Stoichiometry analyses

Chemical Properties. Elemental analysis, impurity content, and stoichiometry are determined by chemical or iastmmental analysis. The use of iastmmental analytical methods (qv) is increasing because these ate usually faster, can be automated, and can be used to determine very small concentrations of elements (see Trace AND RESIDUE ANALYSIS). Atomic absorption spectroscopy and x-ray fluorescence methods are the most useful iastmmental techniques ia determining chemical compositions of inorganic pigments. Chemical analysis of principal components is carried out to determine pigment stoichiometry. Analysis of trace elements is important. The presence of undesirable elements, such as heavy metals, even in small amounts, can make the pigment unusable for environmental reasons. 

Stoichiometry. Analysis of the vapor and condensed products of the overall HP-2 reaction indicate the following stoichiometry. 

Techniques responding to the absolute amount of analyte are called total analysis techniques. Historically, most early analytical methods used total analysis techniques, hence they are often referred to as classical techniques. Mass, volume, and charge are the most common signals for total analysis techniques, and the corresponding techniques are gravimetry (Chapter 8), titrimetry (Chapter 9), and coulometry (Chapter 11). With a few exceptions, the signal in a total analysis technique results from one or more chemical reactions involving the analyte. These reactions may involve any combination of precipitation, acid-base, complexation, or redox chemistry. The stoichiometry of each reaction, however, must be known to solve equation 3.1 for the moles of analyte. 

The accuracy of a standardization depends on the quality of the reagents and glassware used to prepare standards. For example, in an acid-base titration, the amount of analyte is related to the absolute amount of titrant used in the analysis by the stoichiometry of the chemical reaction between the analyte and the titrant. The amount of titrant used is the product of the signal (which is the volume of titrant) and the titrant s concentration. Thus, the accuracy of a titrimetric analysis can be no better than the accuracy to which the titrant s concentration is known. 

Quantitative Calculations In precipitation gravimetry the relationship between the analyte and the precipitate is determined by the stoichiometry of the relevant reactions. As discussed in Section 2C, gravimetric calculations can be simplified by applying the principle of conservation of mass. The following example demonstrates the application of this approach to the direct analysis of a single analyte. 

In a gravimetric analysis a measurement of mass or change in mass provides quantitative information about the amount of analyte in a sample. The most common form of gravimetry uses a precipitation reaction to generate a product whose mass is proportional to the analyte. In many cases the precipitate includes the analyte however, an indirect analysis in which the analyte causes the precipitation of another compound also is possible. Precipitation gravimetric procedures must be carefully controlled to produce precipitates that are easily filterable, free from impurities, and of known stoichiometry. 

In an indirect analysis the analyte participates in one or more preliminary reactions that produce or consume acid or base. Despite the additional complexity, the stoichiometry between the analyte and the amount of acid or base produced or consumed may be established by applying the conservation principles outlined in Section 2C. Example 9.3 illustrates the application of an indirect analysis in which an acid is produced. 

Rios, A. Luque de Castro, M. Valcarcel, M. Determination of Reaction Stoichiometries by Flow Injection Analysis, ... 

Grades, Quality Control. Fluorspar is marketed ia several grades metallurgical fluorspar (metspar) is sold as gravel, lump, or briquettes. The minimum acceptable assay is 60% effective calcium fluoride. The effective value is determined by subtracting from the contained calcium fluoride 2.5% for every percent of Si02 found ia the complete analysis apparently based on the following stoichiometry (1) ... 

When the kinetics are unknown, still-useful information can be obtained by finding equilibrium compositions at fixed temperature or adiabatically, or at some specified approach to the adiabatic temperature, say within 25°C (45°F) of it. Such calculations require only an input of the components of the feed and produc ts and their thermodynamic properties, not their stoichiometric relations, and are based on Gibbs energy minimization. Computer programs appear, for instance, in Smith and Missen Chemical Reaction Equilibrium Analysis Theory and Algorithms, Wiley, 1982), but the problem often is laborious enough to warrant use of one of the several available commercial services and their data banks. Several simpler cases with specified stoichiometries are solved by Walas Phase Equilibiia in Chemical Engineering, Butterworths, 1985). 

Strained set of lattice parameters and calculating the stress from the peak shifts, taking into account the angle of the detected sets of planes relative to the surface (see discussion above). If the assumed unstrained lattice parameters are incorrect not all peaks will give the same values. It should be borne in mind that, because of stoichiometry or impurity effects, modified surface films often have unstrained lattice parameters that are different from the same materials in the bulk form. In addition, thin film mechanical properties (Young s modulus and Poisson ratio) can differ from those of bulk materials. Where pronounced texture and stress are present simultaneously analysis can be particularly difficult. 

Compounds of the same stoichiometry type usually have the same type crystal structure within the row of alkali metals K - Rb - Cs rarely the same type structure with sodium-containing analogues and never ciystallize similarly with lithium-containing compounds. The crystal structure analysis of different fluoride and oxyfluoride compounds clearly indicates that the steric similarity between all cations and tantalum or niobium must be taken into account when calculating the X Me ratio. 

There is an extensive literature devoted to the preparation and structure determination of coordination compounds. Thermal analysis (Chap. 2, Sect. 4) has been widely and successfully applied in determinations [1113, 1114] of the stoichiometry and thermochemistry of the rate processes which contribute to the decompositions of these compounds. These stages may overlap and may be reversible, making non-isothermal kinetic data of dubious value (Chap. 3, Sect. 6). There is, however, a comparatively small number of detailed isothermal kinetic investigations, together with supporting microscopic and other studies, of the decomposition of coordination compounds which yields valuable mechanistic information. 

As was suggested in the preceding discussion, most of the arene complexes isolated by metal-atom techniques are benzene derivatives. However, heterocyclic ligands are also known to act as 5- or 6-electron donors in transition-metal 7r-complexes (79), and it has proved possible to isolate heterocyclic complexes via the metal-atom route. Bis(2,6-di-methylpyridine)Cr(O) was prepared by cocondensation of Cr atoms with the ligand at 77 K (79). The red-brown product was isolated in only 2% yield the stoichiometry was confirmed by mass spectrometry, and the structure determined by X-ray crystal-structure analysis, which supported a sandwich formulation. 

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