Sciencemadness Discussion Board - Low temperature Castner process for sodium?

Low temperature Castner process for sodium?

GammaFunction - 29-4-2013 at 14:14

I used the search engine, and I hope this isn't a repost.

Wikipedia says, without giving a reference, that low-melting sodium hydroxide monohydrate can be electrolyzed to give metallic sodium, at just over 100C.

This inspired the idea of this simple apparatus to make sodium at just over the boiling point of water, by shielding the metallic sodium and hydroxide under mineral oil. Fuming should be reduced or eliminated, and no hazardous pressure could build up.

Glass components wold be nice to observe the process, but I'm not sure how long it would withstand the NaOH even at this low temperature. Even some form of plastic could conceivably work. The mineral oil itself will serve as a top window.

Any thoughts?

elementcollector1 - 29-4-2013 at 14:25

"Hot electrochemical sodium" - stickied in Technochemistry, I suggest you read there!
Glass would withstand molten NaOH for about 5 seconds, and it would turn opaque in 1. No-go.

GammaFunction - 29-4-2013 at 14:33

Quote: Originally posted by elementcollector1

"Hot electrochemical sodium" - stickied in Technochemistry, I suggest you read there!
Glass would withstand molten NaOH for about 5 seconds, and it would turn opaque in 1. No-go.

Guess I didn't read this section. Oh well.

However, this thread seems to be devoted mainly to the high temperature Caster process, not this unusual lower temperature variant, which uses the monohydrate to work just over the BP of water.

BromicAcid - 29-4-2013 at 14:41

Attached are pictures of a similar attempt from me. Starting with the most concentrated solution of sodium hydroxide I could make I began electrolysis on a watch glass. The electrolysis creates plenty of heat from resistance of the solution which drives off water, and it also splits the water to its elements driving it off as well. Eventually it gets to the hydrate. I do find it strange that sodium would form and be isolated from the hydrate. Usually with the high temperature Castner cell the inital bit of electrolysis is to remove the water, no sodium is formed. Still, stranger things have happened and I would chalk this up to overpotental. Nevertheless, without careful control of the temperature and current you will overshoot this window and end up with anhydrous material.

In my attempt water continued to be split off as the mixture heated from the resistance until I was electrolyzing molten NaOH. You see several little drops of sodium forming at the electrode. Afterwards there was a nice scorch mark on the picnic table due to the heat and the watch glass was nicely etched. Would be a neat demonstration save the fine particulates of sodium hydroxide that end up floating around in the air.

Edit: Traced the Wiki source back to August 12, 2010 edit that has made it through revisions to now. I assume however you did the same as someone asked the person who made the edit for a reference within the last few weeks. His response that "it is more of a proof of concept, rather than a source of sodium" rings true considering the reaction simultaneously produces 4 mols of sodium with 6 mols of water and one mol of O2.

[Edited on 4/29/2013 by BromicAcid]

GammaFunction - 29-4-2013 at 16:03

Quote: Originally posted by BromicAcid

Attached are pictures of a similar attempt from me. Starting with the most concentrated solution of sodium hydroxide I could make I began electrolysis on a watch glass. ....

So it looks like you never made sodium from the monohydrate, but conditions were not carefully controlled.

The appeal of this process (if it works) is that the conditions are moderate, and an oil layer can be used to contain it (and any NaOH mist), and an oil bath can be used to moderate the temperature.

Could NaOH hold its water of hydration so strongly that the monohydrate doesn't react with sodium (or reacts very slowly)?

The process seems to depend on the following two assumptions: 1) When Na is formed at the cathode, the liberated water of hydration doesn't react with the sodium much before escaping as a gas; 2) the water in the remaining monohydrate doesn't react with the freshly created sodium.

m1tanker78 - 29-4-2013 at 17:07

Quote: Originally posted by GammaFunction

The appeal of this process (if it works) is that the conditions are moderate, and an oil layer can be used to contain it (and any NaOH mist), and an oil bath can be used to moderate the temperature.

Hot NaOH will turn your oil to a milky white substance in no time. I believe the product is soap but I never got around to researching it.

