Key Instruction Points
A 250 ml glass flask became over pressurized and burst, spraying two laboratory workers with shards of glass.
Approximately ten grams of styrene and a minute quantity of a drying agent were immersed in liquid nitrogen to keep the contents frozen. The laboratory worker then attached the flask to a vacuum pump to evacuate the flask, without success. Thinking the flask might have developed a crack, the laboratory worker removed the flask from the vacuum line and was defrosting it under warm water in the sink, holding it and examining it, when the flask ruptured.
The best guess as to the cause of the rupture is that a small leak, perhaps a pinhole in the flask, developed while it was being frozen and that some liquid nitrogen entered the flask. When the flask was warmed, the liquid nitrogen vaporized (expansion ratio 696:1), over pressurizing the flask and leading to the explosion.
The laboratory worker holding the flask suffered from several lacerations to the face, hands, chest and abdomen. The other worker, who was standing across the room, received lacerations to the abdomen. The worker holding the flask noted shards of glass embedded in his prescription safety glasses.
The procedure was re-written such that under the same conditions, the stopcock will be unscrewed and the flask set in a catch bucket in the hood to allow the contents to warm up and vaporize, if volatile.
Appropriate eye protection helped to avoid a potentially serious eye injury. Consider shielding for operations involving vacuum or pressurization. Be aware of the potential for pressurization when working with liquid nitrogen.