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Flame Test is an analytical technique used to detect the presence of metal ions based on their emission spectrum produced when the sample is in contact with the flame. Different metals show different colours when they are in contact with the heat flame. It is a quick and simple technique and can be performed with very basic lab equipment.
Analytical Chemistry
Analytical chemistry is the branch of chemistry that deals with identifying and quantity of a constituent of a substance. The two main branches of analytical chemistry are
Qualitative Analysis
This branch of analytical chemistry deals with the characteristic and identification of various components present in a substance or sample.
Quantitative Analysis
This branch of analytical chemistry deals with the amount or volume of a component present in the sample or solution.
The Qualitative Analysis Of Inorganic Compounds Involves The Following Steps:
Preliminary Test – This test is performed to differentiate the compounds based on solubilities in water. Compounds that dissolve in water are called soluble compounds, while the insoluble compounds in water are called insoluble compounds.
Dry Test – This test is performed to analyse the physical characteristics of the compounds. The testing parameters are colour, odour, the effect of heating, flame test, borax bead test, etc.
Wet Test for Anions- This test is used to determine the presence of acid radicals in the salt sample.
Wet Test for Cations- This test is used to determine the presence of basic radicals in the salt sample.
The Flame Test
Flame Test is an analytical technique used to visually detect the presence of metal ions based on their emission spectrum produced when the sample is in contact with the flame. The colour produced by the flame is due to the movement of electrons present in metal ions. The blur zone of the flame is used to carry out flame tests. Only a few metal ions give flame tests. Generally, the group 1 metal ions can be differentiated using flame tests. It is one of the simplest and quickest techniques to perform as basic lab equipment.
Principle Involved In Flame Test
Generally, metal halides are used for flame tests as they are highly volatile. A small amount of sample chloride paste is placed on a spatula and is brought near the flame. Due to extreme heating, the electrons present in the metal ion are excited to higher energy levels. When these electrons return to the ground state, they impart energy corresponding to a particular wavelength, and the colour is observed. Only the wavelength of radiation that corresponds to the visible region of the electromagnetic spectrum is visible to the eyes. The rest are not. When they are introduced with the flame, specific metal ions impart specific colour.
Procedure To Carry Out Flame Test
The flame test is one of the quickest and simple procedures in analytical chemistry. It can be carried out by two methods:
CLASSIC WIRE LOOP METHOD
A platinum or nickel-chromium loop is cleaned by dipping it in a solution of hydrochloric acid or nitric acid. The loop is then rinsed with distilled water and brought under the flame of the bunsen burner to check the presence of impurities. If it is clean, no colour is produced in the flame due to the wire. After properly cleaning the wire loop, it is dipped into the solution of salt or a small amount of powdered salt and is brought in contact with the flame of the bunsen burner. The colour imparted by the flame is noted, and the metal ion present in the sample salt is detected.
WOODEN SPLINT OR COTTON SWAB METHOD
Wooden splints or cotton swabs are used as a replacement for platinum or nickel-chromium wire. The wooden splints are soaked in distilled water for six to eight hours. The splint is then rinsed and washed with clean water. The splint, which is sufficiently moistened, is used for the flame test. The splint is dipped in the sample, and then it is waved off the bunsen burner flame within seconds as keeping steady. The splint or cotton swab in the flame will catch fire. The colour of the flame is observed, and the metal ions present in the sample salt are detected.
Interpretation Of Results By Flame Test
The colours produced by the flames in the flame test are observed and matched with known metals’ flame colours. The different colours of different metals are depicted in the table below.
Colour | Metal Ion |
Golden yellow | Sodium |
Violet | Potassium |
Carmine-red | Lithium |
Brick-red | Calcium |
Apple-green | Barium, Molybdenum |
Green | Copper, Titanium |
Crimson-red | Strontium |
Silver-white | Aluminium |
Blue | Arsenic |
Bright green | Boron |
Bright orange | Carbon |
Pale blue | Germanium |
Grey-white | Lead |
Blue-white | Tin |
Conclusion
Flame Test is an analytical technique used to visually detect the presence of metal ions based on their emission spectrum produced when the sample is in contact with the flame. The colour produced by the flame is due to the movement of electrons present in metal ions. Generally, metal halides are used for flame tests as they are highly volatile. Only the wavelength of radiation that corresponds to the visible region of the electromagnetic spectrum is visible to the eyes. The rest are not. When they are introduced with the flame, specific metal ions impart specific colour. The classic wire method and a wooden splint or cotton swab method are used to conduct flame tests.
