Flame tests are useful means of determining the composition of substances. The colors produced by the flame test are compared to known standards. And the presence of certain elements in the sample can be confirmed. The color of the flame and its spectrum (component colors) is unique for each element.
Flame analysis or 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 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. 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.
Special techniques are required 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.
█ FURTHER READING:
Broekaert, José. C. Analytic Atomic Spectrometry with Flames and Plasmas. New York: Wiley-VCH Publishing, 2001.
Helmenstein, Anne Marie. "What You Need To Know About Chemistry-Quantitative Flame Analysis" About, Inc, < http://chemistry.about.com/library/weekly/aa110401a.htm > (March 29, 2003).