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The UCLA site says in step 6 to disconnect the vacuum tubing before turning off the vacuum and the second link from Leeds says in step 11 to turn off the vacuum source first and then disconnect the tubing.

I've been taught both ways and am confused about the difference in methods.

When should one actually disconnect the tubing and what is the rationale for each method?

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  • $\begingroup$ First disconnect the tubing else the vacuum in the erlenmayer flask can suck in water (if you use a water pump). This can be tedious even if your filtered through phase has to be discharged (e.g. solvent wastes separation). If your target is the liquid, than is detrimental. $\endgroup$ – Alchimista Apr 11 '18 at 9:06
  • $\begingroup$ What about a regular vacuum pump? $\endgroup$ – 86BCP2432T Apr 11 '18 at 17:27
  • $\begingroup$ I would do the same but with a normal setup nothing shall happen. It depends on pump, what was used before.... With a line or a pump there should be a trap in between vacuum and erlenmayer. ... $\endgroup$ – Alchimista Apr 11 '18 at 17:31
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If you were to turn off the pump while leaving the object attached then if you have an oil filled pump or a water pump then you will have a suck back which will either damage the pump or contaminate the thing you are filtering, or if you are very unlucky both.

If you have a house vacuum system then you should close the valve / tap between your filtration and the vacuum manifold before you remove the hose to your flask. If you do it the other way around then you will interfere with the use of the house vacuum by others.

I think you need to know if you have your own vacuum pump or if you are using a house vacuum system where a centralised pump serves many people.

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