The determination of oxygen in molybdenum

Apart from a few exceptions, the methods used in the literature - a survey of the situation in 1967 is given by Friedrich and Lassner (153) - can be classified as three different variants of reducing fusion  

The method u i g no ath at 2200 C is based on the M0-M0C2 eutectic, which is liquid at this temperature. Its use for the determination of oxygen in molybdenum was first discussed by Friedrich and Lassner (153)(172) and Friedrich et al. (172). Lassner and Schedle (174) find that it is suitable and yields results agreeing with those given by the procedure employing nickel-iron baths, both for powders and for compact samples. 

Nickel-iron (80 20) baths are preferred by Lassner (167), using temperatures of 1600 to 1700°C. Their suitability is confirmed by Friedrich and Lassner (153)(172) and also by Lassner and Schedle (174). On the other hand, Burzev et al. (180) rates them as better than iron baths, but not nearly as good as platinum baths. Compared with the latter, their yield is stated to be only 60 %. 

Platinum baths appear to be the most suitable auxiliary baths (177)(178). According to Muramatsu (181), 70 % of molybdenum is permissible at 2050°C, only 20 % at 2000 C and 10 % at 1950 C. Burzev et al. (180) also consider this bath superior to the others. 

For the sake of completeness, mention should also be made of a few other suggested bath metals, although these have not been used in practice cobalt or nickel (153), nickel-cerium (153), gold (167), nickel-tin (174), platinum-iron (179) and platinum-nickel (181). 

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F can also be separated by solvent extraction. Faure et al. (223)(224) used solvent extraction with disphenyldichiorosilane in isopropyl ether for the determination of oxygen in molybdenum and lead. The separation of F by solvent extraction using triphenyl antimony (V) derivatives is described by Chermette et al. (225)(226). It is shown that, if fluoride is extracted with an excess of reagent, the extracted salt is triphenyl-antimonyhydroxyfluoride if the pH is not too low. The partition constants of this compound in benzene-water and carbon tetrachloride-water systems are high. The kinetics of the fluoride exchange reaction ... 

Revel and Albert (240) and Fedoroff et al. (220) describe the determination of oxygen in molybdenum by helium-3 activation. Fedoroff et al. (220)... 

Valladon and Debrun (208) used triton activation for the instrumental determination of oxygen in molybdenum. Irradiations of 15 min with a 500 nA beam of 3.5 MeV tritons were carried out. After the irradiation a 5 //m surface layer was removed by a combination of chemical etching and grinding. The result obtained was (15.9 + 1.1) jug/g (n = 12). 

There are a number of possible inhibitors in the glucose determination. Most of them, however, occur in tlie second enzymatic reaction. The glucose oxidase method would be more specific, then, if the hydrogen peroxide were measured directly without the need for a second enzyme. For example, added iodide ion, in the presence of a molybdenum(VI) catalyst, is rapidly oxidized to iodine. The iodine concentration can be followed amperometrically (Chapter 15). An alternative is to measure the depletion of oxygen amperometrically. 

Heteropolyacids contain 10, 11, or 12 molybdenum atoms per phosphorus atom. Indirect methods for the determination of phosphate are given in Figure 97. The most suitable organic solvents for extracting heteropolyacids are oxygen-containing solvents such as alcohols (n-butanol, isobutanol, hexanol, heptanol), ketones (MIBK), and esters (isobutyl acetate, butyl acetate). 

Surface methoxy groups are the intermediate reaction species in the production of partially oxygenated reaction products (formaldehyde, methyl formate, methylal, etc.) during methanol selective oxidation on catalysts containing transition metal oxides (oxides of vanadium, molybdenum, chromium, etc.). Therefore, the knowledge of the amount of surface methoxy species formed during methanol chemisorption is the key for the determination of the number of surface active sites... 

The partial oxidation of propylene occurs via a similar mechanism, although the surface structure of the bismuth-molybdenum oxide is much more complicated than in Fig. 9.17. As Fig. 9.18 shows, crystallographically different oxygen atoms play different roles. Bridging O atoms between Bi and Mo are believed to be responsible for C-H activation and H abstraction from the methyl group, after which the propylene adsorbs in the form of an allyl group (H2C=CH-CH2). This is most likely the rate-determining step of the mechanism. Terminal O atoms bound to Mo are considered to be those that insert in the hydrocarbon. Sites located on bismuth activate and dissociate the O2 which fills the vacancies left in the coordination of molybdenum after acrolein desorption. 

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