Guide
Liquid argon: temperature density, boiling point, hazards and how industry uses it
Liquid argon boils at −185.85 °C (87.30 K) at atmospheric pressure, has a density of about 1,395 kg/m³ and expands roughly 840 times into gas at 21 °C. It is non-flammable and non-toxic but is shipped as UN 1951, refrigerated liquid, because its vapour is a dense asphyxiant and the liquid causes cold burns.
- Boiling point
- −185.85 °C
- Liquid density
- About 1
- Expansion ratio
- About 1:826 at 15 °C and 1:840 at 21 °C
- Narrow liquid range
- Triple point −189.34 °C at 0.689 bar
- Share of air
- 0.934 % by volume
Liquid argon is the bulk form of the shielding gas behind most TIG and much MIG welding, and of the stirring and refining gas used in stainless steelmaking. It is recovered from air, where it makes up 0.934 % by volume, and only a cryogenic air separation plant can make it. Liquid argon is colourless, non-flammable and non-toxic, but its vapour is heavier than air and it is shipped as a refrigerated liquid under UN 1951. Physical figures below are from the NIST Chemistry WebBook; the gas-phase data sheet is argon technical data.
Liquid argon properties
| Property | Value | Source |
|---|---|---|
| Formula | Ar (trade names LAr, LAR) | — |
| Molar mass | 39.948 g/mol | NIST |
| Normal boiling point, 1.013 bar | −185.85 °C (87.30 K) | NIST |
| Triple point | −189.34 °C (83.81 K), 0.689 bar | NIST |
| Critical point | −122.46 °C, 48.63 bar | NIST |
| Liquid density at boiling point | about 1,395 kg/m³ | NIST saturation table |
| Specific gravity of liquid (water = 1) | about 1.40 | From NIST density |
| Gas density, 0 °C and 1 atm | 1.784 kg/m³ | NIST |
| Relative density of gas to air | 1.38 | NIST |
| Liquid-to-gas expansion | about 1:826 at 15 °C, about 1:840 at 21 °C | Computed from NIST densities |
| Latent heat of vaporisation | about 161 kJ/kg | NIST |
| Liquid Cp at boiling point | about 1.12 kJ/kg·K | NIST saturation table |
| Liquid viscosity at boiling point | about 260 µPa·s | NIST saturation table |
| UN number and class | UN 1951, Class 2.2 | UN Model Regulations |
How cold is liquid argon compared with other cryogens?
| Cryogen | Boiling point at 1 atm | Liquid density | Expansion at 21 °C |
|---|---|---|---|
| Liquid nitrogen | −195.8 °C | about 806 kg/m³ | about 1:694 |
| Liquid argon | −185.85 °C | about 1,395 kg/m³ | about 1:840 |
| Liquid oxygen | −182.96 °C | about 1,141 kg/m³ | about 1:860 |
All values from NIST. Two features stand out. Argon's boiling point lies between nitrogen's and oxygen's, only 2.9 K below oxygen, which is why separating argon from oxygen takes a dedicated column with many stages. And liquid argon is the densest of the three, so a liquid argon vessel of a given size is much heavier than the same vessel of liquid nitrogen. Floors, trolleys and lifting gear must be rated for it.
The second feature is argon's very narrow liquid range at atmospheric pressure. Its triple point, at 0.689 bar absolute, is close to atmospheric, so a liquid argon line or tank pulled below about 0.7 bar absolute can freeze solid argon in valves and vaporisers. The argon technical data sheet covers this in more detail.
Rules that matter
Liquid argon is not flammable, not an oxidiser and not toxic. Its dangers are the same physical ones as liquid nitrogen, with one difference that makes argon worse in a confined space: the gas is 1.38 times as dense as air even when warm, so it settles and stays. The three hazards are:
Is liquid argon dangerous?
- Asphyxiation One litre of liquid gives about 0.84 m³ of gas at 21 °C. EIGA guidance treats any fall below about 21 % oxygen as hazardous, notes that performance is impaired without the person noticing below 18 %, and that fainting can follow within minutes below 11 %. Argon from a leak or a stuck relief valve collects in pits, trenches, tank bottoms and the inside of pipes being purged. The detail, including welding bay ventilation, is on argon safety data explained.
- Cold burns and embrittlement At −186 °C the liquid and anything it has cooled will freeze skin and make carbon steel brittle.
- Pressure Like every cryogen, liquid argon boils continuously. Containers need a vent or relief path, and liquid must never be trapped between closed valves.
AIGA 030/13, which covers static liquid oxygen, nitrogen and argon storage at users' premises from 1,000 to 125,000 litres, treats argon and nitrogen together: the extra distance applied to oxygen leak points is zero for both, because neither feeds a fire. The general rules for container types are on cryogenic cylinders, dewars and tanks.
Uses of liquid argon
Bulk supply for welding
Argon for welding is specified by ISO 14175 Group I1 at 99.99 % minimum purity. A shop using a few cylinders a week is best served by high-pressure cylinders. A fabrication plant with many TIG and MIG stations running every shift often moves to liquid: a liquid cylinder or a small vacuum-insulated tank, an ambient-air vaporiser, and a pipeline to the stations. Liquid gives more gas per delivery and fewer cylinder changes, and the gas leaves the vaporiser very dry because almost every impurity has frozen out at cryogenic temperature. The trade-off is boil-off when demand stops, for example over a long shutdown. Choosing the gas for each process is covered on choosing a shielding gas.
