[Kaladiscope]

so that is the question!

I set up my high vacuum and it works until 4.10-1 mbar. That's enought for instance to remove traces of water?

Thanks!

[Honclbrif]

I should say so. I hope you're protecting a pump that good with a decent cold trap.

[Kaladiscope]

Yes! The pump is protected with a cold trap (nitrogen liquid)!

Thanks for your answer! Someone told me that u need minus 2 mbar to remove traces of water...

[fledarmus]

How are you measuring vacuum? Usually, the manometer you use measures absolute pressure - although you can measure very low numbers, an absolute vacuum is a pressure of zero and it is impossible to have negative numbers.

There are gauges that measure vacuum as a drop in pressure. In those cases, atmospheric pressure is usually ~1000 mbar, and a pressure reading of -2 would give a pressure of ~998 mbar - that isn't much of a pressure drop.

[Kaladiscope]

fiedarmus, I didn't say anything about negative number. Or maybe I didn't make myself clear. 1.10-1, which is 0.1 mbar. And someone told me that to remove traces of water u need at least minus 2 (1.10-2).

[Honclbrif]

Maybe I've just never heard of it, but that looks like kind of a strange scale you're using.

Whenever I've seen pressures reported, its always been the absolute pressure, i.e "1 torr", or "1 mbar". Why let other people do the math when you can do it for them. You'll probably have a lot less confusion if you report your pressures as absolute because "4.10-1 mbar" looks like you're saying "a range between 4.10 and 1 mbar"

Again, I'm probably misunderstanding your scale, but 1.10-2 is -0.9.

I've used a pump that only pulls down to about 75 torr (100 mbar) and it will still move water with a little time and heat (about 35°C)

[Kaladiscope]

Honclbrif, sorry.

What I want to say is that I have: 2.10^-1 mbar = 0.2 mbar. It was not a range.

[fledarmus]

Ah - by minus 2, you mean 1 x 10^-2! Got it! Sorry, misunderstood your notation...

1 mbar is already below the triple point of water - you can freeze-dry a compound by adding water, freezing the water by immersion in, say, an acetone/dry-ice bath, then sublime all the ice by lowering the pressure below the triple point. In most cases, this is plenty of vacuum to dry your compounds.

If your compound is actually forming hydrates with the water rather than just absorbing some water, you may need to drop the pressure lower or add some heat to break the hydration. 0.01 mbar, though, is probably overkill.

[Kaladiscope]

fiedarmus,

I would say that the later is my case. My compound is hygroscopic (is a polyethyleneglycol methyl ether chain). For that reason I would think that 0.1mbar is not enough to remove these traces. What I am doing is heating at 60 ºC  and trying to make lower the high vacuum (until 0.09 at least but looks almost impossible).

Will see!

Thanks for your tips!

[Honclbrif]

That does sound like it would be pretty tough to get rid of the water.

Azeotropic distillation, then remove solvent invacuo?