Caesium flame colour

All the cations of Group I produce a characteristic colour in a flame (lithium, red sodium, yellow potassium, violet rubidium, dark red caesium, blue). The test may be applied quantitatively by atomising an aqueous solution containing Group I cations into a flame and determining the intensities of emission over the visible spectrum with a spectrophotometer Jlame photometry). 

Subsequently, in 1860, Bunsen demonstrated that the colouring of the flames used as a means of identifying certain salts was due to the element and not the compound to which it was bound. The power of this new method led him to discover two unknown elements in mineral water caesium and rubidium. Other researchers then soon discovered thallium, indium, gallium and germanium, elements for which lines are emitted in the visible domain. 

The absorption and emission spectra of metals such as copper, lithium, caesium, calcium and potassium involve lines at different wavelengths to those observed in the sodium spectrum. As with sodium, a few lines in each spectrum dominate the colours of compounds of these metals in the gas flame. This is the basis of the flame tests which are used to indicate the presence of compounds of these metals (see page 195). 

Rubidium and caesium salts behave similar to potassium salts. The flame test distinguishes each metal from the others especially if the coloured flame is viewed through a small spectroscope. 

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