Fick's law of diffusion is a law of the diffusion of a gas across a fluid membrane. It was derived by Adolf Fick in the year 1855.

The law gives rise to the formula:

<math>\mathrm{Rate\ of\ diffusion} = \frac{K A (P_2 - P_1)}{D}<math>

It states that the rate of diffusion of a gas is:

• Constant for a given gas at a given temperature by an experimentally determined factor, <math>K<math>
• Proportional to the surface area over which diffusion is taking place, <math>A<math>
• Proportional to the difference in partial pressures of the gas across the membrane, <math>P_2 - P_1<math>
• Inversely proportional to the distance over which diffusion must take place, or in other words the thickness of the membrane, <math>D<math>.

Fick's law is also important in radiation transfer equations. However, in this context it becomes inaccurate when the diffusion constant is low and the radiation becomes limited by the speed of light rather than by the resistance of the material the radiation is flowing through. In this situation, one can use a flux limiter.

The exchange rate of a gas across a fluid membrane can be determined by using this law together with Graham's law.

See also

• Gas exchange
• lung
• alveoli

nl:Wet van Fick sl:Difuzijski zakon