Flame test

Summary

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 emission spectrum. The color of flames in general also depends on temperature and oxygen fed; see flame color.

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.

ProcessEdit

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 samples emit light 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 (used in "eraser" cleaning sponges)[3] 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.

ResultsEdit

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.[4]

Common elementsEdit

 
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[5]  
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[5]
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;[5] 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,[5] 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 alsoEdit

ReferencesEdit

  1. ^ a b Clark, Jim (August 2018). "Flame Tests". chemguide.co.uk. Archived from the original on November 27, 2020. Retrieved January 10, 2021.
  2. ^ Sanger, Michael J.; Phelps, Amy J.; Banks, Catherine (2004). "Simple Flame Test Techniques Using Cotton Swabs". Journal of Chemical Education. 81 (7): 969. Bibcode:2004JChEd..81..969S. doi:10.1021/ed081p969.
  3. ^ Landis, Arthur M.; Davies, Malonne I.; Landis, Linda; Thomas, Nicholas C. (2009). "'Magic Eraser' Flame Tests". Journal of Chemical Education. 86 (5): 577. Bibcode:2009JChEd..86..577L. doi:10.1021/ed086p577.
  4. ^ "Atomic Absorption (AA)". Perkin Elmer. Retrieved 2 May 2013.
  5. ^ a b c d Barrow, R F; Caldin, E F (1949-01-01). "Some Spectroscopic Observations on Pyrotechnic Flames". Proceedings of the Physical Society. Section B. 62 (1): 32–39. doi:10.1088/0370-1301/62/1/305. ISSN 0370-1301.

External linksEdit

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