OXIDATION STATE CHEMICAL ANALYSIS

The most heavily studied high temperature superconductor is YBa2Cus07 x (x = 0 to 1), whose Cu oxidation state is determined by the oxygen content. The parent structure, YBa2Cus07, contains layers 

A striking chemical behavior of YBa2Cu307 is the vigorous evolution of gas when the solid is added to acid or water (10)(11). It was first thought that this represented oxidation of water to 02 by Cu3+, but it was conclusively shown with lsO-enriched superconductor that the evolved 02 is derived from the solid, not the solvent (12)-(14)  

Immediately after the discovery of YBa2Cus07 x, numerous groups employed iodometric titration procedures to measure the effective oxidation state of the material, and therefore the value of x. The procedure described below involves two different titrations (10X17X18) and is more accurate than a procedure in which the first titration is omitted (19). Experiment A measures the total copper content of the superconductor and Experiment B measures the total charge of the copper. The two experiments, together, give the average oxidation state of copper. 

In Experiment A, YBa2Cus07.x is-dissolved in dilute HC104, in which all copper is reduced to Cu2+ (eq. 1). The total copper content is determined by addition of iodide 

Each mole of Cu in YBa2Cus07 x requires one mole of S2Os2 in Experiment A. 

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D. C. Harris, M. E. Hills, and T. A. Hewston, Preparation, Iodometric Analysis, and Classroom Demonstration of Superconductivity in YBt CujOj, A. J. Chem. Ed. 1987, 64, 847 D. C. Harris, Oxidation State Chemical Analysis, in T. A. Vanderah, ed., Chemistry of Superconductor Materials (Park Ridge, NJ Noyes. 1992) B. D. Fahlman, Superconductor Synthesis—An Improvement, ... 

X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), is described in section Bl.25,2.1. The most connnonly employed x-rays are the Mg Ka (1253.6 eV) and the A1 Ka (1486.6 eV) lines, which are produced from a standard x-ray tube. Peaks are seen in XPS spectra that correspond to the bound core-level electrons in the material. The intensity of each peak is proportional to the abundance of the emitting atoms in the near-surface region, while the precise binding energy of each peak depends on the chemical oxidation state and local enviromnent of the emitting atoms. The Perkin-Elmer XPS handbook contains sample spectra of each element and bindmg energies for certain compounds [58]. 

Only slightly less accurate ( 0.3—0.5%) and more versatile in scale are other titration techniques. Plutonium maybe oxidized in aqueous solution to PuO " 2 using AgO, and then reduced to Pu" " by a known excess of Fe", which is back-titrated with Ce" ". Pu" " may be titrated complexometricaHy with EDTA and a colorimetric indicator such as Arsenazo(I), even in the presence of a large excess of UO " 2- Solution spectrophotometry (Figs. 4 and 5) can be utilized if the plutonium oxidation state is known or controlled. The spectrophotometric method is very sensitive if a colored complex such as Arsenazo(III) is used. Analytically usehil absorption maxima and molar absorption coefficients ( s) are given in Table 10. Laser photoacoustic spectroscopy has been developed for both elemental analysis and speciation (oxidation state) at concentrations of lO " — 10 M (118). Chemical extraction can also be used to enhance this technique. 

Chemical appHcations of Mn ssbauer spectroscopy are broad (291—293) determination of electron configurations and assignment of oxidation states in stmctural chemistry polymer properties studies of surface chemistry, corrosion, and catalysis and metal-atom bonding in biochemical systems. There are also important appHcations to materials science and metallurgy (294,295) (see Surface and interface analysis). 

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

Analysis of CEELS line shapes often show chemical shifts that have been used to study FeB alloys after recrystallization, C-H bonding in diamondlike films and multiple oxidation states. 

A second source of plutonium, dispersed more locally, is liquid effluent from fuel reprocessing facilities. One such is the fuel reprocessing plant at Windscale, Cumbria in the United Kingdom where liquid waste is released to the Irish Sea(6). Chemical analysis of this effluent shows that about one percent or less of the plutonium is in an oxidized form before it contacts the marine water(7). Approximately 95 percent of the plutonium rapidly adsorbs to particulate matter after discharge and deposits on the seabed while 5 percent is removed from the area as a soluble component ). Because this source provided concentrations that were readily detected, pioneering field research into plutonium oxidation states in the marine environment was conducted at this location. 

