Alkali metals flame colors

Lithium is presently being recovered from brines of Searles Lake, in California, and from those in Nevada. Large deposits of quadramene are found in North Carolina. The metal is produced electrolytically from the fused chloride. Lithium is silvery in appearance, much like Na and K, other members of the alkali metal series. It reacts with water, but not as vigorously as sodium. Lithium imparts a beautiful crimson color to a flame, but when the metal burns strongly, the flame is a dazzling white. 

Rubidium can be liquid at room temperature. It is a soft, silvery-white metallic element of the alkali group and is the second most electropositive and alkaline element. It ignites spontaneously in air and reacts violently in water, setting fire to the liberated hydrogen. As with other alkali metals, it forms amalgams with mercury and it alloys with gold, cesium, sodium, and potassium. It colors a flame yellowish violet. Rubidium metal can be prepared by reducing rubidium chloride with calcium, and by a number of other methods. It must be kept under a dry mineral oil or in a vacuum or inert atmosphere. 

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. 

Because its outet valence electrons ate at a gteatet distance from its nuclei, potassium is more reactive than sodium or lithium. Even so, potassium and sodium are very similar in their chemical reactions. Due to potassiums high reactivity, it combines with many elements, particularly nonmetals. Like the other alkali metals in group 1, potassium is highly alkaline (caustic) with a relatively high pH value. When given the flame test, it produces a violet color. 

When small amounts of alkali metals, alkaline earth metals, or their salts are introduced into a gaseous flame, flame reactions occur relatively easily at low temperatures.The liberated metal atoms are promoted to excited states and then return to their normal states. Radiation corresponding to the characteristic line spectra of the individual metal atoms is emitted as a result of this energy transition. Colored radiation discernible by the human eye, ranging from red to blue, is dependent on the type of metal atoms, as shown in Table 12AP- i... 

Group V Na+, K+, and Mg2+ The only ions remaining in solution at this point are those whose chlorides, sulfides, and carbonates are soluble under conditions of the previous reactions. Magnesium ion is separated and identified by addition of a solution of (NH4)2HP04 if Mg2+ is present, a white precipitate of Mg(NH4)P04 forms. The alkali metal ions are usually identified by the characteristic colors that they impart to a Bunsen flame (Figure 16.18). 

Where only the determination of alkali metals is desired, a simple instrument can be devised in which selection of a resonance line can be obtained with the help of color filters. However, at present such instruments would not appear to have an appreciable advantage over flame emission methods, considering the sensitivity of the latter and the simplicity of presently available flame photometers. Appreciable advantages, on the other hand, are inherent in absorption over emission methods in the determination of the alkaline earths and magnesium. Since these metals have simple emission spectra, the use of filters, notably interference filters, would be feasible in instruments limited to the determination of these elements. 

Spacings of energy levels are different for different alkah metals. The salts of the alkali metals impart characteristic colors to flames lithium (red), sodium yellow), and potassium... 

Analytical chemistry of the alkalis is difficult. There are a few complex reagents that will precipitate with ions like Na+ and K+, but the reactions are best done using ether or alcohol as the solvent in place of water. It is more common to detect the presence of alkali metals in solutions or compounds by the characteristic colors the alkali metal ions impart to flames. In descending order, lithium salts give a carmine color, sodium salts a yellow color, potassium a violet color, rubidium a bluish red color, and cesium a blue color. 

Flame atomic emission spectroscopy, also called flame photometry, is based on the measurement of the emission spectrum produced when a solution containing metals or some nonmetals such as halides, sulfur, or phosphorus is introduced into a flame. In early experiments, the detector used was the analyst s eye. Those elements that emitted visible light could be identified qualitatively, and these flame tests were used to confirm the presence of certain elements in the sample, particularly alkali metals and alkaline-earth metals. A list of visible colors emitted by elements in a flame is given in Table 7.1. 

The accompanying figure depicts the continuous visible spectrum of the Sun followed by the flame emission spectra of 10 alkali metals and alkaline earth metals (in black and white rather than color as they appear naturally ... 

Group 5. The alkali metal ions and The ions that remain after removing the insoluble phosphates are tested for individually. A flame test can be used to determine the presence of KT, for example, because the flame turns a characteristic violet color if is present. 

Since spectra produced by flames are much simpler (fewer lines) than those produced by arc and spark emission, simple devices for spectral isolation could be used. Developments in Europe in the 1930 s led to simpler burners and made use of colored glass filters for spectral isolation. Read-out systems composed only of a photocell connected directly to a galvanometer were used. Such instruments, many still in use, were adequate for simple liquid samples for the determination of the alkali metals. 

Although alkali metal ions are colorless, each emits a characteristic color when placed in a flame ( Figure 7.22). The ions are reduced to gaseous metal atoms in... 

Sodium is miscible with the alkali metals below it in the periodic table (i.e., K, Rb, and Cs), and it forms a eutectic alloy with potassium (Na22 wt.% K78 wt.%) commercially known as NaK, which melts at -10°C. The eutectics formed in the Na-Rb and Na-Cs binary systems melt respectively at -4.5°C and -30°C. Sodium is the minor component with potassium and cesium of the ternary alloy Na-K-Cs. The composition of this ternary alloy is 3 wt.% Na, 24 wt.% K, and 73 wt.% Cs. This fluid is the lowest melting liquid alloy yet isolated, melting at -78°C. Sodium colors the flame of a Bunsen gas burner with a characteristic yellow color owing to the highly intense D line of its atomic spectra (589 nm). Sodium is ordinarily quite reactive with air, and its chemical reactivity is a function of the moisture content of air. 

Rubidium is a silvery white and very soft metal that colors a flame yeUowish-violet. In chemical behavior rubidium resembles sodium and potassium and reacts violently with water. It is a widely distributed element, usually associated with other alkali metals in minerals. The rate of radioactive decay of the isotope Rb can be used in geological age determination (see Chapter 4 Geochemistry). Rubidium is found in small quantities in tea, coffee, tobacco and other plants. 

Alkali metals are easily detected by flame tests because each metal imparts a characteristic color to a flame. 

---