# Dispersion in Chromatography Columns - Experimental Validation of the Van Deemter Equation > Page 91

In figure 23
the values of (H_{min}_{.}) are plotted against solute diffusivity
and it is seen that the independence of (H_{min.})
on diffusivity is largely confirmed. Close examination, however, shows that
neither of the lines for the two solutes are completely horizontal with the
baseline, but the dependence of (H_{min})
on diffusivity is extremely small for the solute benzyl acetate. The slight
slope of the line for the solute hexamethylbenzene might well result from the
fact that either the (A) term is not completely independent of the diffusivity
(D_{m}) as shown by the results
in figure 21, or the resistance to mass transfer in the stationary phase *does*
make a small but significant contribution to the value of (H).

The curves
relating the optimum velocity with solute diffusivity are shown in figure 24
and it is seen that the straight lines predicted by the Van Deemter equation
are realized for both solutes. It should be noted that similar treatment of the
Knox equation does not predict that values of H_{min} should be independent of the solute diffusivity
neither does it predict that (u_{opt})
should vary linearly with solute diffusivity. This is strong evidence,
supporting the validity of the Van Deemter equation, as opposed to the Knox
equation.

** **** **

*J.Chromatogr*.,**270**(1983)64.

**Figure 24.
Graph of Optimum Velocity against Solute Diffusivity**