Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

Issue link: https://metalfinishing.epubxp.com/i/49721

Contents of this Issue

Navigation

Page 474 of 707

light. When heat, electrical energy, or radiant energy is added to an atom, the atom 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 outer- most 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 photo- graphically (spectrograph) or via direct-reading photoelectric detectors (spec- trophotometers). 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 pre- liminary 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 meth- ods 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 pres- sure and introduced in its entirety into a flame as a fine mist. The temperature of the flame (1,800-3,100O K) 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 radia- tion. 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 adja- cent line emissions when the element being analyzed has nearly the same wave- length as another element. Monochromators or the selection of other spectral lines minimize this interference. Ionization interferences may occur with the higher tem- perature 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 near- ly 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, chromi- um, cobalt, copper, indium, iron, lead, lithium, magnesium, nickel, palladium, 473

Articles in this issue

view archives of Metal Finishing Guide Book - 2011-2012 Surface Finishing Guidebook