Separation of variables method
We will assume that the solution for solute concentration C(y,t) can be separated into two functions, one depending only on position, the other on time, i.e.
If we substitute this into Fick's Second Law then
We then separate the y- and t-dependent terms,
For the first equality in this expression to be valid, each side must be constant. We set this constant to be (—λ2). We now solve each side of the equation separately. The time-dependent equation
can be integrated to yield
where ⊖0 is a constant of integration. The spatial equation
is integrated twice, giving
where A′ and B′ are integration constants. Combining these expressions gives
where A = A′⊖0 and B′ = B′⊖0. In general, there are a multitude of solutions to Fick's Second Law, all of which have this form, but with different values of A, B and λ. Therefore, the most general solution is given by the sum of these
This solution is generally valid for diffusion in one dimension. Similar expressions can be derived for cylindrical or spherical symmetry.
In order to proceed further we need to define the initial and boundary conditions appropriate to the problems at hand. We will consider only one example, namely diffusion out of a plate with an initially uniform concentration, and the concentration fixed at the surface of the plate.