Indifferent adsorption

All the known processes heretofore used for the separation and preparation of pure ergot alkaloids are completely different from the process herein described, as indifferent adsorption media used in the process of Tswett have never been used for this purpose. The possibility of separating and of preparing the very sensitive ergot-alkaloids even from impure mixtures by the chromatographic process and without the use of any chemical reagents, represents a very valuable process and a great improvement of the art. 

The point of zero charge is the pH at which net adsorption of potential determining ions on the oxide is zero. It is also termed the point of zero net proton charge (pznpc). It is obtained by potentiometic titration of the oxide in an indifferent electrolyte and is taken as the pH at which the titration curves obtained at several different electrolyte concentrations intersect (Fig. 10.5). It is, therefore, sometimes also termed the common point of intersection (cpi). The pzc of hematite has been determined directly by measuring the repulsive force between the (001) crystal surface and the (hematite) tip of a scanning atom force microscope, as a function of pH the pzc of 8.5-8.S was close to that found by potentiometic titration (Jordan and Eggleston, 1998). This technique has the potential to permit measurement of the pzc of individual crystal faces, but the authors stress that the precision must be improved. 

Idzikowski, S. (1973) Adsorption of inorganic ions on iron(III) oxide from mixtures of strong electrolytes. IV. Adsorption of silver, cupric and aluminium sulphate in presence of indifferent electrolyte. Rocz. Chem. 231-238... 

Although there is some experimental justification for the conception of a saturation concentration yet this equation containing but one variable was found by Schmidt to conform to the experimental results but very indifferently. Schmidt Zeit. PJiys. Chem. Lxxviii. 667,1912) made the additional hypothesis that the adsorption coefficient k decreases in magnitude with the amount of gas adsorbed in an exponential manner, or... 

Further, in gas-phase adsorption, the solvent for the gas particles, is a vacuum—an inert, indifferent, and noninterfering solvent. In solution, however,... 

Adsorption is enhanced by higher concentration and valency and smaller chao-tropicity of the indifferent ions. 

Qome time ago it was demonstrated conclusively that high heats of adsorption were due to many-sided sites on the surface of the absorbant (5). Previously attempts to predict differential heats of adsorption and the detailed nature of the isotherm up to a monolayer had had only indifferent success (2). It was, however, suggested that the real difficulty lay in the inability to treat lateral interaction on a heterogeneous surface (2). 

Oxide surfaces have zero net charge and zero net surface potential at pH0 when immersed in an indifferent electrolyte solution. Therefore, the second term of the right side of equation 4.14 vanishes. Any change in the concentration of the supporting electrolyte does not change the surface potential and thus pH0 is independent of the electrolyte concentration. This independence is evidenced by a crossing point of the proton adsorption curves and pH0 coincides with PZC. 

When part of the countercharge is bound to the surface by non-electrostatic forces one speaks of specific adsorption. The term derives from the fact that non-electrostatlc binding energies typically depend on the nature of the ion, say on its radius, whereas for purely Coulombic Interactions between point charges, as is the case in diffuse layers, usually the interaction is generic identical for all ions of the same valency. Generically adsorbing ions are caUed indifferent. Non-... 

Elaboration depends on the difference between the magnitudes of Cjjci. c aOH Naci NaA- surfactant adsorption the limiting cases of excess electrolyte, Naci NaA d HO electrol)rte are sometimes distinguished. However, complete absence of indifferent electrolyte renders the definition of a" ambiguous, as discussed before in this chapter. So we have to ensure that the NaCl concentration is high enough to let. Consequently, dPj Q = d/Zjj+ =... 

This book is the first volume of a Treatise on Thermodynamics based on the methods of Gibbs and De Donder. It deals with the following topics fundamental theorems, homogeneous systems, heterogeneous systems, stability and moderation, equilibrium displacements and equilibrium transformations, solutions, azeotropy, and indifferent states. The second volume deals with surface tension and adsorption while the third and last will be concerned with irreversible phenomena. 

To prevent an undesirable degradation of the compounds to be analyzed, glass columns have mostly been used for gas chromatography of alkaloids because they are indifferent to the compounds. Possible catalytic decomposition of sensitive compounds and adsorption phenomena caused by metal columns, e.g. copper, aluminium and stainless steel, may, however, be eliminated in some cases by a simple coating of the tubing material with the stationary phase... 

The overall proton adsorption from an indifferent electrolyte solution on a heterogeneous surface can simply be described as the sum of the local adsorption contributions. The effect of lateral interactions should be taken into account in the local isotherm. For a patchwise surface with a discrete distribution of intrinsic affinity constants the total... 

Use of the classical two-pKn model to describe the local isotherm for proton adsorption on an amphoteric oxide in the presence of an indifferent electrolyte complicates the situation considerably. The two pK values represent, in an artificial way, the amphoteric nature of the oxide (see secs. 4.4. and 4.8.). Equation (77) and (79) have to be extended with a second summation or integration to account for both logK distributions. Only when it is assumed that ApK is the same for all different site types a more simple relation is found. The latter assumption is however rather arbitrary. As there are no physical arguments in favour of using the two-pKn model a good advise is not to use this model. 

For a continuous heterogeneity the expression for the overall complexation of protons with all the different site types is a multiple integral equation. A relevant more simple, but still complicated situation, is that of e.g. proton and metal ion adsorption on oxides. For such a situation and adsorption from an indifferent background electrolyte the integral proton adsorption equation is ... 

For proton adsorption on well studied systems intrinsic affinity distributions can be obtained after conversion of the (T5(pH) curves into curves using the SGC model with a reasonable value for Ci,t. As indicated before the adequacy of the applied double layer model can be checked when adsorption data are available at a series of indifferent electrolyte concentrations. Replotting the surface charge as a function of pHs should lead to merging of the individual curves into a master curve [44, 45]. The master curve reflects the chemical heterogeneity [46-48]. 

Figure 2.9. For Isotherm 2, which is hnear, = 0.08 and 5l = 0, or Cj = 0.08Q. The calculation is presented in Table 2.4, and the locations of different solute concentrations after one PV solution injection are shown in Figure 2.11. For the linear adsorption, the concentration is delayed, while the shape of the concentration front remains unchanged. Such a front is called an indifferent front, which occurs when the slope of the adsorption isotherm is independent of concentration (constant), or the velocities of different concentrations are the same.

FIGURE 2.11 Indifferent front when the adsorption is linear. 

The idea of an inert (indifferent) electrolyte was coined in the context of electrocapillary studies using the Hg electrode. Grahame [34] found a series of electrolytes that produced the same PZC (determined as the electrocapillary maximum) irrespective of the nature or concentration of the electrolyte. Such behavior suggests that the ions of these electrolytes interact with the surface only by a Coulombic force. In contrast, many other electrolytes induced a shift in the PZC, with the magnitude and direction of this shift depending on the nature or concentration of the electrolyte. A shift in the PZC suggests that ions of these electrolytes can be positively adsorbed in spite of electrostatic repulsion that is, they interact with the surface by a noncoulombic force. This phenomenon is termed specihc adsorption. Thus, an inert electrolyte does not show specific adsorption of either ion. 

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