Flame Analysis
Flame analysis is a qualitative test used to identify the components of a substance or mixture. Flame analysis, also known as atomic emission spectroscopy (AES), is based on the physical and chemical principle that atoms--after being heated by flame--return to their normal energy state via the emission of electromagnetic radiation in the form of photons. The frequencies of the photons (i.e., the frequencies of the light) given off are characteristic for each element.
Flame tests are useful means of determining the composition of substances. In general, an unknown substance to be tested by flame analysis is either sprayed into a flame or placed on a thin wire that is then put into a flame. The colors produced by the flame test are compared to known standards to confirm the presence of individual elements in the sample.
Working together, nineteenth-century Prussian physicist Gustav Kirchhoff and German chemist Robert Bunsen (1811-1899) developed methods to analyze matter via flame analysis. In particular, Bunsen's invention of what is now known as the Bunsen burner allowed development of one of the fundamental techniques of spectroscopy. When air is admitted at the base of a Bunsen burner it mixes with the gas to produce a very hot flame at approximately 3,272( F (1,800( C) that is sufficient to cause the emission of light from certain elements. Bunsen used a simple apparatus that consisted of a prism, slits, and a magnifying glass or photo-sensitive film to examine the colors of light emitted and spectra of substances (including known elements) subjected to such intense flame. Bunsen determined that the color of the flame and its component colors or spectra was unique for each element, that is, the individual spectra of elements that emitted light contained only specific portions of the total colors in the spectrum. Using flame analysis techniques, Bunsen discovered the elements cesium and rubidium.
Flame analysis is often used to determine the presence of metal elements in water. The water is vaporized and the emission spectrum of the vaporized metals analyzed.
In some cases, special techniques must be used to properly interpret the results of a flame analysis test. The colors produced by a potassium flame (pale violet), for example, can usually be observed only with the assistance of glass that can filter out interfering colors. Some colors are similar enough that their line spectrum must be carefully examined to make a complete and accurate identification.
The flame test does not work on all elements. Those that produce a measurable spectrum when subjected to flame include, but are not limited to, lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, zinc, and cadmium. Other elements may need hotter flames to produce measurable spectra. In addition, when subjected to flame, very volatile elements (such as chlorides) produce intense colors. The yellow color of sodium, for example, can be so intense that it overwhelms other colors. To prevent this the wire to be coated with the unknown sample is usually dipped in hydrochloric acid and subjected to flame to remove sodium and volatile impurities.
Using the spectroscopic principles pioneered by Bunsen during his development of flame analysis, scientists have been able to determine the chemical composition of stars. Bunsen's observation that flamed elements that emit light only at specific wavelengths and that every element produces a characteristic spectra provided a theoretical base for the subsequent development of quantum theory by Max Planck, Niels Bohr and others.
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