[joepec]

I am having trouble finding high temperature stir bars. Teflon stir bars are only good until ~225C and pyrex stir bars are rated up to 275C. For my applications I need a stir bar than can handle up to 400C. Do they even exist? I will probably end up buying the pyrex and trying it.

[phth]

You do not need to stir a homogeneous reaction.  You must learn to be critical of your own technique *edit*, or whoever told you it was a good idea to stir the solution.

[joepec]

It is not a reaction, it is a bulb to bulb distillation unit.

[kriggy]

Why not use Kugelrohr? With applied vacuum you dont need that high temperature

[clarkstill]

What are you distilling? Kriggy is right - if you have access to a better vacuum that might make the temperature more manageable.

If the mixture is inert towards iron you could always just add a little piece of steel (e.g. a paperclip) to agitate the reaction.

Otherwise I'd just add some anti-bumping granules and let the reflux bubbles do the mixing.

[Enthalpy]

In case the stir bar at +400°C is needed (...I don't have any kind of opinion about that), a difficulty is its Curie temperature, above which a permanent magnet loses its properties. I suspect this is the limit of 275°C, not the Pyrex coating.
https://en.wikipedia.org/wiki/Curie_temperature

Among permanent magnets, neodymium (shorthand for Nd-Fe-B) completely stops working at +310 to +400°C and gets very weak much earlier. Samarium magnets are still sold for that only reason: 720 to 800°C; even more brittle, and prone to corrosion. Some ceramic magnets may fit but are less strong than rare earth magnets.

Would a ferromagnetic bar do the trick, instead of a permanent magnet? The choice would be much broader.

If no stir bar is commercially available, try to make one. If the hardware and skills are within reach to coat a magnet or ferromagnetic bar in glass, fine. If not, and provided the liquids aren't corrosive, I'd just try a ferritic stainless steel, turned to size and anodized as thick and hermetic as possible. Its rotation may scratch you glass pot.

http://www.aksteel.com/markets_products/stainless_ferritic.aspx
the most usual is X6Cr17 = 1.4016 = Aisi 430
http://www.aksteel.com/pdf/markets_products/stainless/ferritic/430_Stainless_Steel_DS_201512.pdf

[P]

Yea - another vote to vac it down from me....   and use bumping granules or sticks.

[clarkstill]

I don't think a Curie transition will be an issue: iron has a Tcurie of 1043K

In case the stir bar at +400°C is needed (...I don't have any kind of opinion about that), a difficulty is its Curie temperature, above which a permanent magnet loses its properties. I suspect this is the limit of 275°C, not the Pyrex coating.
https://en.wikipedia.org/wiki/Curie_temperature

Among permanent magnets, neodymium (shorthand for Nd-Fe-B) completely stops working at +310 to +400°C and gets very weak much earlier. Samarium magnets are still sold for that only reason: 720 to 800°C; even more brittle, and prone to corrosion. Some ceramic magnets may fit but are less strong than rare earth magnets.

Would a ferromagnetic bar do the trick, instead of a permanent magnet? The choice would be much broader.

If no stir bar is commercially available, try to make one. If the hardware and skills are within reach to coat a magnet or ferromagnetic bar in glass, fine. If not, and provided the liquids aren't corrosive, I'd just try a ferritic stainless steel, turned to size and anodized as thick and hermetic as possible. Its rotation may scratch you glass pot.

http://www.aksteel.com/markets_products/stainless_ferritic.aspx
the most usual is X6Cr17 = 1.4016 = Aisi 430
http://www.aksteel.com/pdf/markets_products/stainless/ferritic/430_Stainless_Steel_DS_201512.pdf

[kriggy]

Or if you need it on larger scale and anti bumping granules dont work, then overhead stirrer could work. But unless you have specific equipment it  might be difficult to keep the reaction sealed

[Enthalpy]

I don't think a Curie transition will be an issue: iron has a Tcurie of 1043K.

But are the stirring bars made of iron? The most usual permanent magnets have a Curie temperature <400°C and lose their efficiency much earlier.

[Corribus]

Can you use a mechanical stirrer?

[clarkstill]

I don't think a Curie transition will be an issue: iron has a Tcurie of 1043K.

But are the stirring bars made of iron? The most usual permanent magnets have a Curie temperature <400°C and lose their efficiency much earlier.

Fair point - apparently most stirrer bars are made of 'alnico' - an alloy I'd never heard of until 5 mins ago:

https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/Aldrich/General_Information/1/labwarenews-3-1.pdf?utm_source=redirect&utm_medium=promotional&utm_campaign=insite_labwarenewsv3n1

But on the plus side they have a Tcurie of ~800C:

https://en.wikipedia.org/wiki/Alnico

The aldrich article suggests it's the teflon that becomes a problem at high T - they recommend using pyrex coated magnets.

I've never distilled from an overhead-stirred vessel, but I guess it can be done? Might just depend on your glassware how convenient an option it is.

[Enthalpy]

AlNiCo is very old-fashioned for most uses. Especially, it demagnetizes easily, sometimes just because the magnetic circuit is not closed. But it's less brittle than ceramic and rare earth magnets, and resists corrosion better than rare earth magnets.

Ptfe can withstand +350°C (not 400°C normally) but only if the other constraints are very mild. For instance, its rubbing properties are horrible (abrasion, friction coefficient) above some +50°C already and continue to degrade.

Bare AlNiCo is still available: Bakker Magnetics, Thyssen Krupp... You could buy a part, or turn one to shape and size, and let it cover with electrolytic nickel, cobalt or their alloy, if they are compatible with your reactants and products. Sn can be put over Ni, for instance by dipping the Ni-covered part in liquid Sn. At least, Ni, Co and Sn won't scratch your glass vessels.

Hey, I wrote Ni and Co from my time as a mechanical engineer, but you chemist may prefer to coat them yourself with Ag or Au.

These AlNiCo magnets are meant for electric guitars and their size could match a stirbar:
www.ebay.com/itm/222504778553
www.ebay.com/itm/122047741148
makes an affordable try.

[Enthalpy]

Ahum, please forget about tin at +400°C.