Molten salts Subject

Molten salts are characterized by the formation of discrete complex ions that are subjected to coordination phenomenon. Such complex ions have specific compositions that are related to the rearrangement of their electronic configuration and to the formation of partially covalent bonds. The life time of the coordinated ions is longer than the contact period of the individual ions [293]. 

Good electrical conductance is one of the characteristics of many though not all molten salts. This characteristic has often been employed industrially. Various models have been proposed for the mechanism of electrical conductance. Electrolytic conductivity is related to the structure, although structure and thermodynamic properties are not the main subjects of this chapter. Electrolytic conductivities of various metal chlorides at the melting points are given in Table 4 together with some other related properties. "... 

Ketonate complexes of Ru are reported in a number of papers. The parent complex [Ru(acac)3] has been subject to a polarized neutron diffraction study at 4.18 K, to powder neutron diffraction studies and to single-crystal structure determinations at 293 K, 92 K, and 10.5 K. The structure is disordered at all temperatures. Measurements of the magnetic susceptibilities (at 2.5 K and 300 K) have been made along different crystal axis directions, and the results analyzed. An investigation of the relationships between ionization potentials and half-wave potentials of a series of tris(/3-ketonate)Ru complexes has been reported, and the electrochemical properties of [Ru(acac)3] in chloroaluminate molten salt media have been reported. The reduced species [Ru(acac)3] can react with AICI4 reduction by bulk electrolysis of a small amount of [Ru-(acac)3] in the melt yields [RuClg]. ... 

In aqueous solution, thorium exists as Th(IV), and no definitive data have been presented for the presence of lower-valent thorium ions in this medium. The standard potential for the Th(IV)/Th(0) couple has not been determined from experimental electrochemical data. The values presented thus far for the standard reduction potential have been calculated from thermodynamic data or estimated from spectroscopic measurements. The standard potential for the four-electron reduction of Th(IV) ions has been estimated as —1.9 V in two separate references 12. The reduction of Th(OH)4 to Th metal was estimated at —2.48 V in the same two publications. Nugent et al. calculated the standard potential for the oxidation ofTh(III) to Th(IV) as +3.7 V versus SHE, while Miles provides a value of +2.4 V [13]. The standard potential measurements from studies in molten-salt media have been the subject of some controversy. The interested reader is encouraged to look at the summary from Martinot [10] and the original references for additional information [14]. 

Ionic liquids are a class of solvents and they are the subject of keen research interest in chemistry (Freemantle, 1998). Hydrophobic ionic liquids with low melting points (from -30°C to ambient temperature) have been synthesized and investigated, based on 1,3-dialkyl imidazolium cations and hydrophobic anions. Other imidazolium molten salts with hydrophilic anions and thus water-soluble are also of interest. NMR and elemental analysis have characterized the molten salts. Their density, melting point, viscosity, conductivity, refractive index, electrochemical window, thermal stability, and miscibility with water and organic solvents were determined. The influence of the alkyl substituents in 1,2, 3, and 4(5)-positions on the imidazolium cation on these properties has been scrutinized. Viscosities as low as 35 cP (for l-ethyl-3-methylimi-dazolium bis((trifluoromethyl)sulfonyl)amide (bis(triflyl)amide) and trifluoroacetate) and conductivities as high as 9.6 mS/cm were obtained. Photophysical probe studies were carried out to establish more precisely the solvent properties of l-ethyl-3-methyl-imidazolium bis((trifluoromethyl)sulfonyl)amide. The hydrophobic molten salts are promising solvents for electrochemical, photovoltaic, and synthetic applications (Bon-hote et al., 1996). 

It is usual to operate an aqueous-medium fuel cell under pressure at temperatures well in excess of the normal boiling point, as this gives higher reactant activities and lower kinetic barriers (overpotential and reactant diffusion rates). An alternative to reliance on catalytic reduction of overpotential is use of molten salt or solid electrolytes that can operate at much higher temperatures than can be reached with aqueous cells. The ultimate limitations of any fuel cell are the thermal and electrochemical stabilities of the electrode materials. Metals tend to dissolve in the electrolyte or to form electrically insulating oxide layers on the anode. Platinum is a good choice for aqueous acidic media, but it is expensive and subject to poisoning. 

