Molybdenum-based flame

Several coating techniques are now available to overcome the oxidation problems with molybdenum above 300°C. One of these, based on molybdenum disilicide, is finding increased usage in flame breakout shields for aero-engines where tests have shown (unpublished work) that the coated material can withstand a high pressure torching type flame attack at temperatures in excess of 2(X)0°C. 

If flame emission is based on excitation of atoms formed by combustion in the flame, why does flame emission work well for sodium, potassium, cesium and some of the transition metals but not vanadium, molybdenum or the lanthanides ... 

Molybdenum. Molybdenum can be analyzed by P CAM 173 for total Mo, by S-193 (12) for soluble Mo, or by S-376 for insoluble Mo. The standard nitric wet ashing used in P CAM 173 does not distinghish between soluble and insoluble Mo which have OSHA standards of 5 mg/cu m and 15 mg/cu m. Nitric acid digestion may not dissolve some insoluble Mo that require nitric/perchloric acid or base/nitric acid depending on the solubility properties. Soluble Mo compounds are hot water leached from the cellulose membrane filter used in all three methods. A fuel-rich air/acetylene flame used in P CAM 173 is replaced by an oxidizing nitrous oxide/acetylene flame to achieve total atomization of Mo as detected at 313.3 nm. Aluminum and traces of acid enhance the Mo flame response therefore, 400 ppm A1 is added to the final solution of both S-193 and S-376 and 0.1 N nitric acid is added to the water leach-soluble Mo final solution, S-193. 

CHARMAX LS are zinc-molybdenum complexes targeted as smoke suppressants in PVC wire and cable, PVC plastisols and rigid PVC products. LS Z4A is used in a simple PVC jacket formulation. Aroimd 5% of such additives can replace up to 20% of AOM and reduce the afterglow sometimes seen with antimony oxide containing formulations. Also CHARMAX products are cheaper by weight than primary molybdenum and tin-based additives, AOM and zinc starmate, and lower loadings provide the required level of flame retardancy or smoke suppression that may be specified. The complexes may be used as partial of complete replacements for antimony oxides, whose price and supply has been unpredictable in recent times. 

Finally, indirect determination procedures have been developed recently which are based on the measurement of the molybdenum concentration in the extract of the molybdo-phosphate complex with isobutyl ketone by atomic absorption spectrometry in a N20/C2H2"flame at 313.3 and 320.8 nm (8). 

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