# Dispersion in Chromatography Columns - The Van Deemter Equation > Page 54

Now, if it is
assumed that d_{f} << d_{p }(which in practice will always be true)
and (x) is close to zero (*i.e*.
the diffusivities of the solute in the two phases is about the same) which for
liquid chromatography will also be true, then,

Furthermore, when k'>> 1, that is, for well retained peaks, and taking (g) as 0.6

(36).

Under these
circumstances, it is seen that the optimum velocity is directly proportional to
the solute diffusivity in the mobile phase and inversely proportional to the
particle diameter of the packing. The quality of packing, also plays some
part, but to a very less significant extent. It is now possible to determine
the factors that control the magnitude of H_{min}.
Substituting the function for u_{opt}
from equation (34) in equation (33) an expression for H_{min} is obtained.