Cations common Type

Materials that contain defects and impurities can exhibit some of the most scientifically interesting and economically important phenomena known. The nature of disorder in solids is a vast subject and so our discussion will necessarily be limited. The smallest degree of disorder that can be introduced into a perfect crystal is a point defect. Three common types of point defect are vacancies, interstitials and substitutionals. Vacancies form when an atom is missing from its expected lattice site. A common example is the Schottky defect, which is typically formed when one cation and one anion are removed from fhe bulk and placed on the surface. Schottky defects are common in the alkali halides. Interstitials are due to the presence of an atom in a location that is usually unoccupied. A... 

Pyrazin-2-one (124) has been shown to exist predominantly as such by comparison of its ultraviolet spectrum with those of the fixed alkylated derivatives and by its infrared spectrum. The pK measurements support this conclusion but cannot yield quantitative results since cations of a common type are not formed. ... 

Molecular sieves (zeolites) are artificially prepared aluminosilicates of alXali metals. The most common types for gas chromatography are molecular sieve 5A, a calcium aluminosilicate with an effective pore diameter of 0.5 nm, and molecular sieve 13X, a sodium aluminosilicate with an effective pore diameter of 1 nm. The molecular sieves have a tunnel-liXe pore structure with the pore size being dependent on the geometrical structure of the zeolite and the size of the cation. The pores are essentially microporous as the cross-sectional diameter of the channels is of similar dimensions to those of small molecules. This also contrilsutes to the enormous surface area of these materials. Two features primarily govern retention on molecular sieves. The size of the analyte idiich determines whether it can enter the porous... 

Details of the calculation of Madelung constants for all of the common types of crystals are beyond the scope of this book. When the arrangement of ions differs from that present in NaCl, the number of ions surrounding the ion chosen as a starting point and the distances between them may be difficult to determine. They will most certainly be much more difficult to represent as a simple factor of the basic distance between a cation and an anion. Therefore, each arrangement of ions (crystal type) will have a different value for the Madelung constant. The values for several common types of crystals are shown in Table 7.3. 

Let us consider a multisite mixture of type (A, B, C,. . . ) (M, N, O,. . . ) Z, where cations of types A, B, C and M, N, O, respectively, occupy energetically distinct sites present in one mole of substance in the stoichiometric amounts Vj and V2, and Z is the common anionic group. Applying the permutability concept to each distinct site and assuming random mixing and the absence of interactions on sites, the activity of component A M Z in the mixture may be expressed as... 

Ilmenite structure the stmcture of the mineral FeTiOs Metathetical reaction a common type of reaction, also known as a donble decomposition reaction, in which the anions and cations of the reactants are interchanged in the prodncts (e.g. AX - - BY AY - - BX)... 

Many of the mineralogically important transition-metal oxide phases contain more than one cation species, or more than one type of coordination site for the cations. Commonly, the cations are in more than one oxidation state. Examples include ilmenite (FeTiOj) and the family of minerals with the spinel-type crystal structure, including magnetite (Fe304), chromite... 

Table 2.4 gives both names for many common Type II cations. The system that uses Roman numerals will be used exclusively in this text. 

The term hole transfer is frequently used to describe the specific type of electron transfer which occurs between a cation radical and a neutral molecule. The cation radical is considered to have a missing electron or hole , and transfers this to the neutral molecule, which then becomes a cation radical, while the original cation radical is neutralized. Hole transfer is of one of the simplest and most common types of reaction between cation radicals and neutral molecules. 

In atomic or molecular sohds, common types of point defects are the absence of an atom or molecule from its expected position at a regular lattice site (a vacancy), or the presence of an atom or molecule in a position which is not on the regular lattice (an interstitial). In ionic solids, these point defects occur in two main combinations. These are Schottky defects, in which there are equal numbers of cation and anion vacancies within the crystal, and Frenkel defects, in which there are cation vacancies associated with an equal number of "missing" cations located at non-lattice, interstitial positions. Both are illustrated in Figure 1.1. Point defects are also found in association with altervalent impurities, dislocations, etc., and combinations of vacancies with electrons or positive holes give rise to various types of colour centres (see below). 

FIG. 4.6 Less common types of percentage of uptake (pH) curves (A) the uptake does not reach 100% (cations) (B) the uptake does not reach 0% (cations) (C) two uptake curves with a maximum (anions or cations). 

Adsorption of metal cations that form stable water soluble complexes with anions present in solution has been often interpreted in terms of formation of multiple surface species that differ in the number of anions coadsorbed with one adsorbed cation. When this number is greater than zero these species are termed ternary surface complexes. This can be interpreted as complexation of the adsorbed cation. This type of multiple surface species can be combined with the discussed above species that differ in the number of protons released per one specifically adsorbed cation. Less common (but also physically reasonable for ions that tend to form polymeric species in solution) is the idea of multiple polymeric surface species. For example, adsorption of cobalt on alumina was modeled in terms of formation of =A10Co2(OH)2 and =A10Co4(OH)5 surface species [106],... 

In this equation, Res denotes resin or polymer. Silica-based cation exchangers are generally prepared by reacting silica particles with an appropriate chlorosilane or methoxysilane. A common type of silica catex has the structure ... 

In this type of separation the analyte cations compete with the eluent cation for ion-exchange sites and move down the eolumn at different rates. The ionic eluent selected depends on the cations to be separated, the type of separation column and on the detector. In many cases an aqueous solution of a strong acid such as hydrochloric, sulfuric or methanesulfonic acid is a satisfactory eluent. Sample cations commonly separated include the following alkali metal ions (Li, Na+, K", Rb, Cs ), ammonium, magnesium, alkaline earths (Ca, Sr +, Ba ), and various organic amine and alkano-lamine cations. Most other metal cations are separated with a weakly complexing eluent. 

As stated above, anion extraction of the most common type (water to organic layer) is executed by quaternary ammonium or other onium cations or by crown-ether-masked metal cations. To be useful, at least two factors have to be met ... 

Table 2 (second to fourth columns) summarizes some chemical features of the main sedimentary zeolites. It is evident that the mentioned zeolites are medium- to high-silica types. Phillipsite and chabazite belong to the first category, especially if we consider that the values of 7 Sl in the most common joint occurrences [see, e.g., 40] are in the ranges of 0.71-0.73 and 0.69-0.72, respectively. Instead, clinoptilolite, ferrieritc and mordenite are to be considered siliceous zeolites. This chemical character results naturally in different cation exchange capabilities, as it can be seen in the fourth column of Table 2 (CEC values). It is worth to observe that these values are merely indicative, as they refer to pure zeolite species, not to zeolite-rich rocks. On the basis of the theoretical CEC values (Table 2) and zeolite contents in tuff occurrences (see Sub-sec. 5.1), more realistic CEC values for the most common types of zeolitic luffs may be worked out, as follows ...

A common type of process in water purification involves the use of a mixed bed of cation- and anion-exchenge resins lo deionize water, often following separate cation- and anion-exchange beds. The use of such mixed-bed columns enables very low levels of impurities to be attained. Reganeralion of such columns... 

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