AOD steelmaking
Argon-oxygen decarburisation (AOD) is the main route for refining stainless steel. A mixture of oxygen and argon is blown into the molten metal through tuyeres. Diluting the oxygen with argon lowers the partial pressure of carbon monoxide, which lets carbon be removed while far less chromium is lost to the slag. The argon share is increased as the carbon falls. The process uses large, continuous volumes, so steel plants take argon as liquid or by pipeline from an on-site air separation unit rather than in cylinders. The wider set of argon uses, including ladle stirring, lamps, glazing and laboratory work, is on uses of argon gas.
Other liquid uses
- Silicon crystal growth and titanium and zirconium processing, where an inert atmosphere at very high purity is needed in volume.
- Additive manufacturing and powder production, where argon blankets reactive metal powders.
- Physics research, where large volumes of very pure liquid argon serve as a detection medium.
Where liquid argon comes from
Argon is recovered only by cryogenic air separation. Air is compressed, cleaned, cooled and distilled; argon concentrates in the middle of the low-pressure column, between the nitrogen at the top and the oxygen at the bottom, and is drawn off as crude argon and refined. Because air holds only 0.934 % argon against 20.95 % oxygen, the argon output of a plant is a small fraction of its oxygen output, and argon recovery is normally fitted only to larger plants. How the column works is described on how oxygen is made.
How KIGL supplies argon
KIGL fills argon in high-pressure cylinders at its own PESO-approved works at Noonmati, Guwahati, and Howrah, Kolkata, in two grades published as KIGL specifications. KIGL does not supply liquid argon. Customers whose demand has outgrown cylinders should discuss bulk liquid supply with a cryogenic producer. Cylinder grades, sizes and colour coding are on the argon page, and first aid and spill information is on the argon safety data sheet.
Rules that matter
Rules to apply
- Ventilate at low level wherever liquid argon is stored, decanted or vaporised indoors, and monitor oxygen near the floor.
- Never enter a pit, vessel or trench where argon may have collected without testing the atmosphere.
- Rate floors, lifts and trolleys for the weight of liquid argon, about 1.4 kg per litre.
- Do not pull vacuum on a liquid argon line; it can freeze solid.
- Never seal liquid argon in a container or between valves without a relief path.
Liquid argon — frequently asked questions
What is the temperature of liquid argon?
At atmospheric pressure liquid argon is at its boiling point, −185.85 °C or 87.30 K, from the NIST Chemistry WebBook. In a pressurised liquid cylinder or tank it is a little warmer, because the boiling point rises with pressure. It cannot be colder than its triple point, −189.34 °C, without freezing, and cannot remain liquid above its critical temperature of −122.46 °C at any pressure.
What is the density of liquid argon?
About 1,395 kg/m³ at the normal boiling point, so a litre weighs about 1.4 kg, nearly twice the weight of a litre of liquid nitrogen (806 kg/m³) and heavier than liquid oxygen (1,141 kg/m³). The figure is from the NIST Chemistry WebBook saturation data for argon. That is why a full liquid argon cylinder is far heavier than the same cylinder filled with nitrogen.
Is liquid argon colder than liquid nitrogen?
No, it is about 10 K warmer. Liquid nitrogen boils at −195.8 °C, liquid argon at −185.85 °C and liquid oxygen at −182.96 °C, all NIST figures. Argon's boiling point sitting between nitrogen and oxygen is why it gathers in the middle of an air separation column and is drawn off there, then purified separately in its own column.
What is UN 1951?
UN 1951 is the United Nations dangerous-goods number for argon, refrigerated liquid. It is Class 2.2, a non-flammable, non-toxic gas, the same class as compressed argon (UN 1006) and refrigerated liquid nitrogen (UN 1977). Liquid oxygen is different: UN 1073 carries an oxidiser subsidiary hazard. The class reflects that the dangers of liquid argon are asphyxiation, cold and pressure, not fire.
How much argon gas does one litre of liquid argon make?
About 0.84 m³ at 21 °C and 1 atm, or about 0.83 m³ at 15 °C, from NIST liquid and gas densities. A 180 litre liquid cylinder, emptied, would release about 150 m³, so a single spill or a stuck relief valve in a closed welding bay can lower oxygen substantially. Argon vapour is 1.38 times as dense as air at room temperature and settles in pits and low areas.
Why do large welding shops take argon as a liquid?
Because liquid is far more compact than gas. One litre of liquid argon holds the gas of roughly 840 litres at room temperature, so a liquid cylinder or a small tank with a vaporiser replaces a large bank of high-pressure cylinders, with fewer changeovers. The trade-offs are boil-off losses when demand stops, and the need for a cryogenic supplier.
References
- Asia Industrial Gases Association. (2013). Storage of cryogenic air gases at users' premises (reproduced with EIGA permission) (AIGA 030/13). https://asiaiga.org/uploaded_docs/AIGA%20030_13_Storage%20of%20cryogenic%20air%20gases%20at%20users'%20premises.pdf
- EIGA Safety Info 19/26 https://www.eiga.eu/publications/
- European Industrial Gases Association. (n.d.). Hazards of inert gases and oxygen depletion (EIGA Doc 44). https://www.eiga.eu/publications/
- International Organization for Standardization. (2008). Welding consumables — Gases and gas mixtures for fusion welding and allied processes (ISO 14175:2008).
- IS 5760 Argon, compressed and liquid — Specification, Bureau of Indian Standards
- Linstrom, P. J., & Mallard, W. G. (Eds.). (n.d.). NIST Chemistry WebBook (NIST Standard Reference Database 69). National Institute of Standards and Technology. [thermophysical properties of argon (saturation tables)] https://webbook.nist.gov/cgi/cbook.cgi?ID=C7440371
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