Liquid nitrogen and liquid helium are two common cryogens found in laboratories housing sophisticated analytical instruments. Both exist as harmless gases at room temperature but have very low boiling points. Nitrogen has a boiling point of
[Latex]-196^0C[/Latex] and helium has a still lower boiling point of [Latex]-269^0C[/Latex] which is just 4 degrees the above absolute zero temperature. Such materials at extremely low temperatures pose unique hazards and care needs to be exercised for mishap- free handling.
As the boiling points are well below room temperatures these cryogens may vaporize and boil violently at room temperatures. This feature can cause splashing when transferring from one container to another.
The liquids on evaporation produce colourless and odourless vapours but because of their extreme low temperature they result in condensation of moisture present in the air to form dense clouds of fog. This fog can be seen around equipment or containers holding extremely low temperature materials. The dense fog should not be mistaken for the cryogens but indicates release of vapour from extremely low temperature sources. Such dense fog is not toxic but can reduce visibility to almost zero and can result in serious laboratory accidents.
Due to large expansion ratios from the liquid to gas phase the cold vapour can lead to condensation and freezing of atmospheric moisture and blocking of openings of dewars. Due to pressure buildup of trapped gas inside it can lead to sudden explosions unless pressure relief valves are provided.
As cryogens boil vigourously at room temperature splashing can result when transferring from one container to another. It is advisable to use an appropriate transfer line or a wide mouth funnel. Rapid transfer can result in splashing as the receiving container is at a higher temperature. The transfer rate should be kept low so that the recipient container gets time to cool down. The lower end of the transfer line should not be high but should remain below the liquid level while transferring.
Cryogenic liquids evolve large volumes of gas on vaporization. Liquid nitrogen,for example, vaporizes to nearly Awareness of hazards associated with liquid cryogens helps prevent labora700 volumes on expansion to gaseous state. This rapid expansion readily displaces the oxygen in the air. In case of leakages and spills the affected area should be evacuated to prevent asphyxiation when oxygen levels fall below 20 percent. Installation of oxygen level monitors is advised in .areas housing cryogens
It is true that flames and high temperature surfaces cause burns but you can also get burn injuries through contact with materials at very low temperatures. In addition your skin can stick to the cold surface and when you try to move away the skin will get torn and stick to the surface. It is therefore strongly advised to avoid contact with such surfaces at extremely low temperatures
Liquid nitrogen and liquid helium do not pose combustion hazards but on rapid evaporation the extremely cold vaour can condense oxygen in the air and liquid oxygen is highly reactive and combustible. Materials coming in contact with liquid oxygen can ignite vigourously and even cause explosions. The space around cryogens should be kept free of flammable solvents, oils, grease, etc and any hot surface or heat generating equipments. Such materials can cause spontaneous fires and explosions in oxygen enriched environments.
Cryogens can be used safely provided stipulated precautions are taken in their handling and storage. Such precautions will be discussed in a subsequent article.