Flame Test

Abstract : In this post we intend to explore the flame test analysis technique, revisited through the use of our grating spectrometer which should allow us to highlight the emission lines of the various elements being analyzed.

Introduction

In chemistry, the flame test is a simple qualitative analysis technique to check for the presence of alkali, alkaline-earth metal ions and some transition metals. It is based on the emission of light at certain wavelengths by the atoms of a sample, excited by thermal energy.

A small amount of sample – or a solution of it in hydrochloric acid – is placed on a platinum or nickel-chromium wire (we used a Ni-Cr wire), usually held by a rod, and immersed in the oxidizing flame. the Bunsen burner (we used a portable laboratory gas stove). The use of hydrochloric acid allows the double exchange reaction with the salts to be analyzed, leading to the formation of chlorides which are best observed during the flame coloring process.
It starts from the base of the flame characterized by a lower temperature (about 300 °C) and which allows to observe the cations that need less energy to be observed, up to the melting zone (characterized by a temperature of about 1400 °C) where the remaining cations are observed, of the II group and of the transition metals that need more energy. The atoms of the metal present in the sample, which thanks to the thermal energy have passed to an excited state, give the flame a typical color, from which its presence can be deduced. The color is given by the emission spectrum of the atom or ion. To evaluate the emission spectrum we used our spectrometer: DIY Diffraction grating Spectrometer.

On the following NIST website: https://physics.nist.gov/PhysRefData/Handbook/periodictable.htm you will find the emission lines of the various elements in neutral and ionized form. For each element it is possible to obtain information on the wavelength of the various emission lines and on their relative intensity. The information that interests us for the flame test is the information found on the Persistent Lines tab.

Lithium

At the flame test, the lithium salts give rise to a flame of a beautiful carmine red easily recognizable and characterized by a rather narrow emission line that is located at a wavelength λ = 670 nm, this emission line is associated with the transition 2s → 2p.

Main emission lines of Lithium:
https://physics.nist.gov/PhysRefData/Handbook/Tables/lithiumtable3.htm.

Sodium

At the flame test the sodium salts produce the classic and unmistakable yellow flame at λ = 589 nm, this emission line is associated with the 3s → 3p transition.

Main emission lines of Sodium:
https://physics.nist.gov/PhysRefData/Handbook/Tables/sodiumtable3.htm.

Potassium

At the flame test the potassium salts produce an inconspicuous lilac-colored flame characterized by a wavelength λ = 760 nm, this emission line is associated with the 4s → 4p transition.

Main emission lines of Potassium:
https://physics.nist.gov/PhysRefData/Handbook/Tables/potassiumtable3.htm.

Strontium

On the flame test the strontium salts give rise to an easily recognizable scarlet red flame. The emission detected with our spectrometer is composed of a wide emission band between 600 and 700 nm.

Main emission lines of Strontium:
https://physics.nist.gov/PhysRefData/Handbook/Tables/strontiumtable3.htm.

Copper

In the flame test, the copper salts give the flame a green-blue color that is between 500 and 550 nm.

Main emission lines of Copper:
https://physics.nist.gov/PhysRefData/Handbook/Tables/coppertable3.htm.

Indium

In the flame test, the indium salts color the flame with a beautiful, easily recognizable blue color. The emission detected with our spectrometer is composed of a band at approximately λ = 450 nm, this emission line is associated with the transition 5p → 6s.

Main emission lines of Indium:
https://physics.nist.gov/PhysRefData/Handbook/Tables/indiumtable3.htm.

Conclusions

In this work we have seen how the flame test analysis technique, enriched by the use of a spectrometer, can be used profitably not only for qualitative chemical analyzes, but also to illustrate some properties of atomic spectra such as the emission of the discrete lines corresponding to electronic transitions.

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