Alkali impurities

Hot Corrosion. Hot corrosion is an accelerated form of oxidation that arises from the presence not only of an oxidizing gas, but also of a molten salt on the component surface. The molten salt interacts with the protective oxide so as to render the oxide nonprotective. Most commonly, hot corrosion is associated with the condensation of a thin molten film of sodium sulfate [7757-82-6], Na2S04, on superaHoys commonly used in components for gas turbines, particularly first-stage turbine blades and vanes. Other examples of hot corrosion have been identified in energy conversion systems, particularly coal gasifiers and direct coal combustors. In these cases the salt originates from alkali impurities in the coal which condense on the internal... 

Fireclay. Fireclays consist mainly of the mineral kaolmke, AI2O2 2S1O2 2H2O, with small amounts of other clay minerals, quartzite, iron oxide, titama, and alkali impurities. 

Analysis of data available seems to reveal that the modification of silver by addition of acceptor impurities proceeds by an electronic mechanism. As no experimental data on the effect of alkali and earth alkali impurities on electrical and catalytical properties of silver were available, corresponding studies were undertaken with a silver catalyst. 

FIGURE 1 A representative sample of color centers in alkali halide crystals. The large and small circles represent negative and positive ions, respectively. Colored circles represent alkali impurities. 

Fb centers involve an F-center beside two substitutional alkali impurities. Since the distribution of foreign alkali ions in an otherwise pure lattice is statistical, Fb centers are far fewer in number than Fa centers, and are obtained in substantial quantities only when the impurity dopant concentration is fairly high (approximately 1%). Like the Fa center there are two types of Fb center, also classified by their relaxation behavior. The Fb(I) center is formed when the two substitutional impurities lie along a common (100) axis. The Fb(I) center has optical properties similar to the F-center, which preclude the possibility of lasing action. The Fb(II) center is formed when the two alkali impurities are adjacent to one another along a (110) axis of the crystal (see Fig. 1). 

The former is the older procedure and assumes that the minerals present are kaolinite, feldspar and quartz. It is now known, however, that the alkali impurity in English pottery clays is present as mica and according to the mica convention the rational analysis is calculated on the basis of kaolinite, mica and quartz. (The term proximate ANALYSIS is sometimes applied to the rational analysis of clays.)... 

Finally, moderate amounts of alkali impurities have been intentionally introduced into silica scales growing on both SiC and Si3N4 by vapor techniques (Pareek and Shores, 1991 McNallan et al., 1994 Sun et al., 1994). Alkali halides or alkali salts are vaporized in one portion of a furnace and transported with a carrier gas to the test specimen in another zone of the furnace. Depending on the activity of the alkali vapor species in the test, the oxide scale composition varied from 0.4 (Pareek and Shores, 1991) to 30 (Sun et al., 1994) mole percent alkaline oxide. Oxidation kinetics for the lowest levels of alkali impurity in the scale were parabolic, but elevated over rates ob-... 

In combustion environments, the deposition of alkali impurities in combination with impurities in common fuels, called hot corrosion, results in dramatically increased corrosion rates. This more severe corrosion process is discussed in Section 7.3.9. 

Hydrothermal treatment at high temperatures (e.g., 375-450°C, steam pressure 21-30.5 MPa) can cause crystallization as well as coarsening [31]. The tendency to crystallize is related to the amount of alkali impurity in the gel. Lazarev et al. [32] showed that hydrothermal treatment at 400 C at a pressure of 20 MPa caused densification of alkoxide-derived silica gel with a decrease of surface area from —200 mVg to -1 m /g, but no crystallization occurred. If the original gel, with a total impurity content of 3 ppm, was treated with an alkali-containing mineralizer (procedure not described), hydrothermal treatment would produce cristobalite and then quartz. The kinetics of the transformation could be described by the Avrami equation [33],... 

L. Goodman and F. DiGeronimo, Nematic Liquid Crystal Misalignment Induced by Excess Alkali Impurities in Soft Glass, 5th International Liquid Crystal Conf., Stockholm, Sweden, June 1974. 

Meng J, Pandey R, Vail J M and Kunz A B 1989 Impurity potentials derived from embedded quantum olusters Ag" and Cu" transport In alkali halides J. Phys. Condens Matter 1 6049-58... 

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... 

Dilute sodium hydroxide solution (and also sodium carbonate solution and sodium bicarbonate solution) can be employed for the removal of an organic acid from its solution in an organic solvent, or for the removal of acidic impurities present in a water-insoluble solid or liquid. The extraction is based upon the fact that the sodium salt of the acid is soluble in water or in dilute alkali, but is insoluble in the organic solvent. Similarly, a sparingly soluble phenol, e.g., p-naphthol, CioH,.OH, may be removed from its solution in an organic solvent by treatment with sodium hydroxide solution. 

Suitable inlets commonly used for liquids or solutions can be separated into three major classes, two of which are discussed in Parts A and C (Chapters 15 and 17). The most common method of introducing the solutions uses the nebulizer/desolvation inlet discussed here. For greater detail on types and operation of nebulizers, refer to Chapter 19. Note that, for all samples that have been previously dissolved in a liquid (dissolution of sample in acid, alkali, or solvent), it is important that high-purity liquids be used if cross-contamination of sample is to be avoided. Once the liquid has been vaporized prior to introduction of residual sample into the plasma flame, any nonvolatile impurities in the liquid will have been mixed with the sample itself, and these impurities will appear in the results of analysis. The problem can be partially circumvented by use of blanks, viz., the separate examination of levels of residues left by solvents in the absence of any sample. 

MetaUic ions are precipitated as their hydroxides from aqueous caustic solutions. The reactions of importance in chlor—alkali operations are removal of magnesium as Mg(OH)2 during primary purification and of other impurities for pollution control. Organic acids react with NaOH to form soluble salts. Saponification of esters to form the organic acid salt and an alcohol and internal coupling reactions involve NaOH, as exemplified by reaction with triglycerides to form soap and glycerol,... 

J. T. Keating and K. J. Behhng, Brine, Impurities, and Membrane Chhr—Alkali Cell Pe ormance, presented at the London International Chlorine Symposium, 1988. 

Rubidium metal is commeicially available in essentially two grades, 99 + % and 99.9 + %. The main impurities ate other alkali metals. Rubidium compounds are available in a variety of grades from 99% to 99.99 + %. Manufacturers and suppliers of mbidium metal and mbidium compounds usually supply a complete certificate of analysis upon request. Analyses of metal impurities in mbidium compounds are determined by atomic absorption or inductive coupled plasma spectroscopy (icp). Other metallic impurities, such as sodium and potassium, are determined by atomic absorption or emission spectrograph. For analysis, mbidium metal is converted to a compound such as mbidium chloride. 

Liquid carboxylic acids are first freed from neutral and basic impurities by dissolving them in aqueous alkali and extracting with diethyl ether. (The pH of the solution should be at least three units above the pKg of the acid, see pK in Chapter 1). The aqueous phase is then acidified to a pH at least three units below the pK of the acid and again extracted with ether. The extract is dried with magnesium sulfate or sodium sulfate and the ether is distilled off The acid is fractionally distilled through an efficient column. It can be further purified by... 

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