Properties: Liquid Nitrogen
- Liquid Nitrogen has a boiling point of -195.8°C
- Volume of expansion liquid to gas (at 15°C, 1 atm.) = 682.1
- Sg = 0.808 (at -195.8°C).
- Density of liquid (normal boiling point, 1 atm.) = 0.807 g/cc
- Colourless, odourless liquid similar in appearance to water.
Known or Expected Hazards
a) Temperature Related
- The extremely low temperature of the liquid can cause severe burn-like damage to the skin either by contact with the fluid, surfaces cooled by the fluid or evolving gases. The hazard level is comparable to that of handling boiling water.
- The low temperature of the vapour can cause damage to softer tissues e.g. eyes and lungs but may not affect the skin during short exposure.
- Skin can freeze and adhere to liquid nitrogen cooled surfaces causing tearing on removal.
- Soft materials e.g. rubber and plastics become brittle when cooled by liquid nitrogen and may shatter unexpectedly.
- Liquid oxygen may condense in containers of liquid nitrogen or vessels cooled by liquid nitrogen. This can be extremely hazardous because of the pressure rise on the slightest degree of warming above the boiling point of oxygen ( -183°C ) and the possibility of explosive reaction with oxidisable material.
- Thermal stress damage can be caused to containers because of large, rapid changes of temperature.
b) Vapour Related
- Large volumes of nitrogen gas are evolved from small volumes of liquid nitrogen (1 litre of liquid giving 0.7 m3 of vapour) and this can easily replace normal air in poorly ventilated areas leading to the danger of asphyxiation. It should be noted that oxygen normally constitutes 21% of air. Atmospheres containing less than 10% oxygen can result in brain damage and death (the gasping reflex is triggered by excess carbon dioxide and not by shortage of oxygen), levels of 18% or less are dangerous and entry into regions with levels less than 20% is not recommended.
- Oxygen condensed into leaking containers can explode on heating following resealing or blockage with ice
For an untrained person, the risk of injury is moderate with cryogenic burns the most likely injury. However in exceptional circumstances when large amounts of material are spilled in an enclosed space, asphyxiation may be fatal.
Who is likely to be injured?
The most likely injury is to the person using the material although following major spillage all inhabitants of a room may be affected.
- Liquid nitrogen should never be used except in a well- ventilated area. This is especially true when filling a warm container or transfer tube or inserting a warm object, as large volumes of nitrogen gas are evolved. The safe volume of liquid nitrogen stored or used in any enclosed space is described later.
- Only containers or fittings (pipes, tongs etc.) that have been designed specifically for use with cryogenic liquids may be used as non-specialised equipment may crack or fail. In particular, food type vacuum flasks must not be used as they can implode resulting in flying glass fragments.
- All glass Dewars must be protected against the possibility of flying glass fragments, arising from failure by mechanical or temperature stress damage, by sealing all exposed glass either in an insulated metal can or by wrapping with adhesive tape.
- Warm Dewars should be filled slowly to reduce temperature shock effects and to minimise splashing. Storage Dewars should not be not be over-pressured when filling a globular Dewar. The minimum pressure required to maintain a flow of liquid should be used.
- Containers of liquid nitrogen must be suitably vented and unlikely to block due to ice formation.
- Care must be taken to avoid the formation of liquid oxygen in cold-traps that are open to air or the increase of liquid oxygen content in a flask of liquid nitrogen that has been cold for a long period. (Liquid oxygen has a blue water-like appearance). Solid carbon dioxide (dry ice) should be considered as an alternative coolant in situations where liquid oxygen could accumulate. However, most liquid nitrogen containers are closed except for a small neck area and the nitrogen vapour issuing from the surface forms a barrier which keeps air away from the liquid thus preventing oxygen contamination.
- Skin contact with either liquid nitrogen or items cooled by liquid nitrogen should be avoided as serious burns may occur. Care must be taken with gloves, wrist-bands or bracelets which may trap liquid nitrogen close to the skin.
- Personal Protective Equipment, especially safety glasses, must be worn to protect against splashes, freezing vapour, failure of glass apparatus or brittle failure of items cooled by liquid nitrogen.
Personal Protective Equipment
The following equipment should be worn when handling or dispensing liquid nitrogen:
- Face shield or safety glasses.
- Dry insulated gloves when handling equipment that has been in contact with the liquid. NB there is dispute over the advisability of wearing gloves while handling liquid nitrogen because there is a possibility that gloves could fill with liquid and therefore prolong hand contact which would make burns more severe. If gloves are worn they should be loose fitting and easily removed.
- Lab coat or overalls are advisable to minimise skin contact and also trousers over shoe/boot tops to prevent shoes filling in the event of a spillage.
Avoidance of Oxygen Depletion/ Asphyxiation
Liquid nitrogen should normally be used only in a well-ventilated area. However, there may be occasions e.g. transport of Dewars in lifts, when this may not be possible. To avoid the danger of oxygen depletion, the following should noted:
- Safe limit in an unventilated space: Calculate the room volume in m3 and the max volume of nitrogen in m3 (this can be found from the volume of liquid in litres x0.7). If the volume of nitrogen amounts to >0.15 of the room volume, special precautions or ventilation are required.
- Spillage during filling: during filling assume that 10% of the final volume may be spilled.
- Loss during storage: the boil off loss from a 5l Dewar is expected to be 0.2l per day.
- Transport of liquid nitrogen in lifts. To avoid in possible risks from nitrogen boil off during, for example, a prolonged period of lift breakdown, Dewars of liquid nitrogen must not be accompanied in lifts. Rather, two people should be assigned to transport the Dewars, one to load and one to receive at the destination floor. To prevent others from entering the lift, fitted straps should be pulled across the entrance.
New users of liquid nitrogen should receive instruction in its use from experienced members of the academic or technical staff. Formal training is required before use of the School liquid nitrogen dispensing facility.
Level of Risk Remaining
There remains a significant risk in using liquid nitrogen from the inadvertent condensation of oxygen into a closed system. It is recommended that whenever possible some other coolant is used e.g. solid carbon dioxide/liquid traps or baths - the preferred liquids for such baths are isopropanol or glycols. It is recommended that such baths be used in preference to liquid nitrogen when long term storage is envisaged.
Properties: Solid Carbon Dioxide
- Sublimation point -78.5°C
- Melting point -56.6°C
- Volume of expansion solid to gas ~900
Hazards and Precautions
Apart from being unable to condense oxygen, hazards associated with solid carbon dioxide are similar to those described for liquid nitrogen i.e. temperature related and vapour related. In operation, similar precautions should be taken against cold burns and asphyxiation
- For brief, localised contact with cold material - flush the area with tepid water. (Water is used because of its high heat capacity.) Obtain First Aid assistance.
- More prolonged contact will require medical treatment. Call a First Aider.
- Following a large spillage of liquid nitrogen, evacuate the area and call for help.
- If you suspect that someone is suffering from asphyxiation, do not enter the affected area alone - call for help. Remove the victim to the fresh air and call for a First Aider. If the victim is unconscious, call 6516 2691 or 6516 1616 (after office hours) first.