Beilstein's test

Beilstein's test is a qualitative method for determining halogens (except fluorine ) in a sample. Based on the formation of volatile copper halides, which color the flame green, the detection limit of halogen-containing compounds is less than 0.1 μg. Due to its simplicity, the sample was widely used for rapid analysis of organic compounds.

The method was proposed by the Russian chemical scientist Friedrich Conrad Beilstein in 1872 ^[1] .

The test consists in introducing a sample located on a previously calcined copper wire into the flame of a gas burner and / or spirit lamp . If the flame stains green, the sample is positive, depending on the halogen content of the sample, the color after the sample is introduced into the flame appears momentarily or is visible within 1-2 seconds.

The appearance of a green color is due to the interaction of copper (II) oxide with halogen-containing organic compounds and their oxidation products, which leads to the formation of volatile copper (I) halides coloring the flame:

${\displaystyle \mathsf{R}\mathsf{H}\mathsf{a}\mathsf{l}\mathsf{+}\mathsf{C}\mathsf{u}\mathsf{O}\to \mathsf{C}\mathsf{u}\mathsf{H}\mathsf{a}\mathsf{l}\mathsf{+}\mathsf{C}{\mathsf{O}}_{\mathsf{2}}\mathsf{+}{\mathsf{H}}_{\mathsf{2}}\mathsf{O}}${\ displaystyle {\ mathsf {RHal + CuO \ rightarrow CuHal + CO_ {2} + H_ {2} O}}}

Copper chlorides and bromides color the flame blue-green, copper iodide green.

Copper fluoride under these conditions is non-volatile, therefore, organofluorine compounds by Beilstein breakdown are not detected.

Some organic compounds that do not contain halogens, but form volatile copper compounds, are also able to give a positive Beilstein test ( oxyquinoline , nitriles, urea and thiourea , etc.).