Guide
Liquid oxygen (LOX): temperature density, why it is blue and magnetic, and why it is dangerous
Liquid oxygen (LOX, formula O₂) boils at −182.96 °C at atmospheric pressure, has a density of about 1,141 kg/m³ and expands roughly 860 times to gas at 21 °C. It does not burn, but it makes almost everything else burn fiercely. Oil, grease, asphalt and clothing soaked in LOX can ignite or explode on impact.
- Boiling point
- −182.96 °C
- Liquid density
- About 1
- Expansion ratio
- About 1:843 at 15 °C and 1:860 at 21 °C
- Critical point
- −118.57 °C
- Indian specification
- IS 309:2023 brings liquid oxygen into scope
Liquid oxygen is the form in which most of India's oxygen travels in bulk, from air separation plants to steelworks, glass furnaces and hospital tanks. It is pale blue, slightly denser than water, and drawn to a magnet. It is also the most concentrated oxidiser in general industrial use: a fire that starts in liquid oxygen or in air enriched by it burns hotter and faster than the same fire in air, and is far harder to put out. Physical figures below are from the NIST Chemistry WebBook; the gas-phase data sheet is oxygen technical data.
Liquid oxygen properties
| Property | Value | Source |
|---|---|---|
| Formula | O₂ (trade names LOX, LO₂) | — |
| Molar mass | 31.999 g/mol | NIST |
| Normal boiling point, 1.013 bar | −182.96 °C (90.19 K) | NIST |
| Freezing (triple) point | −218.79 °C, 1.46 mbar | NIST |
| Critical point | −118.57 °C, 50.43 bar | NIST |
| Liquid density at boiling point | about 1,141 kg/m³ | NIST saturation table |
| Specific gravity of liquid (water = 1) | about 1.14 | From NIST density |
| Gas density, 0 °C and 1 atm | 1.429 kg/m³ | NIST |
| Liquid-to-gas expansion | about 1:843 at 15 °C, about 1:860 at 21 °C | Computed from NIST densities |
| Latent heat of vaporisation | about 213 kJ/kg | NIST |
| Liquid Cp at boiling point | about 1.70 kJ/kg·K | NIST saturation table |
| Colour | pale blue | — |
| Magnetic behaviour | paramagnetic | — |
| UN number and class | UN 1073, Class 2.2, subsidiary 5.1 | UN Model Regulations |
Useful conversions follow directly from these numbers. One tonne of liquid oxygen is about 876 litres of liquid and about 700 m³ of gas at 0 °C, or about 740 m³ at 15 °C. A 46.7 litre cylinder filled to 150 bar holds roughly 7 m³, so one tonne of liquid is the gas content of about 100 such cylinders.
The critical temperature explains why liquid oxygen is never sold in an ordinary cylinder. Above −118.57 °C no pressure will keep oxygen liquid, so a cylinder at room temperature always holds gas, and liquid has to be kept cold in vacuum-insulated vessels. The same applies to nitrogen and argon. Carbon dioxide and nitrous oxide, with critical temperatures above 30 °C, behave quite differently.
Why is liquid oxygen blue and magnetic?
The colour comes from light absorbed by pairs of oxygen molecules colliding in the liquid. Each pair removes a little red and yellow light, so a deep layer of LOX looks pale blue. It is a property of the liquid; oxygen gas is colourless.
The magnetism comes from the electron structure of O₂, which has two unpaired electrons. That makes oxygen paramagnetic, and liquid oxygen is attracted strongly enough to bridge the poles of a powerful magnet. Industry uses this every day: paramagnetic and magneto-dynamic analysers measure oxygen purity, and IS 309:2023 added them to its test methods, harmonised with BS 4364:1993.
Rules that matter
Liquid oxygen is not toxic and, unlike liquid nitrogen, it does not asphyxiate. Its danger is fire. Ignition temperatures fall, flames burn faster and hotter, and materials that will not sustain a flame in air burn completely in oxygen. The industry reference is EIGA Doc 04, Fire hazards of oxygen and oxygen-enriched atmospheres, and the threshold used across the industry is 23.5 % oxygen by volume: above that, the atmosphere is treated as oxygen-enriched.
