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An analytical procedure used in the world of chemistry to detect the presence of certain elements, primarily the metal ions, based on each element’s characteristic colour emission spectrum when heated in a Bunsen or gas burner flame at the laboratory is termed as a flame test. The heat supplied to the sample excites the electron present within the metal ions, causing them to release a band of spectrum falling under the visible light band. Although the colour of the flame depends upon the temperature and oxygen fed, every element has its signature emission spectrum that can be used to differentiate between one and another element. When the sample is heated on the Bunsen burner, metal ions that gain energy excites from lower energy to higher energy levels. Since the ions are least stable in their respective higher energy state, it tends to come back to their ground state by releasing the energy they possess. This energy is released in the form of light that varies from each metal and hence falls under the visible band spectrum, resulting in a wide characteristic range of colour.
Process
The test procedure involves introducing a sample or molecular compound to a hot and non-luminous flame and observing the colour of the flame obtained. The main concept behind this test procedure is that an atom evaporates and emits light when subjected to heat. So, when the sample is in bulk, it will also emit radiation under the visible light spectrum band. The sample is usually transformed into a paste by mixing it well with hydrochloric acid to make metal halides volatile, leading to better results. Only single flames are introduced to resist the confusion of contaminated flames and increase the accuracy of the test. While handling such a reactive acid fire, one must ensure their safety as a first option. So some of the safety techniques that need to be mentioned to avoid any sort of accident while performing the test are:
(i) Use of impact goggles/ chemical splash to protect eyes.
(ii) Perform the flame test only under the strict supervision and direction of lab assistants or respective chemistry teachers.
Example:
A very common element, Sodium, has a wide range of use in various compounds. It also tends to dominate the spectrum trends compared to other elements. The flame test is frequently visualised through cobalt blue glass to filter out the yellow of the element and allow easier pathway viewing of different metal ions.
Results:
This test is relatively quick, simple and easy to perform. It can be carried out at any chemistry laboratory with basic apparatus. But this test does not cover the wide range of metal ions to detect its presence like gold, platinum, palladium, and silver do not produce any characteristic colour when subjected to a flame test. It also gives qualitative information, thus cannot give exact information on the amount of the element present within the compound. Some common elements emitting lights are lithium-red; sodium-yellow; potassium-pink; calcium-orange-red; iron-gold and phosphorus-pale blue-green etc.
Limitations of the flame test
The drawbacks of the flame test performed in the laboratory to detect the presence of certain metal ions are stated below. They are:
- This test cannot detect the low concentrated ions within the sample.
- The brightness of the band spectrum coming out varies with the sample change.
- The test results can be manipulated by the impurities added to the sample.
- This test does not cover a wide range of metal ions as some metals like gold, silver, platinum and palladium do not produce any colour or many metals produce the same colour when subjected to this test.
- It gives us qualitative information about the metal ions present within the sample. However, it does not provide any information regarding the quantity of the metal ions. So, to calculate the amount of the metal ion, we have to perform different tests or theoretical calculations.
The quantitative analysis of the metal ions can be detected by flame photometry or flame emission spectroscopy and flame atomic absorption spectroscopy.
Conclusion
An analytical procedure used in the world of chemistry to detect the presence of certain elements, primarily the metal ions, based on each element’s characteristic colour emission spectrum when heated in a Bunsen or gas burner flame at the laboratory is termed as a flame test. The heat supplied to the sample excites the electron present within the metal ions, causing them to release a band of spectrum falling under the visible light band. While handling such a reactive acid fire, one must ensure their safety as a first option. This test is relatively quick, simple and easy to perform. It has a wide range of drawbacks that might rule out when identifying a metal ion’s presence.
