Ficks First and Second Laws

In 1822 Fourier derived an equation for heat conduction. Realizing that the process of transferring heat by induction is analogous to the process of diffusion and that both are due to random molecular motion, in 1855 Pick adapted the Fourier equation to describe diffusion. 

Fick s first law is about the change of concentration (of solute) with respect to coordinates  

Physical Chemistry of Macromolecules Basic Principles and Issues, Second Edition. By S. F. Sun ISBN 0-471-28138-7 Copyright 2004 John Wiley Sons, Inc. 

/ is called the flux (or diffusion flux) or flow (in kilograms per square meter second) and is the rate of transfer per unit of section, c is the concentration of solute (or particles) (in grams per millihter or grams per 100 mL), x is the space coordinate measured normal to the section where the particles cross, and D is the diffusion coefficient (in square centimeters per second). The direction of diffusion is opposite to that of increasing concentration hence, a negative sign is given in the equation. 

Pick s second law describes the change of concentration with time  

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Equation 1 applies to stationary states, and Equation 2 to nonsta-tionary states of diffusion. Equation 2 can be derived from Equation 1 by considering the rate of accumulation of diffusate at a given point in the medium it reduces to Equation 3 when the diffusion coeflBcient is a constant (9). If the latter condition is satisfied, the diffusion process is said to be ideal, or Fickian, and Equations 1 and 3 represent Ficks first and second laws of diffusion respectively. 

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