Flame test

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The flame test carried out on a copper halide. The characteristic bluish-green color of the flame is due to the copper.
Different flame types of Bunsen Burner depending on air flow through the valve:
1. air valve closed
2. air valve nearly fully closed
3. air valve semi-opened
4. air valve maximally opened.
Gas flame

A flame test is an analytic procedure used in chemistry to detect the presence of certain elements, primarily metal ions, based on each element's characteristic emission spectrum. The color of flames in general also depends on temperature; see flame color.

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. Bulk sample emits light too, but its light is not good for analysis. Bulk sample emits light primarily due to motion of the atoms, therefore its spectrum is broad, consisting of a broad range of colors. Separate atoms of sample present in flame can emit only due to electronic transitions between different atomic energy levels. Those transitions emit light of very specific frequencies, characteristic of chemical element itself. Therefore, the flame gets the color, which is primarily determined by properties of the chemical element of the substance being put into flame.

Samples are usually held on a platinum wire cleaned repeatedly with hydrochloric acid to remove traces of previous analytes.[1] 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.[1] 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[2] or melamine foam (eraser)[3] as a support have 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.

The flame test is fast and easy to perform, and does not require any equipment not usually found in a chemistry laboratory. However, the range of detected elements is small, and 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 drown out weaker signals.

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.[4]

Common elements

Some common elements and their corresponding colors are:

Symbol Name Color Image
As Arsenic Blue
B Boron Bright green
Ba Barium Pale/Apple green
Ca Calcium Brick red
Cs Caesium Blue-Violet
Cu(I) Copper(I) Bluish-green
Cu(II) Copper(II) (non-halide) Green
Cu(II) Copper(II) (halide) Blue-green
Fe Iron Gold
In Indium Blue
K Potassium Lilac
Li Lithium Crimson
Mn (II) Manganese (II) Yellowish green
Mo Molybdenum Yellowish green
Na Sodium Intense yellow
P Phosphorus Pale bluish green
Pb Lead Blue/white
Ra Radium Crimson red
Rb Rubidium Red-violet
Sb Antimony Pale green
Se Selenium Azure blue
Sr Strontium Crimson
Te Tellurium Pale green
Tl Thallium Pure green
Zn Zinc Colorless (sometimes reported as bluish-green)

See also

References

  1. ↑ 1.0 1.1 Jim Clark (2005). "Flame Tests". Chemguide. 
  2. ↑ Sanger, Michael J.; Phelps, Amy J.; Catherine Banks (2004). "Simple Flame Test Techniques Using Cotton Swabs". Journal of Chemical Education 81 (7): 969. doi:10.1021/ed081p969 
  3. ↑ Landis, Arthur M.; Davies, Malonne I.; Landis, Linda; Nicholas c. Thomas (2009). ""Magic Eraser" Flame Tests". Journal of Chemical Education 86 (5): 577. doi:10.1021/ed086p577 
  4. ↑ "Atomic Absorption (AA)". Perkin Elmer. Retrieved 2 May 2013. 

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