LABORATORY PROCEDURES: Things to know about the Freeze-Pump-Thaw degasification of liquids


For a long time, Freeze-Pump-Thaw degasification of liquids has been a common laboratory procedure to remove gases out of a reaction mixture, beside others methods.[1,2] In contrast to other procedures like the Schlenk technique of exchanging an unwanted atmosphere against an inert gas or the coloration of a compound via Seebach’s stain, there is no single individual behind it to name. This may not be a further problem during a reaction, but it actually is one when other people try to simulate it.
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In most cases, scientists do not exactly mention how they conduct the degasification, neither in their article, nor in the supporting information (if ever existing). This is in some contradiction to the usual accuracy scientists are faced with in usual reports. Otherwise, it would somehow be easy to solve this problem by having a scientific resource to cite while mentioning only the general procedure of Freeze-Pump-Thaw.
You will find several resources while searching for an instruction regarding Freeze-Pump-Thaw (e.g. a document of Mark Lonergan, a web page hosted by the Oregon State University or one hosted by the University of California).[3,4,1] The mentioned resources do describe the procedure in a pleasant way while they suffer a concluding and scientific form to be printable on the one hand, and nameable on the other hand.
The procedure in a lab will take about 20 minutes for a usual amount of cycles. The following enumeration shows how to deal with this specific but easily manageable process.
(1) Link an evacuated Schlenk tube to a Schlenk line, fill it with the required liquid (with inert gas counterflow) and seal it. (2) Apply vacuum to the hose (keeping the tube valve closed) and freeze the liquid; you often do this by immersing the tube into liquid nitrogen. (3) After freezing (checked via upending the tube), open the tube valve to let the vacuum pass; no gas bubbles may be visible, the liquid must stay frozen. (4) After about three minutes (when a reasonable vacuum is established) close the tube valve and stop cooling. (5) Subsequently, put the whole tube in a warm (not hot) water bath and let the frozen liquid thaw; gas bubbles will evolve. When no more gas is formed, close the valve and take the sealed tube out of the warm bath. Figure 1 describes one full cycle of degassing.

The following figure 2 is a graphical analogue to the one above with the difference that it shows the tubes after each step; therefore, a state zero exists.
Normally, you perform a minimum of four cycles, whereby this depends on how much gas evolves at the end of each cycle, and how clean the resulting mixture has to be. While performing the degasification, there are some risks. You should pay special attention at step number (5); at this step, bursting of the glass is common. It’s not absolutely clear if the filling level of the tube affects the possibility of destroying the glass at this step; to be on the safe side regarding this aspect, the tube should only be filled to 50 % of its volume (normally, a liquid requires more volume when frozen). It is also important to let the frozen mixture thaw slowly (without boiling hot water or heat guns), and to immerse the tube completely into the water instead of only letting the bottom part of the tube melt; a stirring bar might accelerate this step, but the risk of breakage is higher if done carelessly. Unfortunately, the possibility of breakage is also present at other stages (e.g. letting the frozen mixture thaw by itself without a warm bath between number (4) and (5), or freezing the liquid unevenly at number (2) because of handling multiple tubes, what can be quite a challenge with four or more Schlenk tubes).


Figure 2   A graphical demonstration

Although there exist some problems and risks of destroying glassware, for many scientists Freeze-Pump-Thaw is still the number one procedure to degas a liquid. Other methods as ultrasonic or membrane-assisted ones need expensive labware and consumables; that makes them possibly interesting for mass production, but rather not for millimole scale research applications.




Resources
(1)     Bode, J. How to Degas Solvents http://chem.chem.rochester.edu/~nvd/howtodegas.html (accessed May 11, 2014).
(2)     Wikipedia. Degasification http://en.wikipedia.org/wiki/Degasification (accessed May 11, 2014).
(3)     Lonergan, M. Freeze-Pump-Thaw Degassing of Liquids; Washington, 2003; p. 1.
(4)     Beaudry, C. M. STANDARD OPERATING PROCEDURE: Freeze-Pump-Thaw Degassing of Liquids http://chemsafety7.science.oregonstate.edu/content/sop-freeze-pump-thaw-degassing-liquids (accessed May 11, 2014).

Kommentare

  1. Anonym11.1.15

    This is a useful and interesting article, thanks.

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