Flame analysis is a qualitative test, 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 sample subjected to flame analysis is either sprayed into the flame or placed on a thin wire that is then introduced into the flame.
Highly 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 obscuration, 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.
Standard or Bunsen burner based flame tests do 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.
Analysts use 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 a 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 first vaporized to allow observation of the emissions of the subsequently vaporized residual metals.
Flame tests are useful means of determining the composition of substances. The colors produced by flame tests are compared to known standards to identify or confirm the presence of certain elements in the sample.
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K. Lee Lerner