Question

Please explain to me why we use blue cobalt glass in flame tests?

Answer 1

Hint: Cobalt glass is a blue coloured glass which is prepared by including a cobalt compound, usually cobalt oxide or carbonate in a glass melt. It is a very intense colouring agent and is used in a very little amount to observe a noticeable amount of colour.

Complete answer:
A flame test is a qualitative analysis which is used to identify metals and metalloid ions within a given sample. Not all metal ions emit colour when heated on the burner. It is the simplest and easiest way of identifying the presence of alkali metals i.e., group 1 metals in a compound.
In the flame test, when the sample is heater on burner, the metal ion gains energy and shifts from a lower energy level to the higher energy level which is generally termed as excited state. And then they return to the ground state by releasing energy. The wavelength of energy released during this process lies in the visible region i.e., energy is released in the form of light which is responsible for appearance of colour during flame test. Thus, on heating, each metal ion gives a characteristic change of colour.
In flame testing, cobalt blue glass is often used as an optical filter in order to filter out the undesired strong yellow light which is emitted by the traces of metal like sodium and expands the ability to see violet and blue hues and under fluorescent light, the cobalt glass has twice the effect.
Thus, we can conclude that blue cobalt glass is used in flame tests for masking yellow(sodium) emissions.

Note:
It is important to note that cobalt blue glass is coloured blue because it absorbs its complimentary colour i.e., yellow and red-light spectrum. Remember that cobalt blue is especially used for identifying elements which weakly emit violet or blue colour like sodium or potassium.