Adsorption of impurities

There are two cases for which selective adsorption of impurities occurs (i) over the entire surface by epitaxy, and (ii) along the steps of growth layers on the face. 

Polyhedral (principally octahedral) crystals of Type I may have appeared due to selective adsorption of impurities to suppress the growth rate of 111, and therefore pure Type II grew without such an effect, thus resulting in irregular forms. 

One general method may always be used to reduce the effect of impurity adsorption on electrodes, and that is to work only for shorl times. Impurities take substantial times to adsorb. If the time in which the measurement is made is short enough, the adsorption aspect of impurity interference with electrode kinetic measurements can be reduced. Many of the techniques for doing this are described in Chapter 8 (transients). However, this approach does not eliminate the difficulty that at low current densities impurities in the solution may compete with electrons from the electrode. Further, although transient measurements may greatly reduce the adsorption of impurities during the measurement, it is difficult to arrange techniques so that the electrode is in contact with the solution for seconds only. 

With solid electrodes—particularly polyciystals—and less purified solutions, other changes can make the potentiostatic and galvanostatic measurement less clearly defined as to the final time at which the value of the current density at a given potential (or the final potential at a given current) should be taken. What soil of other changes are relevant here (apart from possible changes due to adsorption of impurities) ... 

On a polycrystal (on which most reaction rates are measured), the distribution of the various crystal planes on the surface may vaiy, partly due to time-dependent adsorption of impurities if the solution is not completely clean and partly due to differing dissolution rates of the varying crystal planes in the substrate for an anodic current. Until these unstable factors have been taken care of, the reaction rate will vaiy with time. 

Adsorption of impurities on the particle surface, which act as growth inhibitors. 

Accurate and reproducible Mossbauer spectra of supported iron catalysts require the prevention of the adsorption of impurities including oxygen and water onto the highly reactive sample surface. Cells designed to allow chemical reactions and in vacuo pretreatments at temperatures up to 673 K while the Mossbauer spectrum is being recorded have recently been reported and represent a significant and important development in the application of Mossbauer spectroscopy to catalytic and surface studies (109-111). 

If the concentrations of the stoichiometrically-limiting reactant in the two phases are in equilibrium and if the chemical potential is the driving force, then, from thermodynamics, it is clear that the reaction rate is unaffected by the nature of the phase with which the solid is in contact, provided that no mass- and heat-transfer gradients exist and no blockage of the catalyst sites by the impurities occurs. However, the competitive adsorption of impurities in the liquid, even if these are inert to reaction, can markedly affect catalytic behavior. 

The term aging generally describes a loss in the activity—or selectivity—observed in a catalytic process after a certain period of reaction time. Aging may result from some change in the nature or number of catalyst sites, or in the accessibility of the sites to reactant molecules. Thus, such factors as formation of hydrogen-deficient organic residues ( coke ), selective adsorption of impurities from the charge ad-... 

Hematites with a particular crystal shape (plates, needles, spindles, pseudocubes, peanuts) and a narrow size distribution (monodisperse) can be obtained by adding various chemicals (shape controllers) to the system. The mechanism behind the control of shape is most likely to be the adsorption of impurities on certain crystal faces thereby reducing their growth rate in favor of that of the other faces. Internally these crystals may be either mono- or polydomainic, depending on the type and concentration of the additive. It must be kept in mind that higher additive concentrations may lead to product contamination. Some examples are summarized in the following section. 

It was questioned if permeances were affected by desorption of water and adsorption of impurities during the permeation test. Thus, CO2 and N2 permeances were determined as a function of time. An air-dried membrane was placed in the permeation test unit, the temperature was maintained at 30°C for 30 min, and an equimolar CO2-N2 mixture was introduced. The CO2 and N2 permeances increased by the desorption of water in the initial stage of the measurement, and then gradually decreased. However, the CO2/N2 selectivity did not greatly change with time, ranging from 50 at zero time to 75 after 15 h. 

For adsorption of impurities, the suiTounding phase of interest is typically the liquid phase. The adsorption isotherm can be built by determining the impurity levels in the solution before and after adding the solid (as adsorbent). Based upon the change of impurity levels in the solution, the adsorption isotherm can then be determined. Figiue 2-24 shows a Langmuir-type adsorption isotherm of R-ibuprofen S-lysinate on S-ibuprofen S-lysinate crystals in an ethanol/water solvent mixture. As shown in the figure, a trace amount of impurities can be adsorbed onto the crystal surface even when the impurity concentration in the solution is well below its solubility limit. 

Other crystallization techniques that are less frequently applied in the pharmaceutical industry, such as melt and freeze crystallization, may be applicable for some processes. In Example 11-4, purification of dimethyl sulfoxide (DMSO) is presented. In this case, low-level impurities, primarily dimethyl sulfide, are removed by controlled fractional crystallization from the melt (DMSO is a liquid above 18.45°C), in combination with adsorption of impurities from the unfrozen liquid. In the feed DMSO prior to the crystallization step, the impurities, while unacceptable, are at too low a level to be removable by adsorption alone. 

Allegedly pure materials often contain specifically adsorbing ions as impurities. These impurities induce a shift in the lEP, and special cleaning methods are necessary to remove them. Most likely some unusual pH values reported as pristine lEP for certain materials (Table 3.1, and 3.3) are caused by specific adsorption of impurities, namely, anionic impurities induce a low lEP and cationic—a high lEP. Lack of coincidence between the lEP and CIP (cf. Fig. 4.12) and unsymmetrical shape of the electrokinetic curves corroborates this assertion. The errors caused by specific adsorption of anionic impurities are more common than the cation effects. The COi and SiOj errors in electrokinetic measurements and difficulties in removal of multivalent anions, which are present in solutions used to prepare monodispersed colloids, can serve as a few examples (cf. Section 3.I.B.1). 

Contamination of the droplet by adsorption of impurities from the gas phase tends to reduce 0 if yM and/or ySL is reduced and jSA remains more or less constant. 

When a Pt(100) surface is subjected to ultrahigh vacuum conditions, the clean surface spontaneously reconstructs [84] from the (1 x 1) structure to the reconstructed state at room temperature by the adsorption of impurities, such as water molecules to the (5x20) structure [84]. The present (lxl) structure means that the state of the sample treated this way, which gave an LEED pattern of the (100) plane without further treatment under vacuum, has a satisfactory initial crystallographic surface structure from an LEED point of view for the electrochemical experiment. 

The crystalline nature of zeolites and MOFs allows structures to be designed with very narrow pore size distributions. This opens up another potential application for such materials as porous membranes for the separation, and/ or purification of hydrogen supplies. Size separation could be augmented by selective adsorption of impurities, taking advantage of the inherently low enthalpy of adsorption for hydrogen on these surfaces. 

Surface adsorption of impurities is the most common source of error in gravimetry. It is reduced by proper precipitation technique, digestion, and washing. 

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