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Author Topic: Practical question: forming KOH:NaOH eutectic  (Read 3160 times)

Offline viridian

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Practical question: forming KOH:NaOH eutectic
« on: 31/01/2014 09:42:55 »
So, I have a pot of solid NaOH, and a pot of solid KOH (powders)

I want to combine them into a molten eutectic for use as an etchant.

My plan was to weigh them out in the correct ratio, combine them in a quartz pot, and heat them on a hot plate at 180-200C until they formed said eutectic.

Are there any practical issues with this plan? Will I have to bring them to a higher temperature in order to melt one of the powders before I can form the eutectic? Should I be using a container other than a quartz pot? Are there any safety risks? (Other than the obvious protective gloves and faceguard/ventillated bench/don't stir with an aluminium stick, I mean)

Any advice is appreciated


 

Offline chiralSPO

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Re: Practical question: forming KOH:NaOH eutectic
« Reply #1 on: 31/01/2014 14:06:28 »
Both NaOH and KOH absorb water from the atmosphere (KOH pellets can turn into a caustic puddle pretty quickly on a humid day). There are two reasons I bring this up:

1) this will make it slightly more complicated to get the proportions right, since some of the mass of each component is an unknown amount of water. I don't know how sensitive the melting point is to the proportions of the mixture...

2) when you start heating, the water will boil off--potentially spattering or splashing hot alkali. In addition to whatever eye/face shield and gloves you will be using, I recommend having a shield or screen you can put around the crucible as it is heated, or at least between you and it.

good luck!

 

Offline viridian

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Re: Practical question: forming KOH:NaOH eutectic
« Reply #2 on: 31/01/2014 14:37:16 »
Both NaOH and KOH absorb water from the atmosphere (KOH pellets can turn into a caustic puddle pretty quickly on a humid day). There are two reasons I bring this up:

1) this will make it slightly more complicated to get the proportions right, since some of the mass of each component is an unknown amount of water. I don't know how sensitive the melting point is to the proportions of the mixture...

2) when you start heating, the water will boil off--potentially spattering or splashing hot alkali. In addition to whatever eye/face shield and gloves you will be using, I recommend having a shield or screen you can put around the crucible as it is heated, or at least between you and it.

good luck!

Thanks for the tips, chiralSPO

From looking at the phase diagram, the melting point goes from 170C at the eutectic point to 300ish at either extreme, so I'd be looking at ~200C if my ratios end up a little off, which should be doable with the equipment I have. I'm fortunate enough to have a humidity-controlled lab, so hopefully the caustic solids won't pick up excessive moisture from the air. Is storing them with desiccants advisable, or just an exercise in futility?

I'm going to use a bench which has a perspex screen, which should save me from any splashback, but I'll be mindful of what I leave in the vicinity of the hotplate.
 

Offline chiralSPO

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Re: Practical question: forming KOH:NaOH eutectic
« Reply #3 on: 01/02/2014 02:45:35 »
A desiccator would certainly help, but only reasonable if you're starting with anhydrous material (many labs have plenty of technical grade 80-90% pure KOH, where the remainder is essentially all water.)

It sounds like you have a reasonable setup.

What are you etching?
 

Offline Bored chemist

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Re: Practical question: forming KOH:NaOH eutectic
« Reply #4 on: 01/02/2014 14:44:40 »
Molten alkalies dissolve quartz

If you know that little about the subject you should probably abandon this idea.

Incidentally, NaOH and KOH are both excellent desiccants, they would probably pick up water from the air in most desiccators.

They will pick up CO2 from the air too, and the carbonates have a rather higher melting point.
 

Offline viridian

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Re: Practical question: forming KOH:NaOH eutectic
« Reply #5 on: 03/02/2014 08:39:51 »
A cursory search got me "NaOH etches SiO2" and I wasn't sure to what extent/rate it did, which is why I bothered mentioning that I was planning on using quartz. This is why I was asking for practical tips, since I've not worked with molten alkalis before.
Thanks for the advice re: desiccants, though.

ChiralSPO, I'm etching GaN, hoping for a dislocation-selective etch as I've seen reported in some of the literature.


 

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Re: Practical question: forming KOH:NaOH eutectic
« Reply #5 on: 03/02/2014 08:39:51 »

 

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