Safe Operation of Vacuum System for Avoiding Danger of Liquid Oxygen Sessler Group Safety Talk Ren-Tsung Wu October 23rd, 2012
What Is Liquid Oxygen? • It’s from air. Oxygen makes up ~20%of the earth’s atmosphere. • The BP of oxygen is 90 K, so if air is cooled below 90 K, we start to condense liquid oxygen. • Liquid oxygen is characterized by a light blue colored liquid. • Due to oxygen’s strong oxidizing properties, it is highly explosive in the presence of organic compounds, including the grease used for the system.
Starting Up Vacuum System 1. Wear personal protective equipments: Lab Coat, Gloves, Safety Goggles/Glasses. 2. Ensure that the manifold stopcocks, as well as all joints, are properly greased. 3. Check to see that the vacuum trap is dry and clean, and then insert a trap into an empty Dewar flask. 5. Ensure that all valves are closed. Attach a pressure gauge to the Schlenkline 6. Turn on the vacuum pump. Watch the pressure gauge to ensure that the pressure is dropping (the ultimate pressure of my Edwards pump: 2.0 x 10-3 mbar / ~2 x 10-6 atm). 7. Fill the Dewar with enough liquid nitrogen. Wrap the top with a towel, glass wool, or aluminum foil.
Shutting Down 1. Personal protective equipments are still needed. 2. Carefully remove the liquid nitrogen-containing Dewar flask. 3. Make sure no bluish liquid present inside the cold trap. Turn off vacuum pump and vent the manifold. 4. Once the Dewar flask is taken down, allow the system to thawto room temperature before safely disposing of any solvents in the cold trap. NEVER vent the system to air or turn off vacuum pump when any portion of the system is still at liquid nitrogen temperature. This could condense oxygen which could result in a serious explosion!
What should we do if we have generated liquid oxygen? If the liquid nitrogen Dewar is removed and the liquid in the traps has a blue color or if any liquid is observed, then assume you have liquid oxygen in the trap. Immediately alert others to the danger, and evacuate the area. Place a blast shield around the trap and remove any nearby organic materials. Quickly vent the system and lower the hood sash completely. Rapidly leave the vicinity of the lab and warn others not to enter. After the trap has warmed to room temperature, consider it still dangerous. Liquid oxygen may no longer exist, but some potential peroxides may be formed.
What should we do if we have generated liquid oxygen? (cont’d) Pour the remaining liquid into a clean beaker and flush assembled trap 5 times with water. Do this behind the blast shield and with the sash lowered. Use a potassium iodide test strip to examine the solution. If a purple color forms on the test paper, peroxides are present. In that case, reduce solution with sodium thiosulfate or sodium sulfite before disposing of waster. Make sure your research advisor is informed of the situation.