collect enough oxygen to carry out a reliable gas test.

In one of their experiments the STEM Learning team added bromothymol blue indicator (Edit: no, they didn’t, oops, see below) to the water and used sodium sulfate as (a pH neutral) source of ions. Bromothymol blue is sensitive to slight pH changes around pH 7: it’s yellow below pH 6 and blue above pH 7.6. If you look closely at the photo you can see that the solution around the anode (on the right in the photo above, I think *squint*) does look slightly yellow-ish green, suggesting a slightly lower pH… but… there’s not much in it. This could make sense. The balanced-for-H+ half equations would suggest that, actually, there’s H+ sloshing around both electrodes (being formed at one, used up at the other), but we’re forming more around the anode, so we’d expect it to have the slightly lower pH.

The blue colour does, unfortunately, look a bit like copper sulfate solution, which might be confusing for students who struggle to keep these experiments straight in their heads at the best of times. One to save for A level classes, perhaps.

(After I published this, Louise clarified that the experiment in the photo is, in fact, copper sulfate. Ooops. Yes, folks, it looks like copper sulfate because it is copper sulfate. But I thought I’d leave the paragraph above for now since it’s still an interesting discussion!)

The other practical issue is that you need a lot of tic tac boxes, which means that someone has to eat a lot of tic tacs. There might be worse problems to have. I daresay “your homework is to eat a box of tic tacs and bring me the empty box” would actually be quite popular.

So, there we are. There’s a lot of potential (haha, sorry) here: you could easily put together multiple class sets of this for a few pounds—the biggest cost is going to be a bulk order of 9V batteries, which you can buy for less than £1 each—and it uses small quantities of innocuous chemicals, so it’s pretty safe. Students could even have their own experiment and not have to work in groups of threes or more, battling with dodgy wires and trippy power-packs (we’ve all been there).

Why not give it a try? And if you do, send me photos!


Like the Chronicle Flask’s Facebook page for regular updates, or follow @chronicleflask on Twitter. Content is © Kat Day 2019 (photos courtesy of STEM Learning UK and Louise Herbert). You may share or link to anything here, but you must reference this site if you do. If you enjoy reading my blog, please consider buying me a coffee through Ko-fi using the button below.
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Non-stick toilets, synthetic poo and saving the environment

141 billion litres of water are used to flush toilets every day.

Scientists develop slippery toilet coating that stops poo sticking,” shouted newspaper headlines last week, naturally prompting comments about the state of politics, the usual arguments about the ‘right’ way to hang toilet paper rolls, and puns of varying quality.

There was also more than one person asking WHY, given everything going on at the moment, scientists are spending their time on something which seems, well, not terribly urgent. After all, ceramic toilet bowls are already quite slippery. Toilet brushes exist. We have a myriad of toilet cleaning chemicals. Surely there are higher priorities? Attempting to deal with looming environmental disaster, say?

But here’s the thing, from an environmental point of view, flush toilets are quite significant. If you’re fortunate enough to live somewhere they’re ubiquitous it’s easy to take them for granted, but consider this: flushing even a water-efficient toilet uses at least five litres of water (much more for older models, a bit less if you use a ‘half-flush’ function). Often this is perfectly clean water which has been through water treatment, only to be immediately turned back into, effectively, sewage. Now imagine you have something a bit… ahem… sticky to flush. What do you do? You flush the toilet twice. Maybe more. You break out the toilet brush and the bottle of toilet cleaner, and then you probably flush at least one extra time to leave the bowl clean.

Using toilet cleaning chemicals often results in extra flushes.

Consider that the average person uses the toilet about five times and day and multiply up by the population and, even just in the UK, we’re looking at billions of litres of water daily. Globally, it’s estimated that 141 billion litres of fresh water are used daily for toilet flushing, and in some homes it could account for a quarter of indoor wastewater production. That’s a lot of fresh water we’re chucking, quite literally, down the toilet.

It rains a fair bit in the U.K. so, except for the occasional dry summer, Brits aren’t in the habit of worrying too much about water supply. The opposite, if anything. But we need to change our ways. In a speech in March this year, Sir James Bevan, Chief Executive of the Environment Agency, warned that the U.K. could run into serious water supply problems in 25 years due to climate change, population growth and poor water management.

Even putting those warnings to one side, treating water uses energy and resources. Filters are used which have to be cleaned and replaced, chemical coagulants and chlorine (usually in the form of low levels of chlorine dioxide) have to be added. Sometimes ozone dosing is used. The pH of the water needs to be checked and adjusted. All of these chemicals have to be produced before they’re used to treat the some 17 billion litres of water that are delivered to UK homes and businesses every day. And, of course, the whole water treatment process has to be continuously and carefully monitored, which requires equipment and people. None of this comes for free.

