Flame Analysis
When German chemist Robert Wilhelm Eberhard Bunsen (1811-1999) invented the Bunsen burner--a device used in almost every chemistry laboratory--he also opened the door to the analysis of matter via flame analysis, a technique now grouped with other procedures more commonly known as atomic emission spectroscopy (AES).
Working with Gustav Kirchhoff (1824-1887), Bunsen helped to establish the principles and techniques of spectroscopy. A distinguished scientist, Bunsen discovered the elements cesium and rubidium during a long and productive career. Using the techniques he pioneered, scientists have been able to determine the chemical composition of stars.
Bunsen examined the spectra—the colors of light—emitted when a substance was subjected to intense flame. 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). This temperature is sufficient to cause the emission of light from certain elements. The color of the flame and its spectrum (component colors) is unique for each element.
To examine the spectra of elements, Bunsen used a simple apparatus that consisted of a prism, slits, and a magnifying glass or photo-sensitive film. Bunsen determined that the spectrum of elements that emitted light were different, that is, each did not contain all the colors of the spectrum. Instead, Bunsen determined that only portions of the spectrum were present.
Bunsen's fundamental observation that flamed elements that emit light only at specific wavelengths and that every element produced a characteristic spectra paved the way for the subsequent development of quantum theory by Max Planck (1858- 1947), Niels Bohr (1885-1962) and others.
Flame analysis or atomic emission spectroscopy is based on the physical and chemical principle that atoms--after being heated by flame--return to their normal energy state by giving off the excess energy in the form of light. The frequencies of the light given off are characteristic for each element.
Flame analysis is a qualitative test and not a quantitative test. A qualitative chemical analysis is designed to identify the components of a substance or mixture. Quantitative tests measure the amounts or proportions of the components in a reaction or substance.
The unknown to be subjected to flame analysis is either sprayed into the flame or placed on a thin wire that is then put into the flame. Very volatile elements (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 the volatile impurities and sodium.
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.
It takes some special techniques to properly interpret the results of flame analysis. The colors produced by a potassium flame (pale violet) can usually be observed only with the assistance of glass that can filter out interfering colors. Some colors are similar enough that line spectrum must be examined to make a complete and accurate identification of the unknown substance, or the presence of an identifiable substance in the unknown.
Flame analysis can also be used to determine the presence of metal elements in water by measuring the spectrum produced by the metals exposed to flame. The water is vaporized and then the emissions of the vaporized metals can be analyzed.
Flame tests are useful means of determining the composition of substances. The colors produced by the flame test are compared to known standards. In this way the presence of certain elements in the sample can be confirmed.
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