Sciencemadness Discussion Board

Eutectic salts for heating bath alternative

AnonGuy - 2-2-2024 at 09:11

I'm not sure if this has been posted in the past, but I'm so happy with the results I thought I'd spread the good word.

So as we all know, oil baths are stinky and hazardous at high temps, sand has poor thermal conductivity, copper pellets/beads work well but its high density causes the flask to "float", making it very difficult to remove and replace. Heating mantles are great but not everyone has them.

I've recently started using molten nitrate salts and I'm quite happy with them. Very cheap, reusable, no smell, smoke, fumes, and good past 500C according to the paper. I've only done 280C so far myself, but haven't noticed any degradation or issues.

The only downside is it hardens back into a block when it cools, so storage is a bit of a pain. I just leave it in the pot and re-use. Also because of the calcium nitrate component the salt mix is a bit hygroscopic. It doesn't seem to be an issue however, as the moisture is driven off while its heating up.

https://www.osti.gov/servlets/purl/1142924

Sir_Gawain - 2-2-2024 at 10:50

And what happens when 300C nitrate comes into contact with something combustible?

Rainwater - 2-2-2024 at 12:06

Quote: Originally posted by Sir_Gawain  
And what happens when 300C nitrate comes into contact with something combustible?

A side reaction

AnonGuy - 2-2-2024 at 13:07

At 280C, nothing happens. When I remove the flasks from the molten salt i place it on some newspaper to drip on.

Rainwater - 2-2-2024 at 14:12

Fuel + oxidizer, bad combination.
You'll get lucky 1000s of times, but that one time you dont, you will remimber.

DraconicAcid - 2-2-2024 at 14:23

I'm pretty sure the calcium nitrate is a tetrahydtrate to melt at that temp, and is far less likely to ignite something as, say, molten KNO3.

Twospoons - 2-2-2024 at 15:30

https://www.energy.gov/eere/solar/articles/halide-and-oxy-halide-eutectic-systems-high-performance-high-temperature-heat
Some ternary systems based on chlorides, if molten nitrates seem too risky.

clearly_not_atara - 2-2-2024 at 16:21

Since the methanesulfonates can substitute for nitrate in dissolving silver, I thought I would look up their melting points and eutectic properties. The mp of NaOMs is about 354 C; CsOMs is about 263 C. But I couldn't find any information about eutectics of alkali metal salts or about KOMs. Most methanesulfonates decompose at about 400 C in air.

The chloride eutectics mostly have very high melting points, > 350 C. I assume that the interesting operating range is around 150-350 C, since beyond that there isn't much organic chemistry to do. Eutectics of sodium and potassium bisulfate are stable to around 300 C when they decompose into pyrosulfate.

The tetrachloroaluminates have excellent melting properties but I have always assumed they would be destroyed by moisture. Likewise iron and zinc. Even the chloride mixtures containing MgCl2 are suspect.

IIRC NiBr2 doesn't hydrolyze. Possibly a bromide eutectic containing nickel would be air-stable.

[Edited on 3-2-2024 by clearly_not_atara]

Twospoons - 3-2-2024 at 14:58

T=132 C at NaCl-KCl-AlCl3 mole fraction of 36%-14%-50%.
T=91 C at NaCl-KCl-AlCl3 mole fraction of 26.25%-15%-58.75%.

From the linked presentation above.

DraconicAcid - 3-2-2024 at 17:15

Quote: Originally posted by Twospoons  
T=132 C at NaCl-KCl-AlCl3 mole fraction of 36%-14%-50%.
T=91 C at NaCl-KCl-AlCl3 mole fraction of 26.25%-15%-58.75%.

From the linked presentation above.

But that's going to be really water-sensitive, I think.

Deathunter88 - 3-2-2024 at 19:49

Quote: Originally posted by Rainwater  
Fuel + oxidizer, bad combination.
You'll get lucky 1000s of times, but that one time you dont, you will remimber.


Molten nitrates are more inert than you would expect, and essentially resistant to detonation. Black powder has to be ball milled for hours and hours for intimate contact of the ingredients and even then in the open it is more of a gentle deflagration. Calcium nitrate isn't even used in black powder because of how poorly it performs as an oxidizer.

All that is to say, the OP has nothing to fear from his nitrate heating bath unless he does something incredibly idiotic, and I actually do think it is a good replacement for traditional oil baths.

AnonGuy - 3-2-2024 at 23:33

Quote: Originally posted by Deathunter88  
Quote: Originally posted by Rainwater  
Fuel + oxidizer, bad combination.
You'll get lucky 1000s of times, but that one time you dont, you will remimber.


Molten nitrates are more inert than you would expect, and essentially resistant to detonation. Black powder has to be ball milled for hours and hours for intimate contact of the ingredients and even then in the open it is more of a gentle deflagration. Calcium nitrate isn't even used in black powder because of how poorly it performs as an oxidizer.

All that is to say, the OP has nothing to fear from his nitrate heating bath unless he does something incredibly idiotic, and I actually do think it is a good replacement for traditional oil baths.


If anything i think the nitrate salts are much safer then oil baths. Granted i haven't gone to 500C, and have no intentions of doing so, but at 280C pretty much anything aqueous and probably a whole lot of other stuff will cause an oil bath to turn into a volcano(if you could even reach those temps; synthetic oil perhaps?).

Not so much with this stuff. I carefully added some water expecting a violent reaction. Got just a bit of steam. :D

Now I'm sure there are substances that'd cause a violent reaction if mixed with the nitrate salt, but that's a poor argument against the method, since just about everything and anything will cause that same issue with oil baths, at much lower temperatures in fact. Also most oils are quite flammable, so you have the terrible side effect of a fireball being emitted from the spill as well!

I'm not going to tell you i didn't have trepidation when i first melted up some nitrate and plunged in my 5L flask though. :o

Truth be told, I just ordered a very large heating mantle so I probably wont be using this method much longer, but I found it to be superior to anything else I've tried to date.

The only other thing I considered trying is different low mp metal alloys. However from what i can tell they usually have toxic metals as a component(s), are probably more expensive and may also react violently to contaminants.

[Edited on 5-2-2024 by AnonGuy]