Subject areas for the Series include solutions of electrolytes, liquid mixtures, chemical equilibria in solution, acid-base equilibria, vapour-liquid equilibria, liquid-liquid equilibria, solid-liquid equilibria, equilibria in analytical chemistry, dissolution of gases in liquids, dissolution and precipitation, solubility in cryogenic solvents, molten salt systems, solubility measurement techniques, solid solutions, reactions within the solid phase, ion transport reactions away from the interface (i.e. in homogeneous, bulk systems), liquid crystalline systems, solutions of macrocyclic compounds (including macrocyclic electrolytes), polymer systems, molecular dynamic simulations, structural chemistry of liquids and solutions, predictive techniques for properties of solutions, complex and multi-component solutions applications, of solution chemistry to materials and metallurgy (oxide solutions, alloys, mattes etc.), medical aspects of solubility, and environmental issues involving solution phenomena and homogeneous component phenomena. 

It was John Wilkes who realized that room-temperature molten salts would only experience a widespread interest and uptake if they were stable under environmental conditions. Wilkes group published details of the first such liquid in 1992 using the BF]j" and the PFj anions, the latter showing a miscibility gap with water. Thus these liquids could, in principle, be made water free. (Today we know that ionic liquids containing BFJ and PF are subject to decomposition in the presence of water.) Electrochemical studies showed that even these early ionic liquids had wide electrochemical windows of about 4 V with cathodic limits of-2 to -2.5 V. vs. NHE. This cathodic limit should, from the thermodynamic point of view, be wide enough to electrodeposit many reactive elements. 

In the last few decades, molten salts which melt at low temperature have been the subject of considerable interest, because they retain the advantages offered by molten salts and at the same time make it possible to work at low temperatures. The most important classes of these salts are haloaluminates, chlorocuprates, chlorozincates, etc. 

Some idea of how Raman spectroscopy works—how light from nonelastic scattering on molecules contains information on the vibratory state of the bonds therein— has been given in Section 2.11. Raman spectroscopy can be used to obtain information on the structure of ions in molten salts, as has been shown in the last three sections. Ha-e, two further molten salt systems that eontain eomplexes and that have been subjected to Raman spectroscopy are described. The first one concerns melts of zinc chloride hydrate. 

The Ru(dik)3 complexes have been the subject of numerous electrochemical investigations in different media (from aqueous to non-aqueous solvents, to molten salts) " ... 

After the preliminary selection of 300 series stainless steel, these alloys were subjected to further tests to study the effect of stress in the presence of chloride, oxygen, and water vapor, the effect of sensitization, and the rate at which the alloying constituents are leached and transported under the influence of a temperature gradient. A molten salt loop has been in operation for nearly a year as a part of these tests. The... 

The behavior of an excess electron in dilute metal-molten salt solutions has been the subject of many experimental and theoretical studies [69-72]. The details of the model we employ are exactly the same as the early calculations of Selloni and coworkers [71,72]. Specifically, our simulations have been performed on a periodically repeated system of 32 K+ cations, 31 CP anions, and 1 electron. The mass density was set to p = 1.52 x 10 kg/m. The temperature we use here is T = 1800 K. 

This book should serve as a textbook for all people working in the field of molten salt chemistry. It is aimed at undergraduate students working on their diploma work, for students working on their PhD theses, and also for other graduates, e.g. teachers at universities, scientists in academic instimtes, research institutes, and in industry, i.e. to all who need up-to-date information on this subject. 

The acidity of these organoaluminate molten salts has been investigated in detail by Osteryoung and Wilkes, and reviews on this subject are available.However, since these melts are extremely air and water sensitive, and several organic substrates and organometallic compounds are not chemically inert in these media, their application as fluids for organometallic chemistry is limited. 

Initially, in spite of its complexity, the Houdry process hardware worked satisfactorily. However, the steam cooling tubes were subjected to severe conditions so that after about two years corrosion was sufficiently severe to allow water to contact the catalyst. The steam reacted with the catalyst during regeneration, resulting in severe declines in activity. To overcome this problem, the water/steam fluid was replaced with molten salt to cool the tubes in the case and to act as an intermediate heat transfer agent from the case to the steam generators. This invention has been credited to Socony-Vacuum (27). 

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