STANDARD OPERATING PROCEDURE: Freeze-Pump-Thaw Degassing of Liquids  

Oregon State University, Department of Chemistry
Standard Operating Procedure (SOP): Freeze-Pump-Thaw Degassing of Liquids (ver. 1.0)
Author: Christopher M. Beaudry (christopher.beaudry@oregonstate.edu) 
Safety Web Inception Date: 4/13/2011
Revised: N/A

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A. Introduction

A1. General: Removal of dissolved gases (e.g., oxygen) from liquids is necessary for a variety of sensitive reactions. Alumina-based solvent purification systems (SPS) commonly do not deoxygenate solvents. One of the most reliable methods for deoxygenation of laboratory solvents on a small scale is the freeze-pump-thaw technique. This technique is generally more rigorous in removal of dissolved gases than simple sparging techniques.

B. Hazard identification, PPE, required safety equipment, and emergency response

B1. Hazards: The most significant hazard presented by the freeze-pump-thaw technique is the shattering of the glass vessel. To minimize the chances of accidental vessel rupture, only vessels made from thick-walled glass designed to resist pressure changes should be used. This method is commonly performed in a thick-walled Schlenk or equivalent flask. The use of appropriate engineering controls and personal protective equipment, together with good laboratory housekeeping, will help to minimize the chance of an accidental explosion or implosion of the vessel. Given the significant hazards presented by possible explosions, any researcher performing this technique MUST NOT WORK ALONE.

B2. Personnal protective equipment (PPE): Appropriate eye and skin protection must be worn during all stages of the operation, as follows:

(a) Eye protection: chemical splash goggles or safety glasses that meet ANSI standard Z-87.1 must worn whenever handling pyrophoric chemicals. Ordinary prescription eye glasses will not provide the necessary level of protection unless they also meet the same ANSI standard. A face shield, worn over safety eye wear, is required in addition if there is a possible risk of explosion.

(b) Skin and body protection: thick gloves should be worn when handling sealed vessels. A fully-buttoned knee-length laboratory coat must be worn to protect the body. In addition, fully enclosed shoes which cover the entire foot (with no holes in the top) must be worn.

B3. Engineering controls and other safety equipment: All manipulation of sealed vessels must be conducted inside a well vented fume hood behind a blast shield. Before starting work, clear the fume hood of any unnecessary equipment or chemicals. An eyewash/safety shower station should be within a ten second travel time of the site of the experiment. Familiarize yourself with the location of this important safety equipment and check that the eyewasher is functioning (pass water through it until it runs clear) and that the safety shower passed a recent inspection (within last 12 months). Also before starting work, know the location of the nearest fire extinguisher and fire alarm pull station and check that the fire extinguisher passed recent inspection and that it is not empty.

B4. Emergency response: Shattering of the vessel is the most likely accident, be well prepared for this eventuality and also know the correct response in the event of a glass and liquid spill. If the shattering happened behind the blast shield, turn off the vacuum line, and clean the area. If hazardous liquids exited the hood, call EH&S to alert them of a chemical spill as appropriate. If the shattering threw glass outside the shield and if anyone was injured, administer first aid as appropriate. Call Emergency Medical Services (911) if necessary.

 

C. Procedure for safely conducting a freeze-pump-thaw operation

C1. General: Before commencing a freeze-pump-thaw procedure, ensure that the fume hood to be used is clear of clutter. Appropriate PPE must be worn and other safety equipment as detailed above should be in place and in good working order (Section B). The control of an inert atmosphere and vacuum is best handled by a dedicated gas and vacuum double manifold system provided with exit bubblers and gas and vacuum delivery lines. At all times during operation, the inert gas delivery system must have at least one open bubbler protected vent point to prevent over pressurization and the possibility of explosion.

C2. Set-up: Place the solvent (or solution) in a Schlenk flask. Make sure the stopcock is closed. Be careful not to use more than 50% of the volume of the flask because overfilled flasks frequently shatter during this process. Hook it up to a Schlenk line (leave the attached hose on vacuum throughout this procedure).

C3. Freeze: Freeze the liquid. Liquid nitrogen is usually best for this. Before freezing make sure that the environment in the flask is free of oxygen to prevent condensing liquid oxygen upon freezing.

C4. Pump: When the solvent is completely frozen, open the stopcock to vacuum and pump off the atmosphere for 10-30 minutes, or until the space above the frozen liquid is completely evacuated. Seal the flask to create a static vacuum above the frozen liquid.

C5. Thaw: Thaw the solvent until it just melts using a tepid water bath. You will see gas bubbles evolve from the solution. Try not to disturb the liquid. Note: Letting the frozen solvent thaw by itself, or using a container of water that melts only the bottom of the frozen solvent may cause the vessel to break. Warning: Use of direct heat (e.g. a heat gun) may cause the vessel to explode violently.

C6. Repeat: Remove the water bath and repeat steps C3 to C5 at least twice more for a total of three freee-pump-thaw cycles. Fill the flask with inert gas a seal. The solvent is ready to use.

 

 
This chemical safety advisory document was prepared solely for the use of researchers affiliated to Oregon State University. As stated above (Section A), the content is designed to inform on good working practices and it is not intended to replace hands-on practical training in the techniques described. It is the responsibility of the Principal Investigator to see to it that his/her co-workers are properly trained and informed on hazard management, including the possibility of customization of the information herein as appropriate to meet specific needs. Neither Oregon State University, nor any of its employees (including the author), makes any warranty, express of implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, or represents that its use would not infringe privately owned rights. Reference herein to any specific commerical product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by Oregon State University.