A spill of liquid oxygen enriches the air around it in two ways. The cold, dense vapour hangs at ground level, and anything porous the liquid soaks into holds oxygen at concentrations far above that threshold. Three practical consequences follow.
Is liquid oxygen dangerous? The enrichment fire risk
- Clothing Fabric that has been in oxygen-enriched air, or splashed with liquid, burns fiercely if it meets a spark or a cigarette. Anyone so exposed should keep away from ignition sources and air their clothing thoroughly before going near a flame.
- Ground surfaces Asphalt, tar and wood saturated with liquid oxygen can ignite or detonate under impact, for example a dropped tool or a vehicle wheel. AIGA 030/13, the Asian edition of the EIGA storage document, requires the floor under the equipment, and an area of at least 1 m radius around the tanker filling coupling, to be concrete or another non-flammable, non-porous material.
- Drains and pits Liquid that runs into a drain or trench vaporises there and leaves an enriched pocket. Storage layouts keep oxygen leak points away from drains and below-ground openings for this reason.
The wider workplace picture, including how an enriched atmosphere is detected, is on oxygen enrichment and deficiency in the workplace.
Rules that matter
Oil and grease are the classic ignition source in oxygen systems. A hydrocarbon film in contact with oxygen under pressure, or with liquid oxygen, can ignite spontaneously when the gas is compressed suddenly, for example when a valve is opened fast and the gas slams into a dead end. The heat of compression is enough. Once the oil burns, the fire can spread into the metal itself.
The rules are therefore absolute, and they apply to liquid and gaseous oxygen equally:
The oil and grease rule
- No oil or grease on any valve, regulator, fitting or hose used in oxygen service, and no oily hands or gloves on them.
- Only lubricants and sealants approved for oxygen service, and only where the equipment maker specifies them.
- Equipment cleaned for oxygen service before first use, following EIGA Doc 33 or an equivalent procedure. For pipework, see oxygen pipeline cleanliness.
- Valves opened slowly, so that pressure rises gradually downstream.
Why the same rule applies to flashback arrestors and torch equipment is explained on oil, grease and flashback arrestor elements. The gas-side hazards are summarised on oxygen safety data explained.
Storing liquid oxygen: VIE tanks
Bulk liquid oxygen is stored in a vacuum-insulated evaporator, or VIE: an inner stainless steel vessel holding the liquid, an outer jacket, and an evacuated space packed with insulating powder between them. A pressure-building coil keeps the tank at working pressure, and an ambient-air vaporiser turns liquid into gas for the plant or hospital pipeline.
AIGA 030/13 covers static vacuum-insulated storage of liquid oxygen, nitrogen and argon at users' premises, from 1,000 to 125,000 litres water capacity. Its main requirements are:
| Requirement | What the document asks |
|---|---|
| Location | Safety distances measured from the outer shell, and from leak points such as the fill coupling and relief valve outlets; an extra distance applies from oxygen leak points |
| Ground | Concrete or other non-flammable, non-porous surface under the equipment and for at least 1 m around the fill coupling |
| Relief | Set pressures matched to the maximum permissible operating pressure; where a change-over valve serves two relief devices, at least one must always be open to the tank at full bore |
| Testing | Relief valves tested to EIGA Doc 24; bursting discs may need periodic replacement |
| Vaporisers | Checked for ice build-up; downstream pipework protected against brittle fracture |
In India a static tank of this kind is a pressure vessel under the Static and Mobile Pressure Vessels (Unfired) Rules 2016, administered by PESO. IS 309:2023 clause 6.1 itself points to those Rules, and to the Gas Cylinders Rules 2016, for packing and transport. PESO's own acetylene plant layout rules keep cryogenic storage vessels at least 15 m from an acetylene plant. Container types smaller than a VIE are compared on cryogenic cylinders, dewars and tanks.
Liquid oxygen against gaseous oxygen
Liquid and cylinder oxygen are the same molecule at different temperatures, and quality is specified the same way. IS 309:2023 brought liquid oxygen into its scope and aligned it with Grade 2 (N2.5): 99.5 % minimum, with water vapour at 2.0 ppm, carbon dioxide and carbon monoxide at 0.5 ppm and hydrocarbons at 20 ppm maximum, all measured in the gas phase. Because production is continuous, Annex C allows a sample from any one production day in a week to stand for that week, plus a full test at start-up and after a stoppage. Medical liquid oxygen sits outside IS 309 and is governed by the Indian Pharmacopoeia; that chain is described on liquid medical oxygen in India.
