distribution but does not indicate how the distribution can be managed or controlled. To do this, it is necessary to identify how the magnitude of (K) and (Vs) are controlled. In general, (K) is usually determined by the nature and strength of the intermolecular forces between the solute and the two phases. The availability of the stationary phase (the magnitude of (Vs)) is largely determined by the geometry of the stationary phase. Factors Affecting the Magnitude of the Distribution Coefficient (K) The magnitude of (K) is determined by the relative affinity of the solute for the two phases. Those solutes interacting more strongly with the stationary phase will exhibit a larger distribution coefficient and will be retained longer in the chromatographic system. Molecular interaction results from intermolecular forces of which there are three basic types

distribution coefficient of n-pentanol between carbon tetrachloride, toluene, n-heptane, n-chloroheptane and pure water together with mixtures of n-heptane and n-chloroheptane and pure water. These systems were chosen because the solvents were immiscible and virtually mutually insoluble in each other and thus interactions in one phase were not influence significantly by the presence of the other. The results they obtained are shown in Figure 11. The linear relationship between the distribution coefficient and the volume fraction of the respective solvent is clearly established. The distribution coefficient of n-pentanol between water and pure carbon tetrachloride was found to be about 2.2 and similar distribution coefficient for n-pentanol was predicted by calculation to be obtainable from a mixture containing 82%v/v chloroheptane and 18%v/v of n-heptane. The experiment for toluene was repeated using a mixture of 82 %v/v chloroheptane and 18% n-heptane mixture in place of carbon

taken into account and equation (35) shows that equation (36) is grossly over simplified. From equation (35), a more accurate expression for solute retention would be       Vr = VI(m) + KVI(s) +  K1Vp(1) + K2Vp(2) +  K3VS(A)        (37) where, (K) is the distribution coefficient of the solute between the moving phase and the static portion of the interstitial volume, (K1) is the distribution coefficient of the solute between the moving phase and the static pore contents, (Vp(1)), (K2) is the distribution coefficient of the solute between the moving phase and the static pore contents, (Vp(2)), and (K3) is the distribution coefficient of the solute between the moving phase and the available stationary phase (VS(A

nbsp;                             (28)   where (KAB) is the distribution coefficient of a solute between a mixture of solvents (A) and (B) and a stationary phase, (KA) is the distribution coefficient of a solute between the pure  solvent (A) and a stationary phase, (KB) is the distribution coefficient of a solute between the pure  solvent (B) and a stationary phase, and (b) is the volume fraction of solvent (A) in the mixture.   The distribution coefficients are referenced to the solvent mixture and not the stationary phase and are thus are the inverse of the distribution coefficients employed in the chromatography elution equation.      Now the separation ratio (a(1)(2)) of a pair of solutes, both of

phase consisting of a binary mixture of solvents, as the retention volume will be inversely proportion to the elutive capacity of the mobile phase it will be also inversely proportional to the volume fraction of either component providing there is no strong association between the components. This was experimentally demonstrated by Katz et al. (35) who employed a liquid/liquid distribution system using water and a series of immiscible solvent mixtures as the two phases and measured the absolute distribution coefficient of a solute for different mixtures. The solute they used was n-pentanol and the immiscible solvent consisted of mixtures of n-heptane and chloroheptane, n-heptane and toluene and n-heptane and heptyl acetate.  The two phase system was thermostatted at 25oC and, after equilibrium had been established, the concentration of solute in the two phases was determined by GC analysis. The results they obtained are shown in figure 46. It is seen that linear relationships between

The retention of a solute in a chromatographic system is determined firstly, by the magnitude of the distribution coefficient of the solute between the two phases and secondly, by the amount of stationary phase available to the solute for interaction. This is fully discussed in Plate Theory and Extensions of this series. In addition, the mechanism of distribution has been considered exclusively on the basis of molecular interactions in The Mechanism of Chromatographic Retention . However, the distribution coefficient in chromatography is an equilibrium constant and, consequently, it can be treated rationally by conventional thermodynamics. It follows, that the distribution coefficient can be expressed in terms of the standard energy of solute exchange between the phases employing the traditional and well established Arrhenious relationship,