# The Thermodynamics of Chromatography - Thermodynamics Basics > Page 5

_{} (5)

and _{} (6)

Furthermore, If (V_{r}) is the retention
volume per ml of stationary phase (*i.e.*, V'_{r} = V'_{o(r)})
then , ln(V_{s}) = 0 and,

(7)

It is clear
that a graph of ln(V'_{r}) or ln(k') against 1/T must give a straight
line. From the linear curve, the standard enthalpy (DH^{o}) can be calculated from the slope and the
standard entropy (DS^{o}), from
the intercept. Curves relating ln(K), ln(V'_{r}) or ln(k') against 1/T
are called van't Hoff curves. However, to ensure linearity, the distribution
system must remain *unchanged *throughout
the temperature range examined. If the distribution system does change, then a graph relating ln(V'_{r})
against (1/T) will *not be linear *and the
curves *will no longer be* van't Hoff curves. The conditions, where curves
relating ln(V'_{r}) or ln(k') against 1/T are not linear, will be
considered later.