The practical difference is logistics. Liquid suits continuous, high demand delivered by tanker into a VIE. Cylinders suit intermittent or modest demand. KIGL fills industrial and medical oxygen cylinders at its own PESO-approved works at Noonmati and Howrah; it also operates an air separation plant and supplies liquid oxygen. Cylinder supply is on the oxygen page.
Rules that matter
Rules to apply
- Treat every liquid oxygen spill as a fire hazard first and a cold hazard second.
- Keep oil, grease, asphalt, wood and fabric away from liquid oxygen and from cold oxygen lines.
- Never seal liquid oxygen in a container or between valves without a relief path.
- Stand VIE tanks on concrete, never on tarmac, and keep the fill area clear of drains.
- Open oxygen valves slowly, every time.
Liquid oxygen — frequently asked questions
What is the temperature of liquid oxygen?
At atmospheric pressure liquid oxygen sits at its boiling point, −182.96 °C (90.19 K), from the NIST Chemistry WebBook. In a pressurised storage tank it is warmer: the boiling point rises with pressure, to about −153 °C at 10 bar on NIST saturation data. Oxygen freezes at −218.79 °C and cannot exist as a liquid above its critical temperature of −118.57 °C.
What is the density of liquid oxygen?
About 1,141 kg/m³ at the normal boiling point, so one litre weighs about 1.14 kg and liquid oxygen is slightly denser than water. That figure is from the NIST saturation table (1,141.3 kg/m³ at 1.010 bar). One tonne of liquid oxygen occupies about 876 litres and produces about 700 m³ of gas at 0 °C and 1 atm.
Is liquid oxygen flammable?
No, oxygen itself does not burn. It is an oxidiser, UN 1073 with subsidiary hazard 5.1, and in liquid form it is the most concentrated oxidiser most industries handle. Materials that burn slowly in air burn fiercely in oxygen-enriched air, and organic materials saturated with liquid oxygen, including oil, grease, asphalt and fabric, can ignite or detonate from a blow.
Is liquid oxygen dangerous?
Yes, in three ways: fire, cold and pressure. Spills enrich the surrounding air above the 23.5 % oxygen that EIGA treats as oxygen-enriched; the liquid at −183 °C causes cold burns and embrittles carbon steel; and boil-off at about 860 times the liquid volume will burst any container without a relief path. It is not an asphyxiant, but it is a far greater fire risk.
Why is liquid oxygen blue?
Because it absorbs a little red and yellow light. In the liquid, oxygen molecules are close enough for pairs of them to absorb light together, which removes part of the red end of the spectrum and leaves a pale blue colour. Gaseous oxygen is too dilute for the effect to be seen. Liquid nitrogen, by contrast, is colourless.
Is liquid oxygen magnetic?
Yes. Oxygen is paramagnetic because each O₂ molecule has two unpaired electrons, and liquid oxygen poured between the poles of a strong magnet hangs there as a bridge. The same property is used to measure oxygen: IS 309:2023 added paramagnetic and magneto-dynamic analysers as purity methods, harmonised with BS 4364.
References
- Asia Industrial Gases Association. (2013). Storage of cryogenic air gases at users' premises (reproduced with EIGA permission) (AIGA 030/13).
- Bureau of Indian Standards. (2023). Oxygen, compressed gas and liquid — Specification (IS 309:2023, Fifth Revision).
- European Industrial Gases Association. (n.d.). Cleaning of equipment for oxygen service (EIGA Doc 33). https://www.eiga.eu/publications/
- European Industrial Gases Association. (n.d.). Fire hazards of oxygen and oxygen-enriched atmospheres (EIGA Doc 04). https://www.eiga.eu/publications/
- 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 oxygen (saturation tables)] https://webbook.nist.gov/cgi/cbook.cgi?ID=C7782447
- Static and Mobile Pressure Vessels (Unfired) Rules 2016, Petroleum and Explosives Safety Organisation
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