Cryogens are materials with boiling points below \(– 90^0C\) which find applications in laboratories requiring extremely low temperatures. Such materials boil well below room temperature and on sudden exposure to room temperature can result in uncontrolled boiling.
Cryogenic materials require special handling, storage and transportation. For common laboratory applications a vacuum jacketed dewar is sufficient. A loose fit dust cap permits the escape of gas formed and at same time prevents atmospheric moisture from plugging the cap. The contents of small size dewar can be transferred by simply pouring while taking the prescribed safety precautions.
Commonly used cryogens in laboratories are liquid nitrogen and liquid helium. Solid carbon dioxide or dry ice strictly speaking cannot be termed as a cryogen but deserves mention here as it finds a number of applications in laboratory cooling. Apart from cooling cryogens find applications in advanced spectroscopic techniques such as NMR and Fluorescence spectroscopy.
Solid Carbon Dioxide or Dry Ice
Solid carbon dioxide or dry ice finds use as a cooling agent in laboratories. It sublimates directly to gas at \(-78^0 C\). Though not very toxic it should not be touched with unprotected hands as it can result in instantaneous frostbite.
A mixture of dry ice and acetone serves as a cold bath having temperature of \(-78^0 C\) which can be used for low temperature reaction studies or for condensing of solvents in Rotary evaporators.
Dry ice also finds extensive use in preservation and transportation of biological specimens and temperature sensitive chemicals. The key advantage is its low cost and direct conversion to liquid state without leaving any watery mess.
Liquid nitrogen exists as a liquid at extremely low temperatures. It has a boiling point of \(– 196^0 C\). It finds major applications in preservation of biological tissues, body fluids such as blood and reproductive cells, sperms and other genetic resources for subsequent analysis. It is also used as a low-temperature shield around the primary liquid helium used in superconducting magnets used in NMR spectrometers and imaging systems.
Certain materials normally do not fluoresce at room temperature but become active fluorescence emitters at cryogenic temperatures. The fluorescent bands of such materials could be broad and poorly resolved at room temperatures but significant improvements result in sensitivity and resolution at cryogenic temperatures. Typically some aromatic and uranium compounds show such improvements at cryogenic temperatures.
Liquid helium is the only known material which can exist in liquid state at absolute zero temperature. It has a boiling point of \(4^0K\) which is very close to the absolute zero. When it transforms to a super fluid at temperatures below \(2^0 K\) it acquires free flow and high thermal conductivity which is extremely high in comparison to the conductivity of metallic conductors. Liquid helium is therefore widely used for inducing superconductivity in NMR magnets.
NMR spectroscopy gives us an insight into the structure of complex molecules and imaging is a great boon to the radiologists. Such developments would not have been possible without use of cryogens.