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Chemgineer
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[*] posted on 12-4-2024 at 06:02
High temperature stir bar.


If you have a hot plate that can reach 500 deg C where is a good place to find a suitable stir bar?
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Precipitates
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[*] posted on 12-4-2024 at 06:33


I would probably take my chances and go with something like this on aliexpress:

Magneton High Borosilicate

If it fails it's cheap.
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Jenks
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[*] posted on 12-4-2024 at 06:39


That is the maximum temperature of the surface of the hot plate. The contents of a container on UT will not get that hot, maybe 300C.
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Sulaiman
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[*] posted on 12-4-2024 at 23:42


As above but;
I can't imagine what stirrable liquid would need 500C
The highest temperatures I've used were when boiling conc. sulphuric acid.
I suggest that first you buy a cheap set of ptfe stir bars
which will be adequate for almost all common stirring requirements.
Consider high temp. stirbars when/if you have an actual need.




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Chemgineer
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[*] posted on 15-4-2024 at 04:35


Quote: Originally posted by Sulaiman  
As above but;
I can't imagine what stirrable liquid would need 500C
The highest temperatures I've used were when boiling conc. sulphuric acid.
I suggest that first you buy a cheap set of ptfe stir bars
which will be adequate for almost all common stirring requirements.
Consider high temp. stirbars when/if you have an actual need.


Sulphuric acid is my main use case, I could use boiling chips but I think it would be allot safer with a stir bar. I think they will probably demagnetize though,

Could maybe try and find a piece of steel encapsulated in borosilicate.
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Jenks
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[*] posted on 15-4-2024 at 16:13


Quote: Originally posted by Chemgineer  

Could maybe try and find a piece of steel encapsulated in borosilicate.

That should work up to its Curie temperature of at least 770C.

Most of the references I could find give a limit of 500C for borosilicate laboratory glassware.

[Edited on 16-4-2024 by Jenks]
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Sulaiman
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[*] posted on 16-4-2024 at 03:07


The thought of a (relatively heavy) steel rod spinning around at high speed in a glass flask containing boiling sulphuric acid is not my idea of fun.

A cobalt magnet in a glass envelope looks good,
but I'd expect a lot of wear on the flask and stirbar.

I plan for my next H2SO4 distillation to use a bubbler....




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[*] posted on 16-4-2024 at 05:01


does it actually need to be a stir bar?
Probably not as convenient, but overhead stirrers are a thing





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[*] posted on 16-4-2024 at 08:42


You can enclose a steel rod in quartz glass of sufficient thickness with a pre-calculated thermal gap. But there are several problems here:
-To melt a quartz tube you need high-temperature gas propane + oxygen is not enough. Of course, I bent quartz tubes, but it took so long;
- After soldering, you need to mechanically align the geometry, otherwise there is a risk of flight;
-The bottom of the flask should be almost perfectly flat, although at low stirrer speeds the risk of the stick flying out is minimal. You can make a thickening in the center - this will further reduce the risk of flying out;
- Well, the main problem is that the standard stirrer has a rather weak grip. At least after my homemade one with a neodymium magnet and when I tried to use a homemade steel stick it simply would not be held by the magnetic field. However, mixers are different.
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Jenks
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[*] posted on 16-4-2024 at 09:18


Maybe a paper clip coated in silicone? Or boiling stones or capillary sealed at the top.
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[*] posted on 16-4-2024 at 12:01


What about silica sand? It is inert and insoluble in sulfuric acid. Or glass wool.



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[*] posted on 17-4-2024 at 00:29


You do not need a stirbar or boiling chips for sulfuric acid if your heating is strong enough. From experience, bumping of sulfuric acid is only an issue if you are barely able to boil it.
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[*] posted on 17-4-2024 at 02:05


Interesting.... I can see how that may be true
but don't you get a lot of 'not-distilled' mist going up and into the condenser?
ie impurities (salts etc.) carried over




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[*] posted on 27-4-2024 at 10:31


easy electromagnet made from microwave transformator refresh your demagnetized stirbars. Ill try to do it soon



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[*] posted on 28-4-2024 at 04:30


Quote: Originally posted by LookingForAnswers  
easy electromagnet made from microwave transformator refresh your demagnetized stirbars. Ill try to do it soon
AlNiCo is fairly easy to re-magnetise using dc with coils, iron and small air gaps,
I think that you should consider some type of capacitor-discharging-into-coil arrangement.
That's what I did anyway.




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