Mixtures of molten salts

We define the reference state as the infinitely dilute solution of the component in the solvent, so that yM2A, 7m 7ma- and 7m 7a2- a l 8° t0 unity as m2 goes to zero. This definition fixes the values of the quantities m2a + Mma- )  

The quantity, represents the chemical potential of the undissociated species M2A in its standard state, and not that of the component. The other standard-state quantities represent the chemical potential of the designated species in their standard states, but, for the present, we cannot separate the two standard chemical potentials from the sums, neither is it important to do so. The standard-state quantities appearing in Equations (8.199)—(8.201) are not all independent, because the three equations are equivalent. If we equate Equations (8.199) and (8.200), we obtain an expression that can be evaluated experimentally for the quantity (/ij + + ma- m2a)- Similarly, we obtain an expression that can be evaluated experimentally for the quantity (2— /i 2a) when we equate Equations (81.99) and (8.201). These last two quantities are related to the equilibrium constants for the chemical reactions. This relation is developed in Chappter 11 and the basic experimental methods are discussed in Chapters 10 and 11. 

when in the liquid state, continue to exhibit the ionic properties of the solid state. It is therefore advantageous to consider the pure molten salts and mixtures of molten salts as systems composed of ionic species, and 

For the first case we consider a salt having the general formula Mv+Av and assume that the only species are M+ ions and A ions. The chemical potential of the component can be expressed as 

We choose the pure component to be the reference state for the compound, and therefore also the standard state. We choose the reference state of the M + species to be a fictitious system that contains only M + molecular entities and the reference state of A species to be a fictitious system containing only A molecular entities according to Equation (8.203). The symbols, p t and p% then represent the chemical potentials of the M+ and A species, respectively, in their standard state—the fictitious systems. However, the pure component is also a mixture of the two ions and, according to Equations (8.202) and (8.203), we have 

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The presence of several anions in these ionic liquids has the effect of significantly decreasing the melting point. Considering that the formation of eutectic mixtures of molten salts is widely used to obtain lower melting points, it is surprising that little effort has been put into identifying the effects of mixtures of cations or anions on the physical properties of other ionic liquids [17]. 

Mixtures of molten salts obtain one of the following three equations ... 

In mixtures of molten salts it is necessary to take into account the fact that the interaction forces are very strong and that the nearest neighbors of the cations are anions and the nearest neighbors of the anions are always cations. Therefore, random distributions of anions and cations cannot be conceived. On the basis of these considerations Temkin [18] proposed a model for ideal mixtures of molten salts which assumes the existence of two interlocking sublattices, one of cations and the other of anions. In the case of mixing of two salts, the cations mix on the cation sublattice and the anions mix on the anion sublattice. 

An ideal mixture of molten salts is a mixture for which the heat of mixing, energy of mixing and variation of volume of mixing have the value zero. Certainly, in practice there are no such ideal mixtures. The Gibbs energy of mixing... 

The deviations from ideal behavior can be calculated for all thermodynamic and transport functions of mixtures of molten salts, and these reflect the phenomena which take place at mixing ion associations, formation of complex ions, etc. 

The enthalpy of mixing of some mixtures of molten salts with monovalent ions could be calculated in terms of the quasi-chemical theory given by Guggenheim [19] as follows ... 

Bredig [21] presumed that in a mixture of molten salts the variation of the interaction parameter X [see Eq. (8)] makes evident the formation of complex ions. Thus, in the KCl-CdCl2 and RbCl-MgCl2 systems it is observed variations of the X parameter and with the composition of the melt, and the minimum points of those variations correspond to the composition of complex ions. However, the most convincing data on the formation of complex ions can be obtained from measurements of the enthalpy of mixing [22],... 

These equations show that the sum of the transport numbers for the ions in a molten salt or a mixture of molten salts is always unity. 

The situation is less arbitrary in molten salt mixtures. In that case, the migration of two or more different ions can be defined relative to another ion, often a common ion, in order to avoid the arbitrary reference to a porous plug. That transport number is designated as internal. In a binary mixture of molten salts MX + NX, the internal mobilities of the cations M+ and N+ are referred to the mobility of X and are given by the expressions [39]... 

In the case of mixtures of molten salt, usually only one cation is reduced during electrolysis for example, in the electrolysis of a PbCl2 + NaCl mixture, the cathode reaction is... 

All his life, Temkin contributed to science in many areas, such as diffusion of heavy water into ordinary water, fugacity of gas mixtures, theory of mixtures of molten salts, and mass transfer in chemical engineering. But he left his indelible mark in the fundamentals of catalytic kinetics, on a par with C. J. Christiansen and J. Horiuti. 

Theoretical interpretation of the concentration dependence of equivalent conductivity for simple binary mixtures was first presented by Markov and Shumina (1956). It should be emphasized that this theory, even when considering simple structural aspects, represents rather a method of interpretation of the experimental data than a genuine picture of the structure of the melt. In molten salts generally only ions and not molecules are present, hence the conception of Markov and Shumina (1956) is to be considered also from this aspect. Their theory is based on the assumption that the electrical conductivity of a mixture of molten salts varies with temperature like pure components. In this respect, general character of the electrical conductivity dependence on composition, indicating the interaction of components in an ideal solution, could be expected. 

In a mixture of univalent salts of the type AX-BX, the following interactions should be present AA, BB, AXB, and BXA. The last two interactions are equal, thus they can be written as 2AXB. Considering that the probability of the interactions mentioned is proportional to their molar fractions, Markov and Shumina derived a relation for the composition dependence of the equivalent conductivity in a mixture of molten salts in the form... 

Consider a mixture of molten salts composed of a moles of component A and b moles of component B. Eor the volumes of these components, it holds... 

According to Klemm and Schafer (1996) it is assumed that the binary mixture of molten salts M1X-M2X consists of five kinds of particles, three of which are charged and two are neutral M, mJ, X , MiX, and M2X. The sum of mole fractions of all the particles equals unity... 

Electrolysis, the splitting (lysing) of a substance by the input of electrical energy, is often used to decompose a compound into its elements. Electrolytic cells are involved in key industrial production steps for some of the most commercially important elements, including chlorine, copper, and aluminum. The first laboratory electrolysis of H2O to H2 and O2 was performed in 1800, and the process is still used to produce these gases in ultrahigh purity. The electrolyte in an electrolytic cell can be the pure compound (such as H2O or a molten salt), a mixture of molten salts, or an aqueous solution of a salt. The products obtained depