Air - Diffusion Coefficients of Gases in Excess of Air

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Fick's first law states that a substance will flow from a region with high concentration to a region of low concentration . Fick's law can be expressed as

J = -D*dφ/dx [1]

where

J = diffusion flux - the amount of substance that flows through an unit area per unit time [mass or mol /(m²s)]
D = diffusion coefficient [m²/s]
dφ = change in concentration of substance [mass or mol/m³ ]
dx = change in length [m]

For simple one-dimensional transport, the diffusion coefficient describes the time–rate of change of concentration . The diffusion coefficient varies from substance to substance and with temperature and pressure.

Figure and table below shows the diffusion coefficients, D ₁₂, for binary gas mixtures where air is in large excess.

Air - Diffusion Coefficients of Gases in Excess of Air
Gases in large excess of air		Diffusion coefficient, D ₁₂, [cm²/s] at atmospheric pressure and given temperatures
Gases in large excess of air		0 °C	20 °C	100 °C	200 °C	300 °C	400 °C
Name	Formula	32 °F	68 °F	212 °F	392 °F	572 °F	752 °F
Argon	Ar	0.167	0.189	0.289	0.437	0.612	0.81
Methane	CH4		0.21	0.321	0.485	0.678	0.899
Carbon monoxide	CO		0.208	0.315	0.475	0.662	0.875
Carbon dioxide	CO2		0.16	0.252	0.39	0.549	0.728
Hydrogen	H2	0.668	0.756	1.153	1.747	2.444	3.238
Water vapour	H2O	0.219	0.242	0.399	0.638	0.873	1.135
Helium	He	0.617	0.697	1.057	1.594	2.221	2.933
Sulfur hexafluoride	SF6			0.15	0.233	0.329	0.438

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