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I’ve been working in the lab for some time now, but I’ve found conflicting information when asking this question, so maybe you know the answer.

When working in a Schlenk line with double oblique stopcocks, imagine the following situation. One Schlenk flask needs to be under vaccum, and at the same time another one needs to be filled with argon gas.

Now some colleagues told me that you can’t simultaneously open the argon gas and vacuum even though the schlenk flasks are connected with a different tube and the argon manifold and the vacuum manifold are separate from each other. I’m referring to two different double oblique stopcocks.

My guess would be that it doesn’t matter as long both manifolds are seperate i.e independent from each other. Why should it be a problem? The setup is like this:

enter image description here

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  • $\begingroup$ You can't open a single hose to both as the taps switching the lines won't let you. They either open to one manifold or the other and can't do both at the same time. A vessel attached to two hoses, though, could be open to gas on one line and vacuum on the other. There is no restriction on what you can do with separate hoses/valves/vessels. I suspect your colleagues advice was garbled in translation. $\endgroup$
    – matt_black
    Dec 18, 2023 at 21:23
  • $\begingroup$ Yeah I edited my question to clarify; opening two hoses to two different Schlenk flasks. Not a single one $\endgroup$
    – Mäßige
    Dec 18, 2023 at 22:19
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    $\begingroup$ in which case the answer is there is no problem at all. I've certainly run multiple different operations on a single Schlenk line without any problems. $\endgroup$
    – matt_black
    Dec 19, 2023 at 0:00
  • $\begingroup$ I've learned it as a precaution. Before drawing vacuum on one line, you close all others. If you open the valve the wrong way, the worst that could happen is you ruin one product. Also, depending on what you are doing, the sudden pressure increase (and drop) might have funtastic outcomes for all the other hoses. But then I became a computational chemist, because I have thumbs for fingers... So my advice is probably not the best. $\endgroup$ Dec 19, 2023 at 0:08
  • $\begingroup$ Okay @matt_black just as I figured, thanks. $\endgroup$
    – Mäßige
    Dec 20, 2023 at 7:00

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There are no general reasons to not use vacuum and gas at the same time but there are some subtleties in practice

The design of the valves in Schlenk lines prevent the (obviously bad) idea of opening a single vessel to vacuum and gas at the same time. But should a complete Schlenk manifold only be used for one operation on one vessel at a time?

There is no solid reason why not. The vacuum systems are independent of the gas systems and the use of one does not, in principle, interact with the use of the other. Why, after all, do typical manifolds have multiple valves?

But, in practice, doing too many things at once sometimes can be ill-advised. For example, exposing a vessel to the vacuum side of the manifold is used for a variety of purposes. Sometimes it is to ensure all the oxygen is removed from a vessel before conducting an air-sensitive reaction; sometimes it is to drive out remaining solvent from a completed reaction. And some of these actions need constant attention to prevent accidents. For example, when trying to remove solvents, it is important not to open the vessel to vacuum too quickly or the solvent can froth or boil, contaminating the vacuum line; the same is sometimes true when trying to evacuate gas from finely divided powders where the entrained gas can "blow" the fine powder into the vacuum line (I've done this preparing raney nickel which can be quite dangerous as the dry powder is spontaneously flammable in air!). It is unwise to attempt two such operations simultaneously as divided attention is dangerous when two actions both need fine control of the rate the vacuum is applied. But, once the dangerous stages are complete there is little reason two lines cannot be open at the same time.

When filling vessels with inert gas the major issue is that, if two vessels are both being flushed with inert gas, the overpressure can get too low to do a good job on one if the other is too open.

But the inert gas line is independent of the vacuum line so there is no reason not use both at the same time unless your lack of attention opens a valve the wrong way.

I suspect that the main reason for any advice recommending not to do two different actions at the same time is to prevent attention errors where one action needs to be carefully managed. But this is only a major issue when initiating the action as many reactions are quite safe when started so it is OK to leave them under vacuum or inert gas one the action is initiated (multiple vessels can be evacuated while others are flushed with inert gas as long as fine control is no longer needed).

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