Homogeneity chemical

Precipitatioa (2,13—17) techniques employ a combination of nucleation and growth iaduced by adding a chemical precipitant, or by changing the temperature and/or pressure of the solution. Chemical homogeneity is controlled by controlling the rate of precipitation. FFeterogeneous precipitation iavolves the precipitation of a soHd of different composition from the solution, and the composition of the precipitate may change as precipitation continues. Coprecipitation iavolves the simultaneous precipitation of similar size cations ia a salt as a soHd solutioa. 

Solveat extractioa (2,13—15,17) by freeze dryiag provides another means of maintaining chemical homogeneity ia solutioa-derived powders. Freezing solutioas or emulsions preveats chemical segregatioa and the desired salt is obtained by removing the solvent by sublimation. 

A solid propellant is a mechanical (heterogeneous) or a chemical (homogeneous, or colloidal) mixture of solid-state fuel and oxidizer-rich chemicals. Specially-formed charges of solid propellant (grains) arc placed in the combustion chamber of the solid rocket motor (SRM) at a production facility. Once assembled, the engine does not require additional maintenance, making it simple, reliable and easy to use. 

The entropy of a condensed chemically homogeneous substance vanishes at the zero of absolute temperature ... 

By condensed is meant solid or liquid by chemically homogeneous is meant a pure substance, element or com-... 

The theorem of Nernst applies only to chemically homogeneous condensed phases the entropy of a condensed solution phase has at absolute zero a finite value, owing to the mutual presence of the different components. 

It may reasonably be assumed that the terms in the expression for the entropy which depend on the temperature diminish, like the entropy of a chemically homogeneous condensed phase, to zero when T approaches zero, and the entropy of a condensed solution phase at absolute zero is equal to that part of the expression for the entropy which is independent of temperature, and depends on the composition (Planck, Thennodynamik, 3 Aufi., 279). 

For a system composed of any number of condensed chemically homogeneous phases and a homogeneous gas phase we have the symbol ... 

It must be borne in mind that the equations (11), etc., hold good only when the condensed phases are chemically homogeneous. Thus Foote and Smith, who determined the dissociation pressure of cuprous oxide ... 

To assess homogeneity, the distribution of chemical constituents in a matrix is at the core of the investigation. This distribution can range from a random temporal and spatial occurrence at atomic or molecular levels over well defined patterns in crystalline structures to clusters of a chemical of microscopic to macroscopic scale. Although many physical and optical methods as well as analytical chemistry methods are used to visualize and quantify such spatial distributions, the determination of chemical homogeneity in a CRM must be treated as part of the uncertainty budget affecting analytical chemistry measurements. 

Liebich V (1995) Characterization of the chemical homogeneity of solid-state materials by che-mometric methods Further multivariate aspects. Fresenius J Anal Chem 352 420-425. 

This point analysis is employed to analyse a selected region of chemically homogeneous composition, such as a phase. The electron beam is stopped and positioned carefully on the point selected on the SEM screen, and the composition of the sampling volume is determined by a crystal spectrometer. 

Processes associated with two opposing electrode processes of a different nature, where the anodic process is the oxidation of a metal, are termed electrochemical corrosion processes. In the two above-mentioned cases, the surface of the metal phase is formed of a single metal, i.e. corrosion occurs on a chemically homogeneous surface. The fact that, for example, the surface of zinc is physically heterogeneous and that dissolution occurs according to the mechanism described in Section 5.8.3 is of secondary importance. 

The general principles of testing chemical homogeneity of solids are given e.g. by Malissa [1973], Cochran [1977], and Danzer et al. [1979]. The terms of variation o20tal and o2nal can be separated by analysis of variance (Sect. 5.1.1). According to Danzer and Kuchler [1977] there exists an exponential dependence between the total variance and the reciprocal sample mass... 

Danzer K, Singer R (1985) Application of pattern recognition methods for the investigation of chemical homogeneity of solids. Mikrochim Acta [Wien] 1 219... 

Danzer K, Doerffel K, Ehrhardt H, Geissler M, Ehrlich G, Gadow P (1979) Investigations of the chemical homogeneity of solids. Anal Chim Acta 105 1... 

Ehrlich G, Kluge W (1989) Characterization of the chemical homogeneity of solids affected with periodical concentration fluctuations using the statistics of stochastic processes. Mikrochim Acta [Wien] 1 145... 

Numerous investigators have attempted to control the precursor structure and related solution chemistry effects with varying degrees of success, to influence subsequent processing behavior, such as crystallization tempera-ture.40-42,78,109 110 Particular attention has been given to precursor characteristics such as structural similarity to the desired product and the chemical homogeneity of the precursor species. For multicomponent films, this latter factor is believed to influence the interdiffusional distances associated with the formation of complex crystal structures, such as perovskite compounds. Synthetic approaches have been geared toward the preparation of multimetal species with cation stoichiometry identical to that of the desired crystalline phase.40 42 83 84... 

The ocean is chemically homogeneous and, in addition to carbonates, contains Ca2+ plus enough of inert ions to adjust alkalinity to the values observed in modem seawater ( 2.1 x 10 3eqkg 1). The inert ions, assigned as Na+ and Cl- for convenience, are assumed to be time-invariant in the ocean. 

This model of liquid extraction is symmetrical to fractional crystallization and has attracted renewed interest after the demonstration by Johnson et al. (1990) that REE distributions in abyssal peridotite clinopyroxene cannot be accounted for by equilibrium melting processes. The solid is supposed to maintain its chemical homogeneity while liquid is continuously extracted. Only the last drop of liquid is supposed to be in equilibrium with the residue. 

FIGURE 4.2. Redox and chemical homogeneous catalysis of electrochemical reactions. 

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