Energy, free tungsten oxide

Calcium metal is an excellent reducing agent for production of the less common metals because of the large free energy of formation of its oxides and hahdes. The following metals have been prepared by the reduction of their oxides or fluorides with calcium hafnium (22), plutonium (23), scandium (24), thorium (25), tungsten (26), uranium (27,28), vanadium (29), yttrium (30), zirconium (22,31), and most of the rare-earth metals (32). 

The presence of the monomere, volatile oxide hydrate [WO3 H2O resp. W02(OH)2] was proven by mass spectroscopy [3.28], and thermodynamic data are available for all of the above phase equilibria [3.10,3.29]. Based on these free energy data as well as on those of the solid oxides [3.23], the equilibrium partial pressure of the volatile compound can be calculated as a function of humidity. The result of such a calculation is shown in Fig. 3.2 for a temperature of 1000 °C [3.32]. In addition, the equilibrium pressures of the other volatile tungsten compoimds are also presented. From these calculations it is evident, that the oxide hydrate is by far the most volatile tungsten compound in the W-O-H system. 

The XPS analysis provides a medium binding energy value for the W 4f of 36.4 eV. This value confirms that the metal is in oxidation state four, in agreement with the tungsten coordination sphere of the free complex [18-20]. This result shows that the host matrix environment does not affect the valence state of the metal atom of the complex. A non-covalent interaction between the guest complex and AlTTID-1 is observed. 

The loss of free atoms in the atomizer is also a function of the chemistry of the sample. If the oxide of the analyte element is readily formed, the free atoms will form oxides in the flame and the population of free atoms will simultaneously decrease. This is the case with elements such as chromium, molybdenum, tungsten, and vanadium. On the other hand, some metal atoms are stable in the flame and the free atoms exist for a prolonged period. This is particularly so with the noble metals platinum, gold, and silver. Adjusting the fuel/oxidant ratio can change the flame chemistry and atom distribution in the flame as shown in Fig. 6.17(b). Atoms with small ionization energies will ionize readily at high temperatures (and even at moderate temperatures). In an air-acetylene flame, it can be shown that moderate concentrations of potassium are about 50% ionized, for example. Ions do not absorb atomic lines. 

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