woelen
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Constant gas flow from electrolysis?
I want to do some experimenting with burning hydrogen in different gases and gas mixes. For this purpose I took a U-shaped tube and filled this with a
concentrated solution of NaHSO4, took a copper wire as cathode and a platinum wire as anode:
At the cathode side (copper wire) I connected a PVC tube and at the end of the PVC tube I connected a glass pasteur pipette.
I expected a steady flow of hydrogen from the pasteur pipette (after an initial period during which the air is expelled from the tube and pipette)
when the apparatus is connected to a suitable power supply. Instead of that, the hydrogen, formed at the cathode, pushed the liquid downwards, until
it runs out of the U-tube at the anode side 
Apparently, the pressure at the cathode is more easily released through the U-tube than through the thin open end of the pasteur pipette.
Next, I also connected a PVC tube to the anode side of the apparatus and also connected a pipette to the other end of that PVC tube. The idea behind
this is that both ends now only have a tiny hole for the escaping gas. But still, no steady gas flow is obtained. The apparatus is EXTREMELY sensitive
to how the PVC tubes are held, moving around one of the tubes results in the liquid moving in the U-tube and at one pipette the device then sucks in
air, while at the other pipette a rush of gas is expelled.
I finally put both ends of the pipettes under water and with some tweaking it is possible to have the surface level in the U-tube fairly constant and
getting a stream of bubbles from the pipettes, but this is not a really steady stream. I get bubbles.
What I really want is a steady stream of H2-gas at e.g. 0.3 ml per second (at 3 A there is 0.33 ml of H2 per second), such that it can be ignited and
gives a nice little flame. What I now have is very irregular release of hydrogen and sometimes even suck-back of air into the device (which is
dangerous, as it may cause a flame to flash back into the tube).
What kind of trick can be used to have a truly constant gas-flow?
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Endimion17
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Is the U-tube neccessary? It is very small and really isn't suitable for the production of gasses in the manner you'd prefer. Could you make a crude
Hoffman apparatus? I mean, you have platinum, and you don't need everything to be connected by glass seals. Some tubing and corks will do the work...
a bit higher voltage to overcome the resistivity...
If this was not electrolysis, but a zinc-acid reaction, I'd recommend a T-junction with a baloon and a glass stopcock, where baloon serves as a
buffer, but U-tubes can't use somewhat higher pressures as everything would surge to the other side...
If it isn't a problem, Hoffman is a great solution. You could get a steady stream very easy.
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watson.fawkes
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You have a ΔP problem. The
pressure build-up on each side of the tube isn't constant, and the ΔP is pushing the electrolyte around. The problem is that the way you've
got it set up, changes in gas flow rate on the H2 output side greatly changes the pressure on that side. The pressure on the O2 side was constant. As
a result your ΔP was shifting wildly.
I'd put a pair of gas regulators on each side of the apparatus. They'll provide some flow restriction and create back-pressure on the feed lines. That
back-pressure is your friend, because it damps down pressure variations because of flow rate on the output side of the regulator. Ordinary glassware
can take a few atm of pressure, so there are no particular worries there as long as you maintain a steady-state flow rate. To do your initial
adjustment, open both regulators entirely and then close them in tandem. This procedure will minimize the risk of pressure build-up. You might also
want a pressure gauge and a big red button as a kill switch on the power supply.
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solo
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....interesting, I wonder if methylamine gas can be produced in such a way....that would alleviate the numerous methods involving distillation and
filtration to acquire the said gas.....solo
It's better to die on your feet, than live on your knees....Emiliano Zapata.
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Mr. Wizard
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Is your electrolysis unit open to the air? Containing the electrolyte and electrodes inside a sealed unit will keep the generated gasses from pushing
the electrolyte away from the electrodes, and raise the pressure enough to force the gas through the small opening pipette. The outer container has
to be strong enough to hold the pressure you will need to attain the desired flow through your pipette. You might use a plastic (PET) drink bottle, or
even two of them connected with an electrolyte 'bridge'. The bottles are strong, cheap, and don't throw a lot of fragments in the event of a
flashback.
There is a Henes welder used by jewelers that generates hydrogen and oxygen and allows them to mix. (Stainless electrodes in KOH solution) The mixed
gas was fed through a very solid stainless bottle filled with methanol, which added a small amount of vapor and acted as a flash arrester. The welder
used hypodermic needles as a burner tip. They occasionally show up on ebay for a ridiculous price. They are very simple to make, and use a
pressurized container.
To sum up, seal your electrolysis cell and it will solve your problem if I've understood it correctly.
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