Flame color

Strong green alkyl borate flame .

The flame coloration , also called flame test , is a method for the analysis of chemical elements or their ions ( detection reaction ). The method is based on the fact that the elements or ions in a colorless flame emit light of specific wavelengths that are characteristic of each element. The color of the flame results from the conversion of energy from thermal energy to radiant energy . The conversion comes about through valence electrons , which are raised into an excited state by the thermal energy and fall back again with the emission of light . However, there is no chemical reaction in the actual sense of an electron bond. Due to this property, substances with which flame coloring is possible are used in pyrotechnics .

With flame coloring, the fabric sample is usually simply held on a platinum wire or a magnesia stick in the colorless flame of a Bunsen burner . Based on the color, conclusions can now be drawn about the ions in the sample, but the very intense yellow flame color of sodium often covers all other flame colors. It is only with the help of a spectroscope that it can be decided with certainty which elements are present in the sample, especially since the flame colors of potassium and rubidium , for example, are quite similar.

A distinction must be made between the color of the flame and the light output of the noble gases , which is also based on an excited state , but which is brought about by electricity , not by a flame .

The burning test, in which the smell is also checked, is another method for determining an unknown material, especially for fibers and plastics .

Physical causes

Graphic representation of the electron rise and fall back on the valence shell model

All elements emit light at high temperatures . But for elements that have a flame color, this already happens at the temperatures that prevail in a flame .

The electrons of an atom are raised to an energy level further away from the atomic nucleus - in an excited state - by the addition of thermal energy , which in this case is produced by combustion . These electrons now have a higher potential energy . The negatively charged electrons usually fall back to the lower-energy initial energy level in a fraction of a second. The energy released when falling back is released as a photon . One speaks of a quant . It is characterized by a precisely defined energy and thus also with a single wavelength .

The electrons can also fall back to lower energy levels in stages. Each time this electron falls back to a lower energy level, it emits light of a very specific wavelength , and thus a very specific color and energy .

The released light energy depends on the difference in energy levels . This difference is different for each element. The energy of the photons determines their frequency and thus the color . ${\displaystyle \Delta E}$ ${\displaystyle f}$

If an element has a specific flame color, then many compounds of its ions also have this flame color. However, this is not always the case. Barium sulfate e.g. B. has a greenish flame color, barium phosphate does not. Many elements emit visible spectral lines at high temperatures . Some elements were even named after the color of their spectral lines observed when the flame was colored : cesium (Latin: sky blue), rubidium (Latin: dark red) and indium (indigo blue spectral line).

Examples

Alkali metals

Alkali metals and their salts have a specific flame color:

• Lithium and its salts color the flame red (671 nm).
• Sodium and its salts turn the flame yellow (589 nm).
• Potassium and its salts color the flame violet (768 and 404 nm).
• Rubidium and its salts color the flame red (780 and 421 nm).
• Cesium and its salts color the flame blue-violet (458 nm).

• 

  lithium

• 

  sodium

• 

  potassium

• 

  Rubidium

Alkaline earth metals

The typical alkaline earth metals and their salts have a specific flame color:

• Calcium and its salts color the flame orange-red (622 and 553 nm).
• Strontium and its salts color the flame red (675 and 606 nm).
• Barium and its salts color the flame green (524 and 514 nm).

• 

  Calcium

• 

  strontium

• 

  barium

Other elements

• 

  boron

• 

  copper

• 

  arsenic

• 

  antimony

• 

  lead

Other flame colors:

• Selenium , blue
• Indium , deep blue-violet
• Europium , red
• Thallium , green
• Radium , carmine red

use

The flame color can be used for the Beilstein test.

Modern techniques

The spectroscopic methods of atomic spectroscopy , which represent a kind of further development of this with the help of measuring instruments, offer better possibilities than the classic flame coloring with the help of the eye . The eye is replaced here by the spectrometer , which resolves the position of the spectral lines much better and also uses the invisible areas of the electromagnetic spectrum for analysis, depending on the type of spectroscopy (e.g. IR or UV / VIS spectroscopy ). In addition, unlike the subjective impression by the eye, the spectrometer is able to determine the strength of the spectral lines, which enables quantitative analysis .

literature

• W. Biltz, W. Fischer, Execution of qualitative analyzes of inorganic substances , 16th edition, Harri Deutsch, Frankfurt am Main, 1976.
• G. Jander, E. Blasius, Introduction to the Inorganic Chemical Basic Practical , 14th Edition, S. Hirzel Verlag, Stuttgart, 1995, ISBN 3-7776-0672-3 .

Web links

Commons : Flame Coloring  - Album with pictures, videos and audio files

Wikibooks: Practical course in inorganic chemistry / flame coloring  - learning and teaching materials

• Spectral lines of the alkali metals ( Memento from March 4, 2008 in the Internet Archive )

Individual evidence

1. ↑ ^{a b} Duden Learn Attack GmbH: flame coloration