Foeskes
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Trouble with molten sodium hydroxide electrolysis
Yesterday, I was trying to make sodium metal using electrolysis, how ever I ran into a problem: no sodium was produced and both electrodes were
bubbling.
I was using 12 volts and my batteries and supply around 5 amps. The sodium hydroxide was melted using a alcohol wick lamp, maybe I was supposed to
remove the lamp before electrolyzing the molten salt.
When melting I put a watch glass over the container(made of steel).
My electrodes were galvanized steel (maybe I should remove the zinc first).
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elementcollector1
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The zinc will react with the molten NaOH to make sodium zincate.
I'd suspect an issue with your power supply. Check to make sure it doesn't have short circuit protection (and if it does, put in a high-power
resistor).
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Foeskes
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I'm using 2 6v lead-calcium batteries. There is current flowing since my wires melted and the electrodes bubble, but it doesn't produce sodium. It
bubbled on both sides maybe I should use copper electrodes.
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elementcollector1
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Your wires melted? That's a sign of far too much current. Also, wouldn't that break the circuit?
It may also be that the sodium in your system is either
a) Dissolving in the NaOH melt,
or
b) Reacting with the water in the melt (NaOH is very rarely completely dry) to form fresh NaOH, slowly depleting the system of water but not appearing
to produce any sodium.
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Foeskes
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The rubber melted but the copper just got hot (probably less than 100c).
How do you get rid of the water?
Also what does sodium look like dissolved in sodium hydroxide?
After electrolyzing my melt looks like it fountains chromium3+ due to the color.
Does sodium hydroxide catalyze the reaction of iron and oxygen?
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vmelkon
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You can get rid of the water by melting the NaOH and waiting for a while. That should drive the water off.
If Na dissolves in the NaOH, it would look grey. Keep the temperature of your NaOH at its melting point.
"Does sodium hydroxide catalyze the reaction of iron and oxygen?"
You mean molten NaOH. I don't know but from what I have seen, someone did this experiment and melted NaOH in steel and the NaOH became green. I guess
some the iron does oxidize and particles of iron oxide gets in the NaOH.
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Foeskes
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do I have to remove the zinc coating off of my electrode?
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Broken Gears
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I made a Downs cell a few years back, but never took it for a ride. The design suggested that the sodium should float on top of the molten NaOH.
The alcohol wick lamp could be a problem due to H2 and O gasses produced, unless you isolate them or run it at a well ventilated place.
I would use iron, nickel or copper elektrodes and expect some coloring of the molt due to corrosion.
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Foeskes
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Can molten NaOH absorb H2O?
I'm also running the experiment outside.
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experimental
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Today I tried to make sodium with electrolysis. While I could see the shiny molten sodium droplets forming, I found it nearly impossible to collect
them!
I was using a small metal spoon, but as soon as I touched the sodium, it would solidify. So I tried heating the spoon with the flame while collecting
the sodium. It worked, but as soon as I removed the flame, the sodium exposed to the air, humidity or both, would glow red hot and slowly disappear.
Do you have any suggestions to get past this issue?
Foeskes, in my opinion you need a more powerful flame to properly melt the sodium hydroxide. I used a propane/butane torch, and still had to heat it
quite a bit anyway.
I also suggest to use a multimeter to monitor the current. If the current starts decreasing as soon as the flame is removed, more heating is required.
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Foeskes
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Well I did try it again and got about 4 cubic mm of sodium. Turns out I just needed to keep it hot for a while to remove the water. Anyways I used a
spoon I just pushed the sodium to the side of the container and remove it. What is your container made of? I did mine in a ceramic drying dish at one
point and after cleaning found out that the NaOH dissolved it halfway through.
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experimental
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It's a very shallow flower pot, so I guess it's made of clay. After soaking it in water to dissolve the sodium hydroxide I noticed it was not damaged
in any way.
My yield was something like 2 mm^3 of dirty sodium for 100 g of sodium hydroxide, most of which would flow to the sides once molten, and sit there
doing nothing. I probably need a smaller flower pot.
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elementcollector1
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Two cubic millimeters? That's incredibly tiny, to the point where I'm surprised you can even quantify that. What you need is a steel container that
has not been galvanized and does not contain chromium - I would suggest black iron plumbing pipe that has been thoroughly baked in an oven to remove
the Cu2Se coating on the outer surface.
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experimental
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The causes for such a low yield are multiple
- most of the sodium hydroxide just solidified without participating in the electrolysis
- some of the sodium formed tiny beads that got trapped in the slag and did not coalesce
- when attempting to remove the sodium with a small spoon, the sodium would glow red hot and oxidise, which has been quite frustrating
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MrHomeScientist
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Copper selenide coating on black iron pipe, you say? Do you have a source? That seems like a somewhat exotic material for a pipe coating.
Edit: Found one, via Wikipedia: https://www.epi.com/black-oxide/faqs/
| Quote: | What is black oxide?
Black oxide is a blend of caustic, oxidizers, and additives that is used on ferrous metals. There are three types of metal blackening: hot, mid, and
room temperature blackening. EPi was the first company to offer all three processes.
Hot black oxide (285 F°) is a conventional hot process which produces a true black oxide iron magnetite (Fe3O4) finish. It is a premium-grade salt
mixture which will blacken a wider range of steel alloys than any other process on the market.
Mid-temperature black oxide (225–245 F°) is the latest development in blackening processes, producing a true black oxide iron magnetite (Fe3O4)
finish. It produces no caustic boiling fumes and provides a safer working environment.
Room-temperature blackening (65–85° F) is the preferred metal-blackening process for safe and convenient in-house blackening. It is an excellent
non-bleed black finish for powdered metal and cast iron. It produces super deep blackness and corrosion resistance equal to hot oxide blackeners.
It is important to note this is not a true black oxide process; however, it may be referred to as room-temperature black oxide.
Room-temperature blackening processes are autocatalytic reactions of a black selenium-copper compound that deposits on ferrous parts through
an immersion process.
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Emphasis mine. I guess the benefit of using this material is it can be done at room temperature.
[Edited on 8-23-2017 by MrHomeScientist]
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