Guide
Nitrogen technical data sheet: physical properties IS 1747:2023 grades, PSA versus cryogenic purity and dew point
Nitrogen (N₂, 28.013 g/mol) boils at −195.80 °C, has a gas density of 1.250 kg/m³ at 0 °C and is 0.967 times as heavy as air. IS 1747:2023 sets 4 grades from N 2.5 (99.5 %) to N 5.5 (99.9995 %). PSA plants usually deliver 95 to 99.99 %; cryogenic supply reaches N 5.0.
- Boiling point
- −195.80 °C
- Critical point
- −146.96 °C
- Gas density
- 1.250 kg/m³ at 0 °C
- Liquid expansion
- Liquid at 806 kg/m³ expands about 680 times to gas at 15 °C and about 694 times at 21 °C
- IS 1747:2023 grades
- N 2.5 ≥ 99.5 %; N 4.5 ≥ 99.995 %; N 5.0 ≥ 99.999 %; N 5.5 ≥ 99.9995 %; purity = 100 minus listed impurities
Nitrogen is the inert gas industry uses most, by volume: cheaper than argon, non-reactive with most materials at ordinary temperatures, and available either as cryogenic liquid from an air separation unit or as gas from an on-site PSA or membrane generator. It is a permanent gas, with a critical temperature of −146.96 °C, so it is sold in cylinders by pressure. The figures below are from the NIST Chemistry WebBook and from IS 1747:2023, the Indian specification for compressed and liquid nitrogen. KIGL nitrogen is supplied as IS 1747 grade N 2.5 and as a KIGL high-purity specification.
Physical properties
| Property | Value | Source |
|---|---|---|
| Formula, molar mass | N₂, 28.013 g/mol | NIST |
| Boiling point at 1.013 bar | −195.80 °C (77.35 K) | NIST |
| Triple point | −210.00 °C (63.15 K), 0.125 bar | NIST |
| Critical temperature | −146.96 °C (126.19 K) | NIST |
| Critical pressure | 33.96 bar | NIST |
| Gas density, 0 °C, 1 atm | 1.250 kg/m³ | NIST |
| Gas density, 15 °C, 1 atm | 1.185 kg/m³ | NIST |
| Relative density to air | 0.967 | NIST |
| Liquid density at boiling point | 806 kg/m³ | NIST |
| Liquid to gas expansion | about 680 at 15 °C; about 694 at 21 °C | Computed from NIST densities |
| Latent heat of vaporisation | about 199 kJ/kg | NIST |
| Specific heat Cp, gas, 25 °C | about 1.04 kJ/kg·K | NIST |
| Ratio of specific heats Cp/Cv | 1.40 | NIST |
| Thermal conductivity, gas, 25 °C | about 0.0258 W/m·K | NIST |
| Viscosity, gas, 25 °C | about 17.8 µPa·s | NIST |
| Content of dry air | 78.08 % by volume | Standard atmosphere |
| UN number | 1066 compressed; 1977 refrigerated liquid | IS 4379:2021 Table 1; UN list |
Nitrogen is "inert" only at ordinary temperatures. It forms nitrides with titanium, zirconium, lithium and magnesium, nitrides the surface of steel in heat treatment above about 500 °C, and reacts with molten metals in welding; where that matters argon is used instead. Its relative density of 0.967 means a leak mixes into a room evenly, lowering the oxygen concentration with no warning, which is the subject of nitrogen asphyxiation.
The IS 1747:2023 grades
The 2023 revision removed the "technical" and "pure" grades and replaced them with four purity grades.
| Characteristic | N 2.5 | N 4.5 | N 5.0 | N 5.5 |
|---|---|---|---|---|
| Nitrogen purity, %, min | 99.5 | 99.995 | 99.999 | 99.9995 |
| Oxygen, ppm v, max | 5,000 | 10 | 2 | 1 |
| Hydrogen, ppm v, max | — | 5 | 1 | 0.2 |
| Carbon dioxide, ppm v, max | — | 5 | 1 | 0.5 |
| Hydrocarbons as methane, ppm v, max | — | 5 | 1 | 0.5 |
| Carbon monoxide, ppm v, max | — | 5 | 1 | 1 |
| Moisture, ppm v, max | — | 14 | 2 | 1 |
| Supplied as | Gas only | Gas and liquid | Gas and liquid | Gas and liquid |
Source: IS 1747:2023 Table 1 and clause 4. The Note to Table 1 defines purity as 100 minus the sum of the listed impurities. That is a significant point: a certificate for N 2.5 that reports only oxygen at 4,000 ppm is reporting 99.6 % purity by definition, whatever argon the gas also contains. Argon, about 0.93 % of air, is not a listed impurity in any grade, so argon in nitrogen does not count against its purity under IS 1747.