So, yes, saving fresh water is important. Plugging leaks and using water-saving appliances is vital. And, given that everyone has to go to the toilet several times a day, making toilets more efficient is potentially a really significant saving. An super non-stick toilet surface could mean less flushing is needed and, probably, fewer cleaning products too — saving chemical contamination.

Fresh water is a valuable resource.

The new super-slippery surface was co-developed by Jing Wang in the Department of Mechanical Engineering at the University of Michigan. It’s called a liquid-entrenched smooth surface (LESS) and is applied in two stages. First, a polymer spray, which dries to form nanoscale hair-like strands. The second spray completely covers these ‘hairs’ with a thin layer of lubricant, forming an incredibly flat, and very slippery, surface. The researchers tested the surface with various liquids and synthetic faecal matter and the difference — as seen in the video on this page — is really quite astonishing.

Hold up a moment, synthetic faecal matter? I’ll bet no one embarking on an engineering degree ever imagines that, one day, they might be carefully considering the make-up of artificial poo. But actually, when you think about it, it’s quite important. Quite aside from safety aspects and the sheer horror of the very idea, you couldn’t use the real thing to test something like this. You need to make sure it has a carefully-controlled consistency, for starters. It’s the most basic principle, isn’t it? If you want to test something, you have to control your variables.

Artificial poo is surprisingly important.

Indeed, there’s even a scale. It’s called the Bristol stool scale, and it goes from “hard” to “entirely liquid”. Synthetic poo is a mixture of yeast, psyllium, peanut oil, miso (proof, if it were needed, that miso really does improve everything), polyethylene glycol, calcium phosphate, cellulose and water. The amount of water is adjusted to match different points on the Bristol scale. Aren’t science and engineering fun?

Anyway. Back to the non-stick technology. This new surface can be applied to all sorts of materials including ceramic and metal, and it repels liquids and ‘viscoelastic solids‘ (stuff that’s stretchy but also resists flow: apart from poo, PVA slime is another example) much more effectively than other types of non-stick surfaces. In fact, the researchers say it’s up to 90% more effective than even the best repellent materials, and they estimate that the amount of water needed to clean a surface treated in this way is 10% that needed for ordinary surfaces. They were also able to show that bacteria don’t stick to LESS-coated materials, meaning that even if untreated water is used to flush a toilet, it remains hygienic without the need for extra chemicals.

The potential to cut 141 billion litres of water by a factor of ten is not to be (I’m sorry) sniffed at. Plus, in some areas, ready supplies of water and the facilities to clean toilets just aren’t available. Using LESS could, potentially, reduce the spread of infection.

By Chemystery22 - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=31161897 A graft copolymer has side chains branching off the main chain — these side chains are the “hairs” described by the researchers.

So what IS this surface treatment made of? This information wasn’t widely reported, but it seems quite important, not least because applications of LESS are estimated to last for about 500 flushes, which suggests that re-application will be needed fairly regularly and, perhaps more worryingly, whatever-it-is is passing into the wastewater supply.

Not surprisingly, there’s a certain amount of vagueness when it comes to its exact make-up, but I did find some details. Firstly, it’s what’s known as a graft polymer, that is, a polymer chain with long side chains attached — these are the “hairs” described by the researchers.

Secondly, the polymer strands are based on polydimethylsiloxane, or PDMS. This may sound terrifying, but it’s really not. PDMS (also known as dimethicone) is a silicone — a compound made up of silicon, oxygen, carbon and hydrogen. These compounds turn up all over the place. They’re used contact lenses, shampoos, and even as food additives. Oh, and condom lubricants. So… pretty harmless. In fact, they’re reported as having no harmful effects or organisms or the environment. The one downside is that PDMS isn’t biodegradable, but it is something that’s absorbed at water treatment facilities already, so nothing new would need to be put in place to deal with it.

The problem of better toilets might be more urgent than you thought.

Finally, the lubricant which is sprayed over the polymer chains in the second stage of the treatment to make the surface “nanoscopically smooth” (that is, flat on a 1 billionth of a metre scale) is plain old silicone oil, which is, again, something with a low environmental impact and generally considered to be very safe.

As always with environmental considerations it’s about choosing the least bad option, and using these coatings would certainly seem to be a far better option than wasting billions of gallons of precious fresh water.

In short, silly headlines aside, it turns out that making toilets better might be quite an important problem. Maybe it’s time to rage against the latrine.


Like the Chronicle Flask’s Facebook page for regular updates, or follow @chronicleflask on Twitter. Content is © Kat Day 2019. You may share or link to anything here, but you must reference this site if you do. If you enjoy reading my blog, please consider buying me a coffee through Ko-fi using the button below.
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