Metals in flames

In recent years numerous experiments have been reported on the fluorescence and energy transfer processes of electronically excited atoms. However, for flame studies the rates of many possible collision processes are not well known, and so the fate of these excited atoms is unclear. An interesting example concerns the ionization of alkali metals in flames. When the measured ionization rates are interpreted using simple kinetic theory, the derived ionization cross sections are orders of magnitude larger than gas kinetic (1,2,3). More detailed analyses (4,5) have yielded much lower ionization cross sections by invoking participation of highly excited electronic states. Evaluation of these models has been hampered by the lack of data on the ionization rate as a function of initial state for the alkali metals. 

Per Cent Ionization of Alkali and Alkaline Earth Metals in Flames>... 

This approach was used by Page and Sugden in discussing the ionization of dkali metals in flames. They observed that the electron concentration in a hydrogen flame doped with lithium was independent of hydrogen atom concentration. However when the additive was sodiiun the square of electron concentration was inversely proportional to the hydrogen atom concentration. 

The equation has been shown to be valid thus it can be used to calculate the percentage ionization of metals in flames. Table 9-4 presents results of using the Saha equation to determine percentage ionization of several elements. The validity of the equation also indicates that the assumption that equilibria such as illustrated in equation (9-4) occur is correct. 

The use of a buffer solution containing caesium chloride as a spectroscopic buffer and aluminium nitrate as a physical buffer largely eliminates the potential interferences otherwise caused by alkali metals in flame photometry. Since it is easily ionizable, the caesium chloride has the effect of almost totally suppressing the ionization of the K, Na and Li atoms which are also present and exert a mutual influence on excitation. To this must be added the spectroscopic buffer action of caesium which has a smoothing effect on operational fluctuations of the burner. 

Because chemiionization proceeds more rapidly than collisional ionization, the alkaline earth elements catalyze the ionization of alkali metals in flames via, e.g.,... 

I. R. King, Recombination rates of alkali metal ions, J. Chem. Phys. 36, 553-554 (1962). P. F. Knewstubb and T. M. Sugden, Observations on the kinetics of the ionization of alkali metals in flame gases, Trans Faraday Soc. 54, 372-380 (1958). 

Atomic absorption using either flame or electrothermal atomization is widely used for the analysis of trace metals in a variety of sample matrices. Using the atomic absorption analysis for zinc as an example, procedures have been developed for its determination in samples as diverse as water and wastewater, air, blood, urine, muscle... 

A significant advance in flame retardancy was the introduction of binary systems based on the use of halogenated organics and metal salts (6,7). In particular, a 1942 patent (7) described a finish for utilizing chlorinated paraffins and antimony(III) oxide [1309-64-4]. This type of finish was invaluable in World War II, and saw considerable use on outdoor cotton fabrics in both uniforms and tents. 

D. A. Tillman and C. Leone, "Control of Trace Metals in Flyash at the Tacoma, Washington Multifiiels Incinerator," Proceedings of the Mmerican Flame Research Committee Fall International Symposium, San Francisco, 1990. 

Other uses of oxyacetylene flames in mill operations are in building up or hardfacing metal, lancing (piercing a hole in a metal mass), and a variety of metal cleaning procedures. A minor but interesting fuel use of acetylene is in flame spectrophotometry where oxygen and nitrous oxide are used as oxidants in procedures for a wide variety of the elements. 

The main rises of a sintered metal flame arrester are in the sensing heads of flammable gas detectors and in flame arresters for gas welding (oxyacetylene) equipment. 

A proprietary sintered metal arrester was made by the Linde Division of Union Carbide Corporation (now Praxair) for rise in processes handling acetylene, but is no longer made by Praxair (Dickerman 1999). A sintered metal flashback flame arrester for rise on an acetylene cylinder is made by Western Enterprises of Westlake, OH. Figure 5-4c shows a sintered metal flame-arresting element. 

Protego offers a crimped metal ribbon flame arrester approved in Germany for acetylene service. It is similar in design to their flame arresters for hydrogen service bnt the hydranlic diameter of the flame arrester apertures for qnenching acetylene flames is 0.15 mm rather than 0.20 mm for hydrogen. 

Perchlorates are known for most metals in the periodic table.The alkali-metal perchlorates are thermally stable to several hundred degrees above room temperature but NH4CIO4 deflagrates with a yellow flame when heated to 200° ... 

Metals in contact with timber can be corroded by the acetic acid of the timber and by treatment chemicals present in it. Treatment chemicals include ammonium sulfate and ammonium phosphate flame-retardants. These are particularly corrosive towards steel, aluminum and copper alloys. Preservative treatments include copper salts which, at high timber moisture contents, are corrosive towards steel, aluminum alloys and zinc-coated items. 

---