Flame test

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File:Flame test.jpg
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 is relatively quick test for the presence of some elements in a sample. The technique is archaic and of questionable reliability, but once was a component of qualitative inorganic analysis. The phenomenon is related to pyrotechnics and atomic emission spectroscopy.[1] The color of the flames is understood through the principles of atomic electron transition and photoemission, where varying elements require distinct energy levels (photons) for electron transitions.[2][3]

History

Robert Bunsen invented the now-famous Bunsen burner in 1855, which was useful in flame tests due to its non-luminous flame that did not disrupt the colors emitted by the test materials.[4][1] The Bunsen burner, combined with a prism (filtering the color interference of contaminants), led to the creation of the spectroscope, capable of emitting the spectral emission of various elements.[1] In 1860, the unexpected appearance of sky-blue and dark red was observed in spectral emissions by Robert Bunsen and Gustav Kirchhoff, leading to the discovery of two alkali metals, caesium (sky-blue) and rubidium (dark red).[4][1] Today, this low-cost method is used in secondary education to teach students to detect metals in samples qualitatively.[2]

Process

File:A Student Conducting the Chemical Experiment using Crucible.jpg
A flame test showing the presence of lithium.
File:Flame Tests of Metal Ions, With Labels.webm
Flame test of a few metal ions

A flame 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.[4] The compound can be made into a paste with concentrated hydrochloric acid, as metal halides, being volatile, give better results.[5] Different flames can be tried to verify the accuracy of the color. Wooden splints, Nichrome wires, platinum wires, magnesia rods, cotton swabs, and melamine foam are suggested for support.[6][7][8] Safety precautions are crucial due to the flammability and toxicity of some substances involved.[9][10][11][6] 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 a cotton swab or melamine foam (used in “eraser” cleaning sponges) as a support has also been suggested.[7][8][6] Sodium is a common component or contaminant in many samples,[2] and its spectrum tends to dominate many flame tests others.[5] 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.Script error: No such module "Unsubst".

The color of the flames also generally depends on temperature and oxygen fed; see flame colors.[5] The procedure uses different solvents and flames to view the test flame through a cobalt blue glass or didymium glass to filter the interfering light of contaminants such as sodium.[12]

Flame tests are subject of a number of limitations. The range of elements positively detectable under standard conditions is small. Some elements emit weakly and others (Na) very strongly. 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.[12] The test is highly subjective.

Principle

File:Electron excitation.png
Electron excitation

In flame tests, ions are excited thermally. These excited states then relax to the ground state with emission of a photon. The energy of the excited state(s) and associated emitted photon is characteristic of the element. The nature of the excited and ground states depends only on the element. Ordinarily, there are no bonds to be broken, and molecular orbital theory is not applicable. The emission spectrum observed in flame test is also the basis of flame emission spectroscopy, atomic emission spectroscopy, and flame photometry.[4][13]

Common elements

File:Coloured flames of methanol solutions of metal salts and compounds.jpg
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[5] Image
Al Aluminium Silver-white, in very high temperatures such as an electric arc, light blue
As Arsenic Blue File:FlammenfärbungAs.jpg
B Boron Bright green File:FlammenfärbungB.png
Ba Barium Light apple green Flame resulting from Barium Chloride combustion in a Bunsen burner
Be Beryllium White
Bi Bismuth Azure blue
Ca Calcium Brick/orange red; light green as seen through blue glass. File:FlammenfärbungCa.png
Cd Cadmium Brick red
Ce Cerium Yellow
Co Cobalt Silvery white File:CobaltFlameTestOxyHydrogen.png
Cr Chromium Silvery white File:ChromiumFlameTestOxyHydrogen.png
Cs Caesium Blue-violet File:CaesiumFlameTestOxyHydrogen.png
Cu(I) Copper(I) Blue-green
Cu(II) Copper(II) (non-halide) Green Flame test on copper sulfate
Cu(II) Copper(II) (halide) Blue-green
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 An iron (III) flame, generated using the thermite reaction
Ge Germanium Pale blue
H Hydrogen Pale blue
Hf Hafnium White
Hg Mercury Red
In Indium Indigo blue File:Flammenfärbung Indium (In).jpg
K Potassium Lilac (pink); invisible through cobalt blue glass (purple) File:FlammenfärbungK.png
Li Lithium Carmine red; invisible through green glass File:FlammenfärbungLi.png
Mg Magnesium Colorless due to Magnesium Oxide layer, but burning Mg metal gives an intense white
Mn(II) Manganese(II) Yellowish green File:ManganeseFlameTestOxyHydrogen.png
Mo Molybdenum Yellowish green
Na Sodium Bright yellow; Golden; invisible through cobalt blue glass. See also Sodium-vapor lamp File:Flametest--Na.swn.jpg
Nb Niobium Green or blue
Ni Nickel Colorless to silver-white File:NickelFlameTestOxyHydrogen.png
P Phosphorus Pale blue-green
Pb Lead Blue-white File:FlammenfärbungPb.png
Ra Radium Crimson red
Rb Rubidium Violet red File:Die Flammenfärbung des Rubidium.jpg
S Sulfur Blue File:Sulfur-burning-at-night.png
Sb Antimony Pale green File:FlammenfärbungSb.png
Sc Scandium Orange
Se Selenium Azure blue
Sn Tin Blue-white
Sr Strontium Crimson to scarlet red; yellowish through green glass and violet through blue cobalt glass File:FlammenfärbungSr.png
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 red
Zn Zinc Colorless to blue-green File:Zinc burning.JPG
Zr Zirconium Mild/dull red

See also

References

Template:Reflist

External links

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