Liquid nitrogen

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File:Liquid nitrogen tank.JPG
A tank of liquid nitrogen, used to supply a cryogenic freezer (for storing laboratory samples at a temperature of about -150 Celsius).
File:Nitrogen ice cream 0020.jpg
Liquid nitrogen may be used to prepare "home-made" ice cream, as these students are doing.
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Liquid nitrogen in a cup

Liquid nitrogen (liquid density at the triple point is 0.807 g/mL) is the liquid produced industrially in large quantities by fractional distillation of liquid air and is often referred to by the abbreviation, LN2. It is pure nitrogen, in a liquid state. Liquid nitrogen has the UN number 1977.

Liquid nitrogen boils at -196 °C (Expression error: Missing operand for *. ), and is a cryogenic fluid which is potentially capable of causing rapid frostbite on contact with living tissue. When appropriately insulated from ambient heat, liquid nitrogen can be stored and transported, for example in vacuum flasks. Here, the very low temperature is held constant at -196 °C by slow boiling of the liquid, resulting in the evolution of nitrogen gas. Depending on the size and design the holding time of vacuum flasks ranges from a few hours to a few weeks.

Liquid nitrogen can easily be converted to the solid by placing it in a vacuum chamber pumped by a rotary vacuum pump.[1] Liquid nitrogen freezes at -210 °C (Expression error: Missing operand for *. ). Despite its reputation, liquid nitrogen's efficiency as a coolant is reduced by the fact that it boils immediately on contact with a warmer object, enveloping the object in insulating nitrogen gas. This effect is known as the Leidenfrost effect and applies to any liquid in contact with an object significantly hotter than its boiling point. More rapid cooling may be obtained by plunging an object into a slush of liquid and solid nitrogen, than into liquid nitrogen alone. That said, liquid nitrogen alone is sufficient for most applications.

Applications

Liquid nitrogen is a compact and readily transported source of nitrogen gas without pressurization. Further, its ability to maintain temperatures far below the freezing point of water makes it extremely useful in a wide range of applications, primarily as an open-cycle refrigerant, including:

Safety

Since the liquid to gas expansion ratio of this substance is 1:694,[3] a tremendous amount of force can be generated when liquid nitrogen boils off for whatever reasons. In a well-known accident in 2006 at Texas A&M University, the pressure-relief devices of a tank of liquid nitrogen were sealed with brass plugs. As a result, the tank failed catastrophically, and exploded. The force of the explosion was sufficient to propel the tank through the floor/ceiling immediately above it.[4]

See also

References

  1. Umrath, W. (1974) Cooling bath for rapid freezing in electron microscopy. Journal of Microscopy 101, 103–105.
  2. Wainner, Scott (2003). The Book of Overclocking: Tweak Your PC to Unleash Its Power. No Starch Press. pp. p. 44. ISBN 188641176X. Unknown parameter |coauthors= ignored (help)
  3. "Information Specific to Liquid Nitrogen". Harvard University. 30 Jul 03. Check date values in: |date= (help)
  4. Brent S. Mattox. "Investigative Report on Chemistry 301A Cylinder Explosion" (reprint). Texas A&M University.

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