Liquid O2 condensation

Title

Liquid oxygen condensation in culture tube

Thursday, October 1, 2015

Situation

A researcher was attempting to condense ammonia (1 mL) from a lecture cylinder into an 8 mL, round-bottomed, glass culture tube, using liquid nitrogen (LN2) as the cooling source. The ammonia gas was being introduced through a teflon tube into the vial. The vial was uncapped. After ca. 1 mL of liquid had collected, the tube was capped. Soon after removal from the cooling bath the tube detonated. Post incident analysis revealed that the ammonia tank had only been partially opened and that liquid oxygen had preferentially condensed from the air into the culture tube. This resulted in an extremely large pressure buildup and detonation upon warming.

Suggestions

  • Ammonia condenses at -33 °C. There's no need to use LN2 (-196 °C) when a dry ice +acetone/IPA bath (-78 °C) would suffice. This is the case for many things, not just ammonia—we are too quick to reach for LN2 for many applications (freeze/pump/thaw degassing, for example).

  • Make sure to have a trap to capture ammonia that did not condense. Do not let ammonia gas go into the fume hood as it will oxidize anything in it (including the monitor for air flow).

  • Use of LN2 always introduces an added layer of risk because it has a lower boiling point (-196 °C) than oxygen (-183 °C). An empty vessel exposed to an oxygen headspace, most typically simply open to air, will condense/liquify the oxygen. If there is an oxidizable material already in the flask (e.g., solvent and/or other organic material), the liquid oxygen may react rapidly even at the LN2 cold bath temperature. When using LN2 cooling, never use an experimental set-up that contains air or oxygen.

  • Whenever you're going to be sealing something with the potential for evaporation/expansion, a quick Henry's Law calculation should be carried out to ensure that the system won't reach a critical pressure and rupture the container. Carry out the experiment in a fume hood behind appropriate shielding, wear appropriate PPE for the situation, and alert your lab-mates what you are planning to do.

  • Whenever you make an observation in the laboratory that just doesn't "feel" right, trust your instinct, pause, ensure safety, and more thoughtfully assess the situation before proceeding.