Metal Finishing Guide Book

2013

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becomes excited and emits light. Excitation can be caused by a flame, spark, X-rays, or an AC or DC arc. The electrons in the atom are activated from their ground state to unstable energy shells of higher potential energy. Upon returning to their ground state, energy is released in the form of electromagnetic radiation. Because each element contains atoms with different arrangements of outermost electrons, a distinct set of wavelengths is obtained. These wavelengths, from atoms of several elements, are separated by a monochromator such as a prism or a diffraction grating. Detection of the wavelengths can be accomplished photographically (spectrograph) or via direct-reading photoelectric detectors (spectrophotometers). The measurement of intensity emitted at a particular wavelength is proportional to the concentration of the element being analyzed. An advantage of spectroscopy is that the method is specific for the element being analyzed. It permits quantitative analysis of trace elements without any preliminary treatment and without prior knowledge as to the presence of the element. Most metals and some nonmetals may be analyzed. Spectroscopic analysis is also useful for repetitive analytical work. Disadvantages of spectroscopic analysis include the temperature dependence of intensity measurements, as intensity is very sensitive to small fluctuations in temperature. The accuracy and precision of spectrographic methods is not as high as some spectrophotometric methods or wet analyses. Spectrographic methods are usually limited to maximum element concentrations of 3%. Additionally, sensitivity is much smaller for elements of high energy (e.g., zinc) than for elements of low energy (e.g., sodium). Applications of spectroscopy include the analysis of major constituents and impurities in plating solutions, and of alloy deposits for composition. Flame Photometry In flame photometry (FP), a sample in solution is atomized at constant air pressure and introduced in its entirety into a flame as a fine mist. The temperature of the flame (1,800-3,100OK) is kept constant. The solvent is evaporated and the solid is vaporized and then dissociated into ground state atoms. The valence electrons of the ground state atoms are excited by the energy of the flame to higher energy levels and then fall back to the ground state. The intensities of the emitted spectrum lines are determined in the spectrograph or measured directly by a spectrophotometer. The flame photometer is calibrated with standards of known composition and concentration. The intensity of a given spectral line of an unknown can then be correlated with the amount of an element present that emits the specific radiation. Physical interferences may occur from solute or solvent effects on the rate of transport of the sample into the flame. Spectral interferences are caused by adjacent line emissions when the element being analyzed has nearly the same wavelength as another element. Monochromators or the selection of other spectral lines minimize this interference. Ionization interferences may occur with the higher temperature flames. By adding a second ionizable element, the interferences due to the ionization of the element being determined are minimized. An advantage of FP is that the temperature of the flame can be kept more nearly constant than with electric sources. A disadvantage of the method is that the sensitivity of the flame source is many times smaller than that of an electric arc or spark. FP is used for the analysis of aluminum, boron, cadmium, calcium, chromium, cobalt, copper, indium, iron, lead, lithium, magnesium, nickel, palladium, platinum, potassium, rhodium, ruthenium, silver, sodium, strontium, tin, and zinc. 483

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