Alkali metals, spectra

For many years, flames have been used to excite emission spectra for various elements, and most modern atomic absorption spectrometers are readily adapted for flame emission measurements. Flames are not widely, used for this purpose, however, because for most ingle-element determinations, absorption methods provide as good or better accuracy, convenience, and detection limits. For multielement analyses, plasma sources are far superior to flames in most regards. For these reasons flame emission spectrometry is little used except for the determination of the alkali metals and occasionally calcium. These elements are excited at the relatively low temperatures of flames to give spectra that are remarkably simple and free of interference from other metallic species. Alkali-metal spectra generally consist of a relatively few intense lines, many of which are in the visible region and are well suited to quantitative emission measurements. 

Whereas the emission spectrum of the hydrogen atom shows only one series, the Balmer series (see Figure 1.1), in the visible region the alkali metals show at least three. The spectra can be excited in a discharge lamp containing a sample of the appropriate metal. One series was called the principal series because it could also be observed in absorption through a column of the vapour. The other two were called sharp and diffuse because of their general appearance. A part of a fourth series, called the fundamental series, can sometimes be observed. 

In Pedersen s early experiments, the relative binding of cations by crown ethers was assessed by extraction of alkali metal picrates into an organic phase. In these experiments, the crown ether served to draw into the organic phase a colored molecule which was ordinarily insoluble in this medium. An extension and elaboration of this notion has been developed by Dix and Vdgtle and Nakamura, Takagi, and Ueno In efforts by both of these groups, crown ether molecules were appended to chromophoric or colored residues. Ion-selective extraction and interaction with the crown and/or chromophore could produce changes in the absorption spectrum. Examples of molecules so constructed are illustrated below as 7 7 and 18 from refs. 32 and 131, respectively. 

Licht et al. [17] developed a method of numerical analysis to describe the above-quoted equilibria of the 11 participating species (including alkali metal cations) in aqueous polysulfide solution, upon simple input to the algorithm of the temperature and initial concentration of sulfur, alkali metal hydroxide, and alkali metal hydrosulfide in solution. The equilibria constants were evaluated by compensation of the polysulfide absorption spectrum for the effects of H8 absorption and by computer analysis of the resultant spectra. Results from these calculations were used to demonstrate that the electrolyte is unstable, and that gradual degradation of polysulfide-based PECs (in the long term) can be attributed to this factor (Chap. 5). 

In this Section we want to present one of the fingerprints of noble-metal cluster formation, that is the development of a well-defined absorption band in the visible or near UV spectrum which is called the surface plasma resonance (SPR) absorption. SPR is typical of s-type metals like noble and alkali metals and it is due to a collective excitation of the delocalized conduction electrons confined within the cluster volume [15]. The theory developed by G. Mie in 1908 [22], for spherical non-interacting nanoparticles of radius R embedded in a non-absorbing medium with dielectric constant s i (i.e. with a refractive index n = Sm ) gives the extinction cross-section a(o),R) in the dipolar approximation as ... 

The experimental system for measuring the sonoluminescence spectrum of alkali-metal atom emission from an aqueous solution is similar to that for measuring the MBSL spectrum from water. Degassing the solution is an important procedure because the presence of dissolved air affects the emission intensity. In an air-saturated solution, no observation of alkali-metal atom emission has been reported, whereas continuum emission can be observed. A typical experimental apparatus using ultrasonic standing waves is shown in Fig. 13.3 [8]. The cylindrical sample container is made of stainless steel, and its size is 46 mm in diameter and 150 mm in... 

Abe S, Choi P- K (2008) Effect of frequency on sonoluminescence spectrum from alkali-metal solutions. Nonlinear acoustics- Fundamentals and applications. AIP Confer Proc 22 189-192... 

Increasing the reducing agent/W ratio and using Na-naphthalenide led to the isolation of the two-electron-reduced, diamagnetic compound 19. The C2v symmetry of the h NMR spectrum and the X-ray analysis are in agreement with the cen-trosymmetric structure sketched in Scheme 2 for 19 [W = W, 2.614(1) A]. The six-coordination of the metal and the inclusion of the alkali metal cation removes the planarity ofthe 04 core and the cone conformation of the calix[4]arene. The sodium cation within the calix[4]arene cavity is r 3-bonded to two opposite arene rings. 

The bonding in solids is similar to that in molecules except that the gap in the bonding energy spectrum is the minimum energy band gap. By analogy with molecules, the chemical hardness for covalent solids equals half the band gap. For metals there is no gap, but in the special case of the alkali metals, the electron affinity is very small, so the hardness is half the ionization energy. 

---