The Fick’s second law of diffusion states that the rate of change of concentration at a point in space is proportional to the second derivative of concentration with space.

$\frac{∂N}{∂t}=\frac{∂}{∂x} \bigg(D \frac{∂N}{∂x}\bigg)$

where

D is diffusion coefficient

N is volume concentration

x is the distance in cm

t is diffusion time in seconds.