The Thermodynamics of Chromatography - Thermodynamics Basics > Page 5

                                  (5)

                    

   and                               (6)

Furthermore, If (Vr) is the retention volume per ml of stationary phase (i.e., V'r = V'o(r)) then , ln(Vs) = 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 (DHo) can be calculated from the slope and the standard entropy (DSo), 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.