Most likely, in Bromic's case at least, the large current density (small surface area of electrodes immersed) caused enough local heating to dehydrate the lye around and between the electrodes. By the looks of the burnt table, it got quite hot..

Tank

BromicAcid - 29-4-2013 at 17:32

Quote: Originally posted by m1tanker78

Most likely, in Bromic's case at least, the large current density (small surface area of electrodes immersed) caused enough local heating to dehydrate the lye around and between the electrodes. By the looks of the burnt table, it got quite hot..

Indeed, my point was that one could start out with a hydrated form and then through the electrolysis and heat and resistance end up electrolyzing molten hydroxide and not stop at the hydrate. To do what you are proposing will likely involve the ability to cool the setup appropriately once at temperature to keep it in range or else it will quickly turn into a traditional Castner cell.

GammaFunction - 29-4-2013 at 18:37

Quote: Originally posted by m1tanker78

Hot NaOH will turn your oil to a milky white substance in no time. I believe the product is soap but I never got around to researching it.

Is this true even for a mineral oil, rather than an organic oil? Mineral oil should not saponify. I can't find any evidence that hydrocarbons react with hydroxides.

elementcollector1 - 29-4-2013 at 20:03

It's not true in my experience - KOH heated to 200+ C with mineral oil and kerosene had no reaction that I could discern (at least, nothing forming a milky white substance).

ScienceSquirrel - 30-4-2013 at 02:50

If your mineral oil or kerosene is saturated hydrocarbons then it will not react with potassium hydroxide under any conditions that you are likely to be able to achieve in a home lab.

Fantasma4500 - 30-4-2013 at 05:49

IIRC 316 steel can withstand hydroxides

using it as cathode might work aswell as heating element / container

but i believe this is more fitting for technochemistry anyhow

blogfast25 - 30-4-2013 at 08:43

Quote: Originally posted by GammaFunction

Wikipedia says, without giving a reference, that low-melting sodium hydroxide monohydrate can be electrolyzed to give metallic sodium, at just over 100C.

Where precisely does it say that? If this is indeed possible, countless experimenters would have reported on it.

GammaFunction - 30-4-2013 at 12:48

Quote: Originally posted by blogfast25

Where precisely does it say that? If this is indeed possible, countless experimenters would have reported on it.

It has been edited out.

It seems vaguely plausible only if the sodium were at least fractionally protected against the water of hydration. Maybe in the immediate neighborhood of the cathode, the current density and heat are high enough to perform the anhydrous reaction.

blogfast25 - 30-4-2013 at 13:26

Gamma:

Perhaps. It'll be like walking a tight rope, I think. This kind of thing could be OK for demo purposes (making micro quantities of Na) but I doubt if it can be reasonably scaled up.

kristofvagyok - 30-4-2013 at 13:35

Quote: Originally posted by GammaFunction

Wikipedia says, without giving a reference, that low-melting sodium hydroxide monohydrate can be electrolyzed to give metallic sodium, at just over 100C.

Just a thought. If you electrolyze NaOH.H2O to get sodium, it will immediately react with the water from the monohydrate to form back the NaOH and producing some H2. It would be a really expensive and hard way to produce anhydrous NaOH

P.S.: Anhydrous NaOH/KOH could be made by melting it in a silver pot till it boils (the water comes out). And be careful, because it eats everything.

GammaFunction - 30-4-2013 at 13:51

Quote: Originally posted by kristofvagyok

Yes. That's the most evident thing that would happen. It might work if the immediate zone around the cathode were hotter and water-depleted (because the current density is highest there). One could imagine a partly efficient process in which some of the sodium reverted back to the hydroxide, and some water was forced out of the system where it didn't react with the sodium.

So far, there was exactly one unsupported reference to this process (wikipedia), and it has been edited out in the past day, probably because this thread drew attention from a justified skeptic.

elementcollector1 - 1-5-2013 at 10:19

I have heard that if you take regular NaOH, stick in your electrodes (w/o melting yet), turn on the power, and add a drop of water or two, ohmic heating will do the rest - no heat required!
I have not tested this yet, but it would be interesting to try someday.