# Dispersion in Chromatography Columns - Experimental Validation of the Van Deemter Equation > Page 87

Figure 22
discloses, in more detail, the factors that control the magnitude of the (A)
term and the effect of particle diameter on the mobile phase velocity at which
the Giddings equation simplifies to the Van Deemter equation. For very small
particles (*e.g.* 3 m) the Giddings
equation simplifies to the Van Deemter at a velocity of about 0.2 cm/sec but
for the larger particles (*e.g.,* 10 m)
it occurs at about 1 cm/sec. However, at the optimum velocity, irrespective of
the particle diameter, the contribution from the coupling term is very small
and so the Van Deemter equation can be used with confidence in column design.

** **

*J.Chromatogr*.,**270**(1983)62.

** Figure
23. Graph of the (B) Term against Diffusivity**

In summary,
the Data of Katz *et al.* shows some slight dependence of the (A) term on
D_{m}_{,} (which can be explained on the basis of the calculations
given above). However, as a result of the curve fitting procedure to the
equation it is shown not to be dependent on
(u) and thus, supports the Van Deemter equation as opposed to the Knox
equation. It does, however, also support the idea that the Van Deemter equation
is a special case of the Giddings equation.