There are three methods used in perfume chemistry to test for the presence of chlorine compounds: the Beilstein test, the Lime test, and the Combustion test.
Among these, the Beilstein’s Test is considered the most sensitive and, if no chlorine is detected through this method, the substance is considered completely free of chlorine.
The Beilstein’s Test is the simplest method for detecting the presence of halogens, however, it cannot distinguish between chlorine, bromine, and iodine as a positive result can be obtained for any of these elements.
Principle of Beilstein Test
When heated in a burner flame, chlorine reacts with copper compounds to produce excited copper atoms or ions that display a brilliant green or blue-green flame.
Procedure of Beilstein Test
- Take a copper wire approximately 1mm thick and fuse it into a glass rod.
- Beat the end of the wire to form a spatula with a width of 2-3mm.
- Heat the loop end of the wire in a Bunsen burner flame until it glows brightly.
- Allow the wire to cool.
- Dip the wire directly into a sample of the unknown substance or place a small amount of the substance on a watch glass and wet the copper wire in distilled water before placing it into the sample.
- Heat the wire in the Bunsen burner flame again.
- Observe the burning of the substance followed by the appearance of a green flame if a halogen is present.
- Hold the wire in the flame either just above the tip of the flame or at its outside edge near the bottom to obtain the best results.
When organic compounds containing hydrogen and halogens are subjected to ignition, they decompose and produce the corresponding hydrogen halide.
If the sample is mixed with copper oxide and then heated, it will form copper halide, which imparts a distinctive green or blue-green color to a non-luminous gas flame.
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Applications of Beilstein Test
The Beilstein test is the simplest method for detecting the presence of halogens. However, it does not have the ability to distinguish between the different halogens – chlorine, bromine, and iodine – as all of them will give a positive result.
But, if the identity of an unknown substance has been narrowed down to two options, one with a halogen and one without, the Beilstein test can often be used to differentiate between the two.
Limitations of the Beilstein Test
The Beilstein test is not always a dependable method for detecting halogens, as certain substances that do not actually contain halogens, such as urea hydroxyquinoline, thiourea, substituted pyridines, guanidine carbonate, can still yield a positive result.
As a result, it is generally considered more reliable to consider a negative Beilstein test as evidence of the absence of halogens, rather than a positive test being proof of their presence.
What is Beilstein’s test indicating how the test was done?
The appearance of green or blue-green color indicates that the test is done.
What reagents are in Beilstein’s test?
Two main reagents are used in Beilstein’s test.
i) Organic sample containing halogen.
ii) CuO which acts as Catalyst.
Why urea gives green color in Beilstein’s test?
This is because urea forms cuprous cyanide, which is volatile, and upon decomposition releases copper that burns with a green flame.
Which of the compounds give positive Beilstein’s?
Hydroxyquinoline, urea, thiourea, pyridines, guanidine carbonate, mercaptobenzothiazole, and salicylaldoxime.
Can fluorine be detected by Beilstein test?
A positive result of Beilstein’s test is signified by the appearance of a green flame, which is a result of the formation of a copper halide. However, the test is not capable of detecting fluorine or fluorides.
What is the difference between Beilstein’s test and the flame test?
The flame test is utilized to identify the existence of halogens in organic compounds. Meanwhile, the Beilstein test is commonly employed to determine the presence of Polyvinyl chloride. The procedure involves using a piece of copper wire or copper gauze, which is burned in a flame to remove any impurities.
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