Sciencemadness Discussion Board - Glassware safety with 22.5 micron vacuum pump

Glassware safety with 22.5 micron vacuum pump

itsafineday - 17-1-2019 at 10:07

I bought a 2 stage vacuum pump from harbor freight and it will pull down to 22.5 microns hg .

How much vacuum is safe to apply to glassware like rbf , rbf with flat bottom , cold trap and erlenmeyer flask?

Sulaiman - 17-1-2019 at 12:37

There is negligible difference between 1mm Hg pressure and a perfect vacuum - in terms of stress on glassware.

unionised - 17-1-2019 at 12:42

Quote: Originally posted by itsafineday

... rbf with flat bottom... erlenmeyer flask?

I'd think twice about using anything with a flat bottom, apart from a filter flask which is designed to stand the load.

Incidentally, you can't have a round bottomed flask with a flat bottom.
You can have one of these.
https://en.wikipedia.org/wiki/Florence_flask

itsafineday - 17-1-2019 at 16:12

Thanks Sulaiman and unionised .

Sulaiman - 17-1-2019 at 17:37

This is a useful resource for b.p. vs. pressure
https://www.sigmaaldrich.com/chemistry/solvents/learning-cen...

I bought my cheap hvac dual-stage rotary four years ago and I've not yet used its vacuum capability because;
. the only high b.p. liquids that I've used were mercury and conc. sulphuric acid, neither of which I want in my pump
. common solvents need cryogenic condensers at full vacuum
. I've been lazy and not set up a workable pressure regulation system
. I have no instrument to measure lower pressures, only a plan
. my little 12Vdc vacuum pump is good enough for most things that I do, mostly filtration and rarely (twice) a reduced pressure simple distillation.
So, the most expensive single item that I bought for my chemistry hobby has spent most of its time in its box

Pumukli - 18-1-2019 at 05:33

Flat surfaces and vacuum are not a friendly combo as I heard. I'd never put a flat bottom anything to strong vacuum.

And it includes filter flasks too! My uncertainty arises from the fact that I have a 500 ml filter flask which explicitely states that "Vacuum > 0.3 bar" - which is not a hefty vacuum at all!

Sulaiman - 18-1-2019 at 06:50

I filter at 0.2 to 0.3 bar, depending on the condition of my little vacuum pump on the day,
lower pressures cause solvents to boil and/or froth.

Mateo_swe - 11-12-2019 at 00:41

Are you sure the common rb flasks with a small flat surface on the bottom cant be used with vacuum?
With vacuum i mean low enough preassure to boil off solvents and most vacuum destillations.
It seem strange to me that there isnt a warning or a vacuum pressure limit specified on the salesite if there is a high risk of the flask imploding.
I would think many people use these flasks with common vacuum destillations.
I have read somewhere that only round flasks should be used with vacuum but isnt that very old info and not for modern flasks?

G-Coupled - 11-12-2019 at 02:04

I don't think it's to do with 'old' vs 'modern' glassware, and more to do with physics.

S.C. Wack - 11-12-2019 at 04:00

Quote: Originally posted by Mateo_swe

Are you sure the common rb flasks with a small flat surface on the bottom cant be used with vacuum?
With vacuum i mean low enough preassure to boil off solvents and most vacuum destillations.
It seem strange to me that there isnt a warning or a vacuum pressure limit specified on the salesite if there is a high risk of the flask imploding.

Are you talking about glass made by amateurs, for amateurs? Are you talking about thin or thick glass? Are you talking about glass that is not heated?

Herr Haber - 11-12-2019 at 04:19

Quote: Originally posted by G-Coupled

I don't think it's to do with 'old' vs 'modern' glassware, and more to do with physics.

And the sphere is of course the best possible shape

That doesnt mean a flat bottom flask will necessarily implode. It's just more likely.

That said Deschem glassware is thicker than my Duran Schott, Simex etc. I used it several times for distillations.
At the moment I have two hotplates and no mantle but with a mantle I'd use round bottom flasks.

happyfooddance - 11-12-2019 at 10:23

I use 500ml flat bottom "round bottom" boiling flasks all the time, they are great as recieving flasks (can set them on workbench easily), but also great as boiling flasks (heat better on hotplates). I have used them under vacuum countless times without incident.

I would probably not go any bigger than 500 ml though.