Flame test

The flame test carried out on a copper halide. The characteristic bluish-green color of the flame is due to the copper.

A flame test showing the presence of lithium.

A flame test is an analytical procedure used in chemistry to detect the presence of certain elements, primarily metal ions, based on each element's characteristic flame emission spectrum (which may be affected by the presence of chloride ion). The color of flames in general also depends on temperature and oxygen fed; see flame color.

The test involves introducing a sample of the element or compound to a hot, non-luminous flame and observing the resulting flame color. The flame test is quick, simple, and can be performed with basic laboratory equipment. However, it only provides qualitative information, and its effectiveness depends on the subjective experience of the experimenter. For quantitative data, related techniques such as flame photometry or flame emission spectroscopy can be used.

Process

The test involves introducing a sample of the element or compound to a hot, non-luminous flame, and observing the color of the flame that results. The idea of the test is that sample atoms evaporate and since they are hot, they emit light when being in flame. The solvent of the solution evaporates first, leaving finely divided solid particles which move to the hottest region of the flame where gaseous atoms and ions are produced through the dissociation of molecules. Here electrons are excited by the heat, and the spontaneously emit photon to decay to lower energy states.

Bulk sample emits light too, but its light is not good for analysis. Bulk samples emit light with hydrochloric acid to remove traces of previous analytes. The compound is usually made into a paste with concentrated hydrochloric acid, as metal halides, being volatile, give better results. Different flames should be tried to avoid wrong data due to "contaminated" flames, or occasionally to verify the accuracy of the color. In high-school chemistry courses, wooden splints are sometimes used, mostly because solutions can be dried onto them, and they are inexpensive. Nichrome wire is also sometimes used. When using a splint, one must be careful to wave the splint through the flame rather than holding it in the flame for extended periods, to avoid setting the splint itself on fire. The use of cotton swab or melamine foam (used in "eraser" cleaning sponges) as a support has also been suggested.

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.

Results

The flame test is relatively quick and simple to perform and can be carried out with the basic equipment found in most chemistry laboratories. However, the range of elements positively detectable under these conditions is small, as the test relies on the subjective experience of the experimenter 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 cause fainter colors to not appear.

Although the flame test only gives qualitative information, not quantitative data about the proportion of elements in the sample, quantitative data can be obtained by the related techniques of flame photometry or flame emission spectroscopy. Flame atomic absorption spectroscopy Instruments, made by e.g. PerkinElmer or Shimadzu, can be operated in emission mode according to the instrument manuals.

Common elements

Coloured flames of methanol solutions of different compounds, burning on cotton wool. From left to right: lithium chloride, strontium chloride, calcium chloride, sodium chloride, barium chloride, trimethyl borate, copper chloride, cesium chloride and potassium chloride.

Some common elements and their corresponding colors are:

Symbol	Name	Color	Image
Al	Aluminium	Silver-white, in very high temperatures such as an electric arc, light blue
As	Arsenic	Blue
B	Boron	Bright green
Ba	Barium	Pale/Apple Green
Be	Beryllium	White
Bi	Bismuth	Azure
C	Carbon	Bright orange
Ca	Calcium	Brick red, light green as seen through blue glass.
Cd	Cadmium	Brick red
Ce	Cerium	Yellow
Co	Cobalt	Silver-white
Cr	Chromium	Silver-white
Cs	Caesium	Blue-violet
Cu(I)	Copper(I)	Bluish-green
Cu(II)	Copper(II) (non-halide)	Green
Cu(II)	Copper(II) (halide)	Blue-green
Ge	Germanium	Pale blue
Fe(II)	Iron(II)	Gold, when very hot such as an electric arc, bright blue, or green turning to orange-brown
Fe(III)	Iron(III)	Orange-brown
H	Hydrogen	Pale blue
Hf	Hafnium	White
Hg	Mercury	Red
In	Indium	Indigo/Blue
K	Potassium	Lilac; invisible through cobalt blue glass (purple)
Li	Lithium	Carmine red; invisible through green glass
Mg	Magnesium	Colorless due to Magnesium Oxide layer, but burning Mg metal gives an intense white
Mn(II)	Manganese(II)	Yellowish green
Mo	Molybdenum	Yellowish green
Na	Sodium	Intense yellow; invisible through cobalt blue glass. See also Sodium-vapor lamp
Nb	Niobium	Green or blue
Ni	Nickel	Silver-white (sometimes reported as colorless)
P	Phosphorus	Pale bluish green
Pb	Lead	Blue/white
Ra	Radium	Crimson
Rb	Rubidium	Red-violet
Sb	Antimony	Pale green
Sc	Scandium	Orange
Se	Selenium	Azure
Sn	Tin	Blue-white
Sr	Strontium	Crimson to Scarlet, yellowish through green glass and violet through blue cobalt glass
Ta	Tantalum	Blue
Te	Tellurium	Pale green
Ti	Titanium	Silver-white
Tl	Thallium	Pure green
V	Vanadium	Yellowish Green
W	Tungsten	Green
Y	Yttrium	Carmine, Crimson, or Scarlet
Zn	Zinc	Colorless (sometimes reported as bluish-green)
Zr	Zirconium	Mild red

Gold, silver, platinum, palladium, and a number of other elements do not produce a characteristic flame color, although some may produce sparks (as do metallic titanium and iron); salts of beryllium and gold reportedly deposit pure metal on cooling.

See also

• Bead test
• Spark testing
• Colored fire
• Emission spectrum
• Photoelectric flame photometer
• Qualitative inorganic analysis
• Inductively coupled plasma atomic emission spectroscopy

External links

Wikimedia Commons has media related to Flame test.

• Flame Test - Coloring Fire - Pictures of Several Flame Tests, Includes Instructions
• WebMineral.com - Flame Coloration by Element