While these calculations provide information about the ultimate equilibrium conditions, redox reactions are often slow on human time scales, and sometimes even on geological time scales. Furthermore, the reactions in natural systems are complex and may be catalyzed or inhibited by the solids or trace constituents present. There is a dearth of information on the kinetics of redox reactions in such systems, but it is clear that many chemical species commonly found in environmental samples would not be present if equilibrium were attained. Furthermore, the conditions at equilibrium depend on the concentration of other species in the system, many of which are difficult or impossible to determine analytically. Morgan and Stone (1985) reviewed the kinetics of many environmentally important reactions and pointed out that determination of whether an equilibrium model is appropriate in a given situation depends on the relative time constants of the chemical reactions of interest and the physical processes governing the movement of material through the system. This point is discussed in some detail in Section 15.3.8. In the absence of detailed information with which to evaluate these time constants, chemical analysis for metals in each of their oxidation states, rather than equilibrium calculations, must be conducted to evaluate the current state of a system and the biological or geochemical importance of the metals it contains. 

The STEM Is Ideally suited for the characterization of these materials, because one Is normally measuring high atomic number elements In low atomic number metal oxide matrices, thus facilitating favorable contrast effects for observation of dispersed metal crystallites due to diffraction and elastic scattering of electrons as a function of Z number. The ability to observe and measure areas 2 nm In size In real time makes analysis of many metal particles relatively rapid and convenient. As with all techniques, limitations are encountered. Information such as metal surface areas, oxidation states of elements, chemical reactivity, etc., are often desired. Consequently, additional Input from other characterization techniques should be sought to complement the STEM data. 

Thus we shall be concerned with properties that furnish information about the nature of the ligands, the oxidation state of the metal, and the geometry of the field of ligands. Techniques such as radio-isotope tracer studies, neutron-activation analysis, and electron microscopy are powerful methods for locating a metal within constituents of the cell and are particularly suited to heavy-metal rather than organic drugs but since they do not provide information about the chemical environment of the metal they will not concern us here. After each section below we shall give an example, not necessarily from platinum chemistry, where the method has been used with success in biochemistry. 

A structural classification of 8 is difficult due to the fact that an arrangement of metal atoms as in 8 is uncommon in the whole field of molecular metal clusters. For this reason, detailed understanding of the bonding properties in 8 requires quantum chemical calculations. Theoretical analysis seems to be especially applicable to learning more about the bond between the two tetrahedra, which appears at first to be an isolated metal-metal bond between two metal atoms in the formal oxidation state zero. 

Direct measurement of the absolute binding energy and widths of core (X-ray) and valence (UV) bands. The core levels do not participate in bonding, hence each element gives a characteristic XPS spectrum electron spectroscopy for chemical analysis (ESCA). ESCA gives the elemental composition of the surface of a solid sample (except H), the relative amounts of each element present, its oxidation state and some information on the chemical environment around each element. In addition, it is capable of providing an estimate of the depth of a deposited overlaycr... 

Evidences about the successful intercalation of the Rh-TPPTS complex (qualitative and quantitative) between the layers of both LDHs were also provided by the XPS and DRIFTS results. XPS composition was in a very good concordance with chemical analysis. As shown in Table 2, the binding energies of the constitutive elements in both LDHs are typical for their oxidation states, while for Rh it corresponds to (I) state [13] that is again in accordance with the oxidation state of the expected complex. 

Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium precursors (acetate, chloride, nitrate, sulphate and ammonium dichromate) were studied in the selective ammoxidation of ethylene to acetonitrile. Cr-ZSM-5 catalysts were characterized by chemical analysis, X-ray powder diffraction, FTIR (1500-400 cm 1), N2 physisorption (BET), 27A1 MAS NMR, UV-Visible spectroscopy, NH3-TPD and H2-TPR. For all samples, UV-Visible spectroscopy and H2-TPR results confirmed that both Cr(VI) ions and Cr(III) oxide coexist. TPD of ammonia showed that from the chromium incorporation, it results strong Lewis acid sites formation at the detriment of the initial Bronsted acid sites. The catalyst issued from chromium chloride showed higher activity and selectivity toward acetonitrile. This activity can be assigned to the nature of chromium species formed using this precursor. In general, C r6+ species seem to play a key role in the ammoxidation reaction but Cr203 oxide enhances the deep oxidation. 

Sample Effects The recovery of an analyte from a complex matrix may be affected by other components of the matrix. The homogeneity of the sample will also influence the results. This is related to the issue of sampling mentioned above. Physical or chemical form can lead to incomplete recovery of the analyte. For example, an element may exist in more than one oxidation state in a sample and hence be incompletely determined by a method that requires it to be in one particular state only (speciation). The sample and/or analyte may be unstable, causing a change in the composition of the sample during the course of the analysis. 

Is the chemical analysis used sufficiently accurate to support the modeling study The chemistry of the initial system in most models is constrained by a chemical analysis, including perhaps a pH determination and some description of the system s oxidation state. The accuracy and completeness of available... 

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