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.