Gas densities and optical properties
For an ideal gas, properties such as density and refractive index
scale as p/T. This approximation is valid if atomic spacing is
large compared with atomic size and the gas temperature is far above its
This may not be the case, for example, for CO2
or acetylene at room temperature.
Tables of gas properties must be used with caution, since they vary in
reference temperature and pressure:
In these pages, we follow the early density-effect papers of Sternheimer,
Barger, and Seltzer, who tabulate the gas density at NTP, (20°, 1 atm).
Note that the Handbook of Chemistry and Physics reference
conditions are 25°, 1 atm.
Standard Temperature and Pressure (STP) for a gas is defined by IUPAC as
0° C (273.15 K), 100 kPa. This differs from an earlier definition that used
1 atm (101.325 kPa) as the pressure reference. It also is different in
Imperial and USA systems of units.
Normal Temperature and Pressure (NTP) for a gas is defined
as 20 ° C, 1 atm. This standard is often more
convenient, e.g. for testing fans.
Other standards abound, and the definitions are not necessarily consistent.
Indices of refraction are taken from
where they are given for dry gases at 0° and 1 atm, evaluated at the
mean of the Na D doublet, 589.3 nm.
A calculator for the index of air can be found at
where the wavelength, temperature, pressure, humidity, and
CO2 content can
also be varied.
Index and dispersion for some gases can be found at refractiveindex.info,
The reference pressure is usually not given
(assumed to be 1 atm?). T is usually (but not
always) 0° C. In some case, e.g. Kr, only UV data are given.
Written 2014 April 1
web pages as of Apr 2014