Flame test

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The flame test is a procedure used in chemistry to detect the presence of certain metal ions. It involves introducing a sample to a flame, and observing the color that results. The test is based on the fact that each element has its own characteristic emission spectrum. Samples are often held on the looped end of a clean, nonreactive wire or on the tip of a wood stirring stick soaked in water.

Sodium is a common component or contaminant in many compounds and its spectrum tends to dominate over others. The test flame is often viewed through cobalt blue glass to filter out the yellow of sodium and allow for easier viewing of other metal ions.

The flame test is fast and easy to perform, and does not require any special equipment. However, the range of detected elements is small, and the test relies on the subjective experience of the experimentalist rather than any objective measurements. The test has difficulty detecting small concentrations of some elements, while too strong a result may be produced for certain others, which tends to drown out weaker signals. Finally, the test only gives qualitative information, not quantitative data about the actual proportion of elements in the sample.

Related methods

The flame test is also the predecessor of Flame Atomic Absorption Spectrometer. There is also a similar, though more complicated but precise test - the borax bead test.

Common Metals

Some common metals and corresponding colors are:

Symbol	Name	Color
As	Arsenic	Blue
B	Boron	Bright Green
Ba	Barium	Yellowish green
Ca	Calcium	Red-orange
Cs	Caesium	Pale violet
Cu	Copper (non-halide)	Emerald green
Cu	Copper (halide)	Blue-green (bluish green)
In	Indium	Blue
Li	Lithium	Crimson
K	Potassium	Lilac
Mg	Magnesium	Colorless
Mo	Molybdenum	Yellowish green
Na	Sodium	Intense yellow
P	Phosphorus	Pale bluish green
Pb	Lead	Pale green
Rb	Rubidium	Pale violet
Sb	Antimony	Pale green
Se	Selenium	Azure blue
Sr	Strontium	Crimson
Te	Tellurium	Pale green
Tl	Thallium	Pure green
Zn	Zinc	Bluish green

Procedure

1. Use a wire loop, usually made of Platinum or Nichrome.
2. If the wire loop has been previously used for a flame test, it must be cleaned. Dip it into concentrated hydrochloric acid and then hold it in a non-luminous flame until no discolouration of the flame can be observed.
3. Rinse the wire loop in distilled or deionized water.
4. Dip the clean loop into an ionic salt or solution and pass it into the non-luminous flame.
5. Blue cobalt glass may be desirable to filter sodium's light if sodium ions are present in the solution.

External links

• [Flame Test - Coloring Fire] - Pictures of Several Flame Tests, Includes Instructions
• [WebMineral.com] - Flame Coloration by Element
• [All Info About Chemistry] - How to Do Flame Tests

 

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