Spectroscopic methods, molten salts

Spectroscopic methods, molten salts, 702 Spectroscopy detection of stmctnral nnits in liquid silicates, 747 and structure near an ion, 72 Standard partial gram ionic entropies, absolute, II Thermodynamics, applied to heats of solvation, 51 of ions in solution, 55 Time average positions of water near ions. 163 Tools, for investigating solvation, 50 Transformation, chemical, involving electrons, 8 Transition metals... 

The formation of complex ions is an important problem for the study of the structure and properties of molten salts. Several physicochemical measurements give evidence of the presence of complex ions in melts. The most direct methods are the spectroscopic methods which obtain absorption, vibration and nuclear magnetic resonance spectra. Also, the formation of complex ions can be demonstrated, without establishing the quantitative formula of the complexes, by the variation of various physicochemical properties with the composition. These properties are electrical conductivity, viscosity, molecular refraction, diffusion and thermodynamic properties like molar volume, compressibility, heat of mixing, thermodynamic activity, surface tension. 

Such problems can be tackled by spectroscopic means, as shown later. Raman spectra, in particular, would indicate new lines having characteristic frequencies when Br is added to CdlNOjjj in KNOj-LiNOj, and in the preceding section it has been shown that an analysis of the variations of the electrode potential of Cd(N03)2in KNOj-LiNOjWith Cr addition has given reason to believe in complex ions in the cases quoted. However, there is a nifty electrochemical method that allows one to also obtain the lifetime of the individual ions and hence remove doubt as to the real existence of complex ions in molten salts. 

Nuclear Magnetic Resonance and Other Spectroscopic Methods Applied to Molten Salts... 

Spectroscopic methods provide specific information on the structure of molecules, on the chemical environment of atoms and their oxidation state. The basis of spectroscopy consists in monitoring changes that occur at the interaction of radiation with substances, eventually at radiation after the interaction of the excitation energy with the substance. Quite recently, an excellent review on the methods and recent results of Raman scattering from molten salts was given by Papatheodorou and Yannopoulos (2002). 

As a final example, the anodic response of sulfur electrodes in molten salts can be considerably more complicated even than that of oxygen electrodes, not least because both sulfur atoms and ions have a tendency toward catenation. Hence, it is always necessary to consider families of polynuclear species like and Sx, rather than their dimeric persulfide, 82 and supersulfide, 82 , derivatives. Frequently, it becomes impossible to resolve the electrochemical behavior of these species and spectroscopic methods are required in support of data interpretations. 

Evidence for the formation of the first dizinc(I) salt was observed in 1967 by introducing Zn into molten ZnClj [16]. The existence of Zn2Cl2 was proven by UV-vis, Raman, and ESR (Electron Spin Resonance) spectroscopy as well as magnetic susceptibility measurement and chemical methods however, no crysttJ structure has been obtained yet. Dizinc(l) dihydride, ZnjHj, was claimed to be produced in 1995 by the matrix-isolation technique and could be identified by spectroscopic methods involving deuterium substitution and comparisons with quantum mechanical calculations [17]. 

The alkali metals were discovered by several special individuals using some new techniques. The daredevil showman Davy isolated potassium and sodium by electrolysis of molten salts, the young Swede Arfwedson identified lithium by quantitative analysis, Bunsen and Kirchhoff identified rubidium and cesium spectroscopically, and Perey prepared minute quantities of francium by radiochemical methods. 

In addition to the more or less classical electroanalytical methods discussed above, the application of thin-layer electrochemical cells, spectroscopic characterization coupled with electrochemical generation, and the use of small digital computers coupled to electrochemical instru-mentation - for molten salt studies have been reported. 

---