A 99.9 % product, the common "high purity" of Indian trade literature, falls between N 2.5 and N 4.5 and is not an IS grade. KIGL publishes its high-purity nitrogen as a KIGL specification: at least 99.9 % with oxygen below 1,000 ppm and dew point at or below −60 °C.
Sampling under Annex G treats as one lot all cylinders filled in one shift from one manifold from one source, with 3 samples from a lot of up to 25 rising to 25 from a lot of 301 or more; every tanker or liquid cylinder is its own lot, and the lot passes only if every sample does. Contents are stated in cubic metres at 27 °C and 760 mm Hg (cl 6.2).
PSA, membrane or cryogenic: what purity each delivers
| Source | Typical purity range | Residual impurities | Dew point | Suits |
|---|---|---|---|---|
| Membrane generator | 95 to 99.5 % | Oxygen, argon, some moisture | Depends on feed-air drying | Tyre inflation, fire prevention, coarse blanketing |
| PSA on carbon molecular sieve | 95 to 99.99 % | Oxygen and argon | −40 to −70 °C with a dryer | Food packaging, blanketing, laser cutting, heat treatment |
| Cryogenic liquid nitrogen | 99.999 % and above | Trace oxygen, argon | −70 °C or drier | Electronics, pharmaceuticals, N 5.0 and N 5.5 duties |
Oxygen is the main residual in PSA nitrogen and argon passes through almost untouched, so a PSA product at 99.99 % by IS 1747's definition can carry close to 1 % argon if argon is not counted. The cost of purity in a PSA plant is flow: output at 99.99 % is well under half the output at 99 % for the same adsorber beds and feed air. The choice of purity is the subject of choosing PSA nitrogen purity, and the generators are on the PSA nitrogen plant page. Feed air quality for these plants is classed under IS/ISO 8573-1, described on compressed air quality classes.
Dew point and moisture
Moisture limits in IS 1747:2023 are in ppm by volume. At atmospheric pressure, 14 ppm (N 4.5) corresponds roughly to a −58 °C frost point, 2 ppm (N 5.0) to about −71 °C and 1 ppm (N 5.5) to about −76 °C. Dew point measured at line pressure is higher than at atmospheric pressure for the same gas, so a dew point quoted without its pressure is incomplete. IS 1747 Annex F makes the dew-formation method, with nitrogen held at 21.0 kg/cm² over a polished cooled surface, the referee in a dispute, and asks for moisture reported in grams per cubic metre.
Cylinder and pressure data
Nitrogen behaves almost ideally at cylinder pressures at room temperature: its compressibility factor is about 1.02 at 150 bar and 15 °C (NIST), so a cylinder holds slightly less than the ideal-gas figure.
| Water capacity | Fill pressure at 15 °C | Approximate content, 15 °C | Same at 27 °C (IS 1747 basis) |
|---|---|---|---|
| 46.7 L | 150 bar | about 6.8 m³ | about 7.1 m³ |
| 46.7 L | 200 bar | about 9.0 m³ | about 9.4 m³ |
| 10 L | 150 bar | about 1.5 m³ | about 1.6 m³ |
Worked examples only; the method is on gas cylinder sizes and contents in India. Under GCR 2016 rule 2(xxviii), nitrogen is one of the gases allowed in cylinders up to 3,000 litres water capacity if the diameter is within 60 cm, which is how nitrogen tubes on trailers are licensed. The cylinder is French grey shade 630 with a Graphite Black shoulder and a Bottle Green RAL 6007 band under IS 4379:2021; an air cylinder is French grey throughout, so the shoulder and band are what distinguish them.
Rules that matter
Handling notes
- Nitrogen is a simple asphyxiant: an atmosphere below 19.5 % oxygen is oxygen-deficient and below about 10 % causes collapse without warning. Monitor oxygen where nitrogen is used indoors and follow inert gas confined space entry for vessels.
- Liquid nitrogen at −196 °C causes cold burns and embrittles carbon steel; vent dewars outdoors.
