Alkali metals flame colours

All the alkali metals have characteristic flame colorations due to the ready excitation of the outermost electron, and this is the basis of their analytical determination by flame photometry or atomic absorption spectroscopy. The colours and principal emission (or absorption) wavelengths, X, are given below but it should be noted that these lines do not all refer to the same transition for example, the Na D-line doublet at 589.0, 589.6 nm arises from the 3s — 3p transition in Na atoms formed by reduction of Na+ in the flame, whereas the red line for lithium is associated with the short-lived species LiOH. 

Excitation of the outer ns electron of the M atom occurs easily and emission spectra are readily observed. We have aheady described the use of the sodium D-line in the emission spectrum of atomic Na for specific rotation measurements (see Section 3.8). When the salt of an alkali metal is treated with concentrated HCl (giving a volatile metal chloride) and is heated strongly in the non-luminous Bunsen flame, a characteristic flame colour is observed (Li, crimson Na, yellow K, lilac Rb, red-violet Cs, blue) and this flame test is used in qualitative analysis to identify the M ion. In quantitative analysis, use is made of the characteristic atomic spectrum in flame photometry or atomic absorption spectroscopy. 

The metals and their compounds display characteristic flame colours. If a moistened platinum wire is dipped into a substance containing the alkali metal and introduced into a Bunsen flame, the flame becomes coloured. The colours are shown in Table 12.1. 

Table 12.1 Flame colours of the alkali metals and their compounds ...

The coloured radiations produced when the salts of certain elements are introduced into a flame have been used for identification purposes for many years. In flame photometry a solution of a salt is introduced continuously and at a constant rate so that the intensity of the radiation is a constant value which when measured forms the basis of a quantitative procedure. Many instruments have been built for this purpose and the range of work which can be accomplished depends on the design. In the simplest equipment a cool air/coal gas or air/propane flame is used so that few elements are excited and from the simple spectra the required radiations can be isolated by means of filters. The intensity of the transmitted radiation is then measured with a photocell coupled to a galvanometer or amplifier and meter. Such instruments are used for the determination of the alkali metals and in certain cases the alkaline earths. The more versatile instruments employ a monochromator for isolating the required wavelengths and so hotter flames can be used which excite many more elements and therefore produce much more complex spectra. A sensitive detector is required and a photomultiplier coupled to an amplifier satisfies this need. 

Lithia is not quite so soluble in water as soda or potash, nor is it so caustic but it very much resembles these alkalies. Its solution attracts carbonic acid as readily as theirs from the atmosphere. When lithia is fused on platinum, it corrodes and stains the metal. Lithia and all its salts give a blood-red colour to flame. The carbonate of lithia is sparingly soluble, and its phosphate is nearly insoluble. Lithia occurs too rarely to admit of any useful application / but it is important to know that lithion-mica, which is recognised by its easy fusibility before the blow pipe, and by its tinging the outer flame red, has hitherto been only found associated with albite and topaz, or pycnite, in tin districts, and its occurrence, thus associated, may be looked on as a sure indication of the existence of tin in the locality. 

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