Stationary-mobile phase equilibria

Here we consider briefly the effect of chemical reactions on two types of phase equilibrium ion exchange resin-liquid solution equilibrium stationary medium-mobile liquid solution equilibrium, as in chromatography (Section 7.1.5). For the first case, the resin phase is generally stationary in a fixed bed however, the resin phase may also be mobile. 

There is an extensive literature in this area. Helfferich (1962, 1995) has provided a comprehensive description of the effect of chemical reactions on ion exchange equilibria. Three types of chemical reactions of interest are complex formation in solution complex formation with the ion exchanger ionization/dissociation in the solution. 

The expressions for the selectivity of A over B are available in Helfferich (1962). The net result in this case is that 

Acidic carboxylic acids naphthenic acids Shell Chemical Co. copper-nickel separation 

Acid chelating hydroxyoximes LDC63, LIX64N, LK65N, LIX70 Henkel Corporation copper and nickel extraction 

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Finite speed of equilibration, inability of solute molecules to truly equilibrate in one theoretical plate, the C term, present in all chromatographic columns. This term is also called the resistance to mass transfer term and, in more contemporary versions, consists of two mass transfer coefficients Cs, where S refers to the stationary phase, and Cm, where M refers to the mobile phase. Equilibrium is established between M and S so slowly that a chromatographic column always operates under nonequilibrium conditions. Thus, analyte molecules at the front of a band are swept ahead before they have time to equilibrate with S and thus be retained. Similarly, equilibrium is not reached at the trailing edge of a band, and molecules are left behind in S by the fast-moving mobile phase (23). 

Another source of drift results from incomplete mobile phase equilibrium with the stationary phase or incomplete mixing in the mobile phase preparation. Such drift always occurs on changing the composition of the mobile phase and to eliminate this drift, mobile phase should be pumped through the column-detector system until a stable base line is obtained. This problem can be... 

The distribution of a solute, S, between the mobile phase and stationary phase can be represented by an equilibrium reaction... 

In ion-exchange chromatography (lEC), the mobile phase modulator is typically a salt in aqueous solution, and the stationary phase is an ion-exchanger. For ddnte conditions, the solute retention faclor is commonly found to be a power-law function of the salt uormahty [cf. Eq. (16-27) for ion-exchange equilibrium]. 

The concentration profiles of the solute in both the mobile and stationary phases are depicted as Gaussian in form. In due course, this assumption will be shown to be the ideal elution curve as predicted by the Plate Theory. Equilibrium occurs between the mobile phase and the stationary phase, when the probability of a solute molecule striking the boundary and entering the stationary phase is the same as the probability of a solute molecule randomly acquiring sufficient kinetic energy to leave the stationary phase and enter the mobile phase. The distribution system is continuously thermodynamically driven toward equilibrium. However, the moving phase will continuously displace the concentration profile of the solute in the mobile phase forward, relative to that in the stationary phase. This displacement, in a grossly... 

Now, as equilibrium is maintained in the plate (p) by definition, the mass (dm) will bfe distributed between the two phases, resulting in a solute concentration change of dXm(p) in the mobile phase and dXs(p) in the stationary phase. Then,... 

The profile of the concentration of a solute in both the mobile and stationary phases is Gaussian in form and this will be shown to be true when dealing later with basic chromatography column theory. Thus, the flow of mobile phase will slightly displace the concentration profile of the solute in the mobile phase relative to that in the stationary phase the displacement depicted in figure 1 is grossly exaggerated to demonstrate this effect. It is seen that, as a result of this displacement, the concentration of solute in the mobile phase at the front of the peak exceeds the equilibrium concentration with respect to that in the stationary phase. It follows that there is a net transfer of solute from the mobile phase in the front part of the peak to the... 

Other modes of LC operation include liquid-liquid partition chromatography (LLC) and bonded phase chromatography. In the former, a stationary liquid phase which is immiscible with the mobile phase is coated on a porous support, with separation based on partition equilibrium differences of components between the two liquid phases. This mode offers an alternative to ion exchange in the fractionation of polar, water soluble substances. While quite useful, the danger exists in LLC that the stationary phase can be stripped from the column, if proper precautions are not taken. Hence, it is typical to pre-equil-ibrate carefully the mobile and stationary phases and to use a forecolimn, heavily loaded with stationary phase 9). 

One of the most crucial influencing factors in planar chromatography is the vapor space and the interactions involved. The fact that the gas phase is present, in addition to stationary and mobile phases, makes planar chromatography different from other chromatographic techniques. Owing to the characteristic of an open system the stationary, mobile, and vapor phases interact with each other until they all are in equihbrium. This equilibrium is much faster obtained if chamber saturation is employed. This is the reason for differences in separation quality when saturated and unsaturated chambers are used. However, the humidity of the ambient air can also influence the activity of the layer and, thus, separation. Especially during sample application, the equihbrium between layer activity and relative humidity of the... 

Apart from the choice of an appropriate stationary and mobile phase, the essential problem for PLC is to attain equilibrium in a three-phase system — between the stationary, mobile, and gas phases. In a nonequilibrated system, the velocity of the mobile phase in a thicker layer (i.e., the effect of solvent evaporation) is less in a lower part of an adsorbent. Such a situation leads to the diffusion of bands and deterioration of the adjacent bands separation. This can be minimized or avoided by prerunning the plate with the mobile phase before spotting of the sample and the saturated chromatographic chambers. 

Multiple solvent fronts are also observed with mobile phases containing solvents of different strength [8,27,41]. As the mobile phase rises through the layer it becomes depleted in the component with the greatest affinity for the stationary phase. Eventually a secondary front is formed that separates the equilibrium solvent... 

Mass transfer in either the stationary or mobile phase is not instantaneous and, consequently, complete equilibrltui is not established tinder normal separation conditions. The result is that the solute concentration profile in the stationary phase is always displaced slightly behind the equilibrluM position and the mobile I se profile is similarly slightly in advance of the equilibrium position. The combined peak observed at the column outlet is broadened about its band center, which is located where it would have been for instantaneous equilibrium, provided the degree of nonequllibrluM is small. The stationary phase contribution to Mass transfer is given by equation (1.25)... 

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