- Nitrogen is used to purge acetylene plant and pipelines before start-up and maintenance; IS 15190 (Part 1) cl 12.6.3 sets the purge end point at not more than 2.5 % oxygen in the outgoing gas.
- Uses by grade are on uses of nitrogen gas, the standards on nitrogen standards, and first aid on the nitrogen safety data sheet.
Nitrogen technical data sheet — frequently asked questions
What are the grades of nitrogen under IS 1747?
IS 1747:2023 abolished the old technical and pure grades and set four: N 2.5 at 99.5 % with oxygen up to 5,000 ppm (gas only), N 4.5 at 99.995 %, N 5.0 at 99.999 % and N 5.5 at 99.9995 %, the last three with limits on oxygen, hydrogen, CO₂, hydrocarbons, CO and moisture. Purity is calculated as 100 minus the listed impurities, not measured directly.
Is 99.9 % nitrogen an IS 1747 grade?
No. It sits between N 2.5 (99.5 %) and N 4.5 (99.995 %), so a 99.9 % product must be described as a supplier specification, not as an IS 1747 grade. KIGL publishes its high-purity nitrogen this way: at least 99.9 % with oxygen below 1,000 ppm and dew point at or below −60 °C, as a KIGL specification.
What purity does a PSA nitrogen generator give?
Pressure-swing adsorption on carbon molecular sieve is usually specified between 95 and 99.99 %, with the residual mostly oxygen and argon. Output falls steeply as purity rises: a plant delivering a given flow at 99 % delivers well under half of it at 99.99 %. For N 4.5 and above with tight hydrogen, CO and moisture limits, cryogenic liquid nitrogen is the usual source.
What dew point does nitrogen need?
It depends on the use. IS 1747:2023 sets moisture at 14 ppm for N 4.5, 2 ppm for N 5.0 and 1 ppm for N 5.5, roughly −58, −71 and −76 °C frost point at atmospheric pressure. Heat treatment and electronics usually want −60 °C or drier; tyre inflation and blanketing tolerate much wetter gas. The referee method in IS 1747 is dew formation at 21.0 kg/cm².
What is the boiling point of nitrogen gas?
−195.80 °C (77.35 K) at 1.013 bar, according to the NIST Chemistry WebBook. Below that temperature nitrogen is a liquid, and it freezes at its triple point of −210.00 °C. Its critical temperature is −146.96 °C, so at room temperature nitrogen cannot be liquefied by pressure alone and is sold in cylinders as a compressed gas, by pressure.
What is the density of nitrogen, and is it lighter than air?
Nitrogen gas has a density of 1.250 kg/m³ at 0 °C and 1 atm, or 1.185 kg/m³ at 15 °C, and liquid nitrogen 806 kg/m³ at its boiling point (NIST). The gas is slightly lighter than air, at a relative density of 0.967, so at ambient temperature it mixes with air rather than separating. That is what makes it dangerous: a leak lowers the oxygen concentration in a room evenly, with no layer to see or smell. Cold nitrogen vapour from liquid is denser than air and flows to low points until it warms.
What is the expansion ratio of liquid nitrogen?
About 1 to 694 at 21 °C and 1 atm, and about 1 to 680 at 15 °C, from a liquid density of 806 kg/m³ at the boiling point. One litre of liquid nitrogen spilled or vented in a room of 20 m³ releases nearly 0.7 m³ of gas, enough to lower the oxygen concentration by about 0.7 percentage points if it mixes evenly; a dewar emptied indoors does far more.
How is the oxygen content of nitrogen measured under IS 1747?
IS 1747:2023 Annex A lists four methods for oxygen in the purer grades, including the pyrocatechol colorimetric method, which works only below 0.005 % oxygen, and the Hersch galvanic cell with ranges of 0 to 10 and 0 to 100 ppm. In a dispute, Method 2 is the referee. Moisture is referred to the dew-formation method.
References
- Bureau of Indian Standards. (2021). Identification of the contents of industrial gas cylinders (IS 4379:2021, Second Revision).
- Bureau of Indian Standards. (2023). Nitrogen, compressed gas and liquid — Specification (IS 1747:2023, Second Revision).
- EIGA — asphyxiation and inert gas documents
- IS/ISO 8573-1 Compressed air purity classes, 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. [nitrogen thermophysical properties] https://webbook.nist.gov/cgi/cbook.cgi?ID=C7727379
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