Sciencemadness Discussion Board - Porcelain crucible and molten KOH

Porcelain crucible and molten KOH

Parakeet - 29-4-2023 at 06:13

Can a porcelain crucible (glazed) withstand molten hydroxide? Or would it be better to use a steel soup can?
I want to use it for making dichromate and maybe permanganates and cyanides.

Admagistr - 29-4-2023 at 08:02

Quote: Originally posted by Parakeet

Can a porcelain crucible (glazed) withstand molten hydroxide? Or would it be better to use a steel soup can?
I want to use it for making dichromate and maybe permanganates and cyanides.

Porcelain and molten hydroxide is not a very good choice

,use definitely an iron/steel crucible and preferably a nickel or silver crucible are the best choice,but not everybody has those at hand...

Keras - 29-4-2023 at 10:29

I just made p-cresol out of molten KOH in a porcelain crucible, it worked perfectly well.
I, however, cracked a porcelain crucible after fusing KOH with MnO₂. I suppose this was caused by thermal expansion coefficient difference.
So either use a porcelain crucible, but unload it as soon as you’re done, to buy a stainless steel crucible that won’t break even on cooling (that’s what I ended up doing).

Admagistr - 29-4-2023 at 10:44

Quote: Originally posted by Keras

I just made p-cresol out of molten KOH in a porcelain crucible, it worked perfectly well.
I, however, cracked a porcelain crucible after fusing KOH with MnO₂. I suppose this was caused by thermal expansion coefficient difference.
So either use a porcelain crucible, but unload it as soon as you’re done, to buy a stainless steel crucible that won’t break even on cooling (that’s what I ended up doing).

I was surprised that your crucible survived without corrosive damage...Porcelain consists mainly of the mineral mullite, which is an aluminium silicate.Thus, with prolonged melting and high temperatures, it is almost certain that the product will be contaminated with potassium silicate and possibly a small amount of potassium aluminate.

[Edited on 29-4-2023 by Admagistr]

Osmiridium - 29-4-2023 at 12:09

A porcelain crucible will always be etched by molten hydroxides (also carbonates) and impurities will be introduced, that's inevitable.
But it doesn't dissolve quickly so it may still be used for this purpose if the impurities can be removed easily or don't interfere with what you are doing. So if you dissolve the residue and acidify there will be a silica crystals that may be filtered off. Still there would be kations in solution. Mostly aluminium.

Another problem is the fact that the crucible itself will be contaminated with substances and is often almost impossible to clean.

Parakeet - 29-4-2023 at 20:47

Hmm, splashing potassium hydroxide (molten) must be super nasty. I don’t want to risk it…
Then how about steel cans for food/juices? I’m thinking something like a soup can, as it’s cheap. I’ve never used it because it has thin walls and seams, but has anyone?

Texium - 29-4-2023 at 21:10

Cans aren’t great since they usually have polymer liners that char and make a mess of things. I’ve found that stainless steel condiment cups make excellent crucibles though. Like these: https://www.amazon.com/Stainless-Commercial-Individual-Condi...

I used a similar one as a crucible for preparing potassium manganate several years ago.

Keras - 29-4-2023 at 23:34

Quote: Originally posted by Admagistr

The porcelain is enamelled. I’m not sure if that counts. In any case, since the desired product was organic rather than water-soluble, I didn't care much about possible ion contamination.

There was no real sign of deep corrosion inside the crucible, so the attack must have been fairly superficial, if any.

Sulaiman - 30-4-2023 at 01:36

Quote: Originally posted by Texium

Cans aren’t great since they usually have polymer liners that char and make a mess of things. I’ve found that stainless steel condiment cups make excellent crucibles though. Like these: https://www.amazon.com/Stainless-Commercial-Individual-Condi...

I used a similar one as a crucible for preparing potassium manganate several years ago.

I too use this type of s/s pot - very useful.
But, I once poured molten iodine into one and it ate right through the metal!

Small graphite pots with lids are also quite useful,
as are the small porcelain crucibles with lids.
Putting a 'tin' can on a bbq grill will burn away plastic linings.
Just choose the most suitable material for the required use.

metalresearcher - 30-4-2023 at 02:48

Stainless steel soup dippers are the best. Available for a few dollars at a common household supply shop.
I have used them successfully with both NaOH and KOH. But on the longer term the lye will dissolve some of the metal oxides on the steel.

And ALWAYS use leather (e.g.welding) gloves when handling molten lye, as the lye of over 300 C will leave nasty burns !

unionised - 30-4-2023 at 03:27

Interesting aspect of stainless steel; chromium will dissolve in hot alkali if there's oxygen present.
Iron won't.
So ordinary steel is actually less susceptible to corrosion than stainless, in these conditions.

It may be worth a preliminary "wash" with molten caustic to strip off tin plating.

Parakeet - 30-4-2023 at 04:26

Sauce cups seem like a good idea! They are cheap enough that I can throw away when they get destroyed.

Quote: Originally posted by unionised

Interesting aspect of stainless steel; chromium will dissolve in hot alkali if there's oxygen present.
Iron won't.
So ordinary steel is actually less susceptible to corrosion than stainless, in these conditions.

Unfortunately, steel cups do not seem to be as common as stainless ones. And I think I can tolerate Cr contaminations for my purposes.

Thank you for your ideas everyone!

Texium - 30-4-2023 at 11:16

Quote: Originally posted by unionised

Interesting aspect of stainless steel; chromium will dissolve in hot alkali if there's oxygen present.
Iron won't.

True, but from my experience using them, I don’t think a significant amount of chromium is leached from the alloy under typical molten-alkali-and-oxidizer conditions. There was no sign of pitting on the cup, it just discolored to a darker brown/black tint.