Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4.[dead link][1] Liquid nitrogen is often referred to by the abbreviation, LN2 or "LIN" or "LN" and has the UN number 1977.
At atmospheric pressure, liquid nitrogen boils at −196 °C (77 K; −321 °F) and is a cryogenic fluid which can cause rapid freezing on contact with living tissue, which may lead to frostbite. 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 77 K 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.[2] Liquid nitrogen freezes at 63 K (−210 °C; −346 °F). Despite its reputation, liquid nitrogen's efficiency as a coolant is limited by the fact that it boils immediately on contact with a warmer object, enveloping the object in insulating nitrogen gas. This effect, known as the Leidenfrost effect, 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.
Nitrogen was first liquefied at the Jagiellonian University on 15 April 1883 by Polish physicists, Zygmunt Wróblewski and Karol Olszewski.[3]
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[edit] Uses
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:
- in cryotherapy for removing unsightly or potentially malignant skin lesions such as warts and actinic keratosis
- as a coolant for CCD cameras in astronomy
- to store cells at low temperature for laboratory work
- in cryogenics
- as a source of very dry nitrogen gas
- for the immersion freezing and transportation of food products
- for the cryopreservation of blood, reproductive cells (sperm and egg), and other biological samples and materials
- as a method of freezing water pipes in order to work on them in situations where a valve is not available to block water flow to the work area
- in the process of promession, a way to dispose of the dead
- for cooling a high-temperature superconductor to a temperature sufficient to achieve superconductivity
- for the cryonic preservation in the hope of future reanimation.
- to preserve tissue samples from surgical excisions for future studies
- to shrink-weld machinery parts together
- as a coolant for vacuum pump traps and in controlled-evaporation processes in chemistry.
- as a coolant to increase the sensitivity of infrared homing seeker heads of missiles such as the Strela 3
- as a coolant to temporarily shrink mechanical components during machine assembly and allow improved interference fits
- as a coolant for computers[4]
- in food preparation, such as for making ultra-smooth ice cream.[5]
[edit] Safety
Since the liquid to gas expansion ratio of nitrogen is 1:694 at 20 °C (68 °F), a tremendous amount of force can be generated if liquid nitrogen is rapidly vaporized. In an incident in 2006 at Texas A&M University, the pressure-relief devices of a tank of liquid nitrogen were malfunctioning and later sealed. As a result of the subsequent pressure buildup, the tank failed catastrophically and exploded. The force of the explosion was sufficient to propel the tank through the ceiling immediately above it.[6]
Because of its extremely low temperature, careless handling of liquid nitrogen may result in cold burns.
As liquid nitrogen evaporates it will reduce the oxygen concentration in the air and might act as an asphyxiant, especially in confined spaces. Nitrogen is odorless, colorless and tasteless, and may produce asphyxia without any sensation or prior warning.[7] A laboratory assistant died in Scotland in 1999, apparently from asphyxiation, possibly caused by liquid nitrogen spilled in a basement storage room.[8]
Oxygen sensors are sometimes used as a safety precaution when working with liquid nitrogen to alert workers of gas spills into a confined space.[9]
Vessels containing liquid nitrogen can condense oxygen from air. The liquid in such a vessel becomes increasingly enriched in oxygen (boiling point 90 K; −183 °C; −298 °F) as the nitrogen evaporates, and can cause violent oxidation of organic material.[citation needed]
[edit] See also
[edit] References
- ^ "Dielectric Constants". http://www.apgsensors.com/ltr2/access.php?file=pdf/dielectric-constants.pdf.
- ^ Umrath, W. (1974). "Cooling bath for rapid freezing in electron microscopy". Journal of Microscopy 101: 103–105. doi:10.1111/j.1365-2818.1974.tb03871.x.
- ^ William Augustus Tilden (2009). A Short History of the Progress of Scientific Chemistry in Our Own Times. BiblioBazaar, LLC. p. 249. ISBN 1103358421. http://books.google.com/books?id=8SKrWdFLEd4C&pg=PA249.
- ^ Wainner, Scott; Robert Richmond (2003). The Book of Overclocking: Tweak Your PC to Unleash Its Power. No Starch Press. pp. 44. ISBN 188641176X.
- ^ Liquid Nitrogen Ice Cream Recipe, March 7, 2006
- ^ Brent S. Mattox. "Investigative Report on Chemistry 301A Cylinder Explosion" (reprint). Texas A&M University. http://ucih.ucdavis.edu/docs/chemistry_301a.pdf.
- ^ British Compressed Gases Association (2000) BCGA Code of Practice CP30. The Safe Use of Liquid nitrogen Dewars up to 50 litres. ISSN 0260-4809.
- ^ Inquiry after man dies in chemical leak, BBC News, October 25, 1999.
- ^ Liquid Nitrogen - Code of practice for handling. United Kingdom: Birkbeck, University of London. 2007. http://www.bbk.ac.uk/so/policies/liqn2. Retrieved 2012-02-08.
