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Author: Subject: Hydrogen production for Al + Na/KOH + H2O - what is the best method for control?
RogueRose
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[*] posted on 26-9-2020 at 10:20
Hydrogen production for Al + Na/KOH + H2O - what is the best method for control?


I was trying to make some H2 for an experiment to reduce lead oxide (and some "sponge lead" from lead acid batteries) and I created a NaOH solution and added all of it to a test tube of Al pieces (cut up soda bottle tabs). Well the reaction eventually ran away and there was no way to control it once the temp got up past about 120-130F.

So I figure there are two other options I could try:
1 - place Al into a flask with side vacuum filtration port, and then drip in a solution of NaOH through the stoppered neck of the flask (through hole through stopper) though IDK what concentration would be best for this - 10-50%? or what.
2 - place Al in flask as well as NaOH in stoichiometric amounts, connect gas collection tube to side port, and drip water in through the stoppered neck of the flask.

I'm thinking that #1 might be the best solution b/c I can then wash off the unused Al and not waste the NaOH that didn't get used up. But maybe I'm missing something in this process, but, again, I'm guessing a NaOH/water solution dripping in would be the best bet?

What do you think and do you have any other methods that might work. Also, should I try to heat either the flask with the Al (to 120-150F) to allow the reaction to take place faster, or is room temp sufficient?
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Twospoons
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[*] posted on 26-9-2020 at 13:05


3rd method: have your Al in one solid piece that can be lowered into your NaOH as needed. Or flip that and arrange the liquid level to be adjustable via a separate liquid flask and a couple of siphon hoses.
You could almost make that self-regulating - if you had a flow restrictor on the hydrogen outlet, then the pressure build up could be used to force the NaOH soln out of the reaction flask into a reservoir if the reaction was running too fast. And vice versa - allowing soln to flow back in as the pressure dropped.



[Edited on 26-9-2020 by Twospoons]




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Sulaiman
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[*] posted on 26-9-2020 at 17:49


One problem is the ink on the outside of the cans and the plastic lining inside.
You can melt aluminium cans in a steel can using eg burning bbq charcoal and a small fan.
Pour the molten Al into water to get clean blobs of aluminium,
which are more suitable for a controlled reaction rate.




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MidLifeChemist
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[*] posted on 26-9-2020 at 18:30


4th Method - dissolve Aluminum in liquid Gallium, and simply to water. Works great, I don't remember it getting too hot and it's not difficult to separate them afterwards in solution and recover your Gallium.
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[*] posted on 27-9-2020 at 01:51


What Twospoons suggested (self regulating thingy) is called a Kipp's Apparatus - You might be able to fashion one out of glassware you have with some ingenuity.
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kmno4
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[*] posted on 30-9-2020 at 12:51


I spent many hours thinking about the same issue.
Such a purely chemical way of H2 production seems simple at the first sight. Of course, the most important is controling rate of H2 evolution. The assumption was to use one-vessel apparatus.
So I was thinking about some tilted vessels, aluminium mechanically joined with steel and movable by magnet from outside of the vessel.. etc.
It turn so complicated, that I abandoned the idea and built electrolyser. Easy to control, the only thing to maintain is adding water from time to time. My first construction was distroyed - blown up the plastic lid and turn it into pices. It happened because of my stupidity :P The second construction (from PP plastic containers, carefuly welded, SS net as electrodes) still works. Useful thing, may be running even for days, practically without supervision (up to 5 A).
Very important thing in such H2 generators is to keep dead-space above the liquid as small as posibble. It is because air from the space must be first removed, so the generation should be free running to replace this air with hydrogen.
It was not done in previous version - the effect was small explosion inside, destroing - fortunately - only the (welded) plastic lid. Of course, it was not self ignition :D




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TriiodideFrog
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[*] posted on 1-10-2020 at 02:15


To be honest, I feel that there could be easier methods to produce hydrogen gas. You could just drip HCl onto some zinc metal, which not only produces hydrogen gas but also zinc chloride, which is hygroscopic.
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[*] posted on 1-10-2020 at 02:18


However, if you still want to follow your method, maybe a separatory funnel might work for you.
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njl
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[*] posted on 1-10-2020 at 07:05


An addition funnel dripping your base solution onto aluminum foil would be even better. That way you can keep the vessel "closed" and still precisely control H2 evolution. You most likely do not want to heat the reaction. This reaction is extremely exothermic by itself, and as soon as the oxide layer on your aluminum is etched away the reaction will proceed vigorously.
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SaccharinSlayer157
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[*] posted on 1-10-2020 at 15:35


I find aluminum foil reacts too fast for me, and I have the hot water burns to prove it.:cool: Aluminum shot, made by pouring the molten metal into water, reacts at a much slower but still appreciable and consistent rate. The only trouble is melting the aluminum in the first place.
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[*] posted on 12-10-2020 at 11:13


if you are comfortable with the calculations, adding aluminum chunks to caustic in a cylinder works really well, its then essentially on tap, via a regulator, just ensure your calculations don't result in anywhere near the burst pressure of the cylinder, add the chunks of aluminun to the cylinder, pull a few vacuum/argon cycles or just pull a vacuum, then use the vacuum to draw in the amount of caustic solution you need, seal, it will still be under vacuum, but not for very long. i always have a large vessel full of water in case things get hot, but if you use large chunks it's fine...
if it begins to go over pressure....run...just kidding this where your diligence means, that your burst disks are ok because you assessed them previous to beginning
i find small steel oxygen cylinders are perfect and in abundance at scrap metal places.
i think most of my instruction came from magpie who has an extensive post on this somewhere..
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