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 boiling point.
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:

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. See
www.engineeringtoolbox.com/stp-standard-ntp-normal-air-d_772.html
en.wikipedia.org/wiki/Normal_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.

Indices of refraction are taken from www.kayelaby.npl.co.uk/general_physics/2_5/2_5_7.html 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 emtoolbox.nist.gov/Wavelength/Documentation.asp, 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