Sciencemadness Discussion Board

Electrolysis of alkali hydroxide eutectic mixtures

Zyklon-A - 23-4-2014 at 11:59

Sodium, potassium and lithium hydroxides can form eutectic mixtures which melt at much lower temperatures then any of the pure hydroxides.
Quote:
Originally posted in this patent The flux is a non-aqueous molten salt formulated on eutectic mixtures of KOH (potassium hydroxide), NaOH (sodium hydroxide) and LiOH (lithium hydroxide), with melting temperatures in a range from about 170° C. to about 226° C.

There is not a lot of information on this subject, but I found some.
Quote:
Although the melting points of both
pure sodium hydroxide and potassium hydroxide are over
300 °C, (323 °C for NaOH and 360 °C for KOH,)
the eutectic point at NaOH/KOH ) 51.5:48.5 is only about
165 °C.

This is very good, of course 323°C isn't that hard to reach, there are other problems, such as the temp must be just above the melting point or else the sodium will dissolve/react with the melt.
If this indeed does work the metals will be plated out as a mixture of course, and separation will be a pain in the ass. This will not be practical for elemental isolation as far as I can see.
Here is some more info:http://www.basf.com/group/corporate/en_GB/function/conversio...
http://www.chem.sc.edu/faculty/zurloye/JCG-97-172-183.pdf
http://www.nanoscience.gatech.edu/paper/2006/06_NL_4.pdf
http://books.google.com/books?id=ViOMjoLKB1gC&pg=PA36&am...

I don't have any potassium hydroxide right now, but Does anyone who does have some want to try this?

[Edited on 23-4-2014 by Zyklonb]

HgDinis25 - 23-4-2014 at 12:41

Potassium is more reactive than Sodium (Reactivity Series). Wouldn't any Potassium formed simply react with NaOH to form Sodium?
K + NaOH --> KOH + Na
I believe this is plausible. It would also affect the melting point of your mixture because the concentration of NaOH would greatly decrease.

This is discussed at length here:
http://www.sciencemadness.org/talk/viewthread.php?tid=2103

Zyklon-A - 23-4-2014 at 12:50

Reactivity series have nothing to do with mixtures, only aqueous solutions. Heat of Formation and Gibbs free energy decide all that.
This is why reactions like this can proceed: 6 NaOH + 4 Al → 2 Al2O3 + 6 Na + 3H2.


[Edited on 23-4-2014 by Zyklonb]

HgDinis25 - 23-4-2014 at 13:11

Quote: Originally posted by vulture  
I think if you were to electrolise that when molten, firstly all the sodium would be produced because of it's higher electronegativity and lower reactivity.
This will ofcourse have its effect on melting point, because the mixture will shift to Na + KOH.

Na/K alloys are often liquids at roomtemperature and very reactive, that is more reactive than pure sodium or potassium metal.


"Potassium hydroxide can be found in pure form by reacting sodium hydroxide with impure potassium" (http://en.wikipedia.org/wiki/Potassium_hydroxide#As_a_base)


What you said maks sense, but I've seen some references stating that K does react with NaOH.

Zyklon-A - 23-4-2014 at 13:24

Yes, your right, Interestingly sodium and potassium hydroxide have the exact same Std enthalpy of formation: -425 kJ·mol. So I'm not sure why it reacts, but apparently it does. I guess this is better though, as it will allow pure sodium to be plated out.

HgDinis25 - 23-4-2014 at 16:01

Quote: Originally posted by Zyklonb  
Yes, your right, Interestingly sodium and potassium hydroxide have the exact same Std enthalpy of formation: -425 kJ·mol. So I'm not sure why it reacts, but apparently it does. I guess this is better though, as it will allow pure sodium to be plated out.


Problem is your NaOH concentration will get lower while the KOH concentration won't be affected. Therefore your eutetic mixture is gone. Probablye the melting point of the mixture will start increasing.

Metacelsus - 24-4-2014 at 05:01

Well, you could just add more NaOH as it gets depleted.

Marvin - 24-4-2014 at 05:27

Electrolysis of a Na/K eutectic salt mix will produce a NaK alloy. NaK is a nightmare. Industrially it's possible to push this to pure Na or pure K with counter current methods.

Alloy formation can release a lot of energy, thermite like heat and light. Think of it as forming a metal metal compound.

Zyklon-A - 26-5-2014 at 08:34

Marvin, not necessarily, I will be using an inert atmosphere either way, molten sodium or potassium would be scary too.
But it wouldn't be hard to control it, pump it out as a liquid. Ampule it under argon, that's what I'd do with either sodium, potassium or even cesium. Sure it's much more reactive, but it'll never make contact with air, so the only difference is it's melting point. Also as previously discussed, it probably won't even isolate NaK, just sodium.
So basically it's just like the calcium chloride and sodium chloride mixture used to lower the melting point, in the method used industrially to isolate sodium. The calcium stays out if the equation, and only the sodium chloride needs to be replaced. Of course in this case we're using hydroxide.