Harry Metcalfe

The Hoffman Voltameter: Mark II

Following on from my first attempt, I’ve built a new version:

Hoffman Voltameter Mark II (it's a bit better)

This version has smaller bottles, reducing the amount of electrolyte needed, and goes back to using silicone sealant to attach the PEX piping to the bottles.

The glue gun was just too leak prone. Although it was much quicker to work with, the glue is very rigid when dry, so handling the apparatus (eg when filling or emptying) tended to create leaks. So I decided to be patient, and use sealant, which is much more flexible when dry. I also sanded the area around the holes in the bottles, and the ends of the PEX pipes, so that the sealant would adhere more effectively.

This version works much better! It’s leak-free, and I succeeded in generating about 100ml of hydrogen and collecting it by releasing the clamp and using a makeshift gas bubbler. I passed it over a flame, and heard the squeaky pop — perhaps the best one I’ve *ever* heard, considering the effort it took!

Unfortunately, I had a little less luck with the Chlorine. A small quantity of gas was generated at the anode, but I could not discern any particular colour. Conversely, the electrolyte in the anode bottle had a definite green tinge. I suspect that the Chlorine is largely dissolving in the water, and that the gas I collected at the top was probably a small amount of Oxygen. This feels more plausible than it otherwise would given that the electrolyte was not a saturated solution. I’ll test this out later.

However, this version isn’t without its problems. There’s some visible corrosion around the join between the electrode and the wire, which I tried to insulate using heat shrink. As it’s clear that the steel connection is being oxidised, that’s obviously not working. I also don’t like that the electrodes are dangling from cables. It feels… bad. So, I’m going to take the plunge and mount them permanently to the bottles from the outside, which will remove the steel connection from the electrolyte entirely as well as making it all a bit neater.

Another very strange problem is that the water level in the three containers is not always the same, even when the tubes are open to the atmosphere. This is a bit baffling, to say the least. I think that the tubes are probably at fault: their internal diameter is very small (about 2mm) and I think there’s some kind of capillary effect at work. I’m going to replace these with some normal 5mm PVC tubing and see if that solves the problem.

This version is much better than its predecessor, and I’ve experienced some success. But I think there’s better to come.

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Making a Hoffman Voltameter

I’ve been dabbling with odds & ends of chemistry in my spare time recently (mostly trying in vain to remember the stuff I learnt at school). I’ve been partly inspired by this excellent book, and partly by a young cousin who’s recently become fascinated by all things chemical. Hmm. Is that a redundancy?

Anyway. For a fun project, and to start understanding electrochemistry better (which, as soon as you scratch the surface, becomes rather complicated), I’ve been building a Hofmann Voltameter. They’re made for electrolysing things, typically water, in such a way that one can easily collect the gases. The proper ones look like this:

A Hofmann Voltameter

And are rather expensive. I figured I could make one. So, I set about it last weekend, and this was the result (forgive the clutter!):

My Hoffman Voltameter, Mark I!

The two bottles at either side are 500ml diet coke bottles. They’re joined by a push-fit T-junction and some PEX pipe to the central bottle, an M&S orange juice bottle, which is used to add the electrolyte. The base has a shaped grove to hold the bottles in place with a pole at the back to support the central bottle so it doesn’t swing forwards and spill. The lids of the coke bottles each have a tube which is used to collect the gases, and a cable to which is connected the electrode. The electrodes themselves are made of carbon fibre — somewhat cheaper than platinum and, unlike graphite, does not readily fall apart. Unfortunately, this didn’t work so well. There were problems:

• This leaked, quite significantly, where the pipes enter the coke bottles. Eventually I managed to get the major ones but even when it appeared to have stopped leaking it would grow new ones.
• The gases were not effectively collected. The electrodes sat at an angle and tended to produce a circular current within the electrolyte which drew a large proportion of the bubbles back into the bottle rather than letting them rise to the surface. The bubbles were also very small, which — combined with the textured surface of the bottles — tended to make the bubbles stick to the inside of the bottle.

The combination of a slowly leaking system and a relatively small amount of gas reaching the top meant that no gas was ultimately collected at all. To compound the problem, my glue gun broke, so I couldn’t deal with the leaks. At this point, slightly frustratedly, I decided that it wasn’t really worth fixing anyway — as it would take a very, very long time to produce a usable amount of anything.

So, on to Mark II: I’ve built a new version using square 330ml Copella bottles. These are much less textured, and as they’re square they’re easier to glue to the pipes than the round coke bottles. They’re also smaller, therefore requiring less electrolyte than the 500ml coke bottles, and allowing the height of the central bottle to be reduced. As I write, the Mark II is bing leak tested: sitting in my kitchen full of bright red water on a bed of white tissue paper!

I’ll post again when I’ve had a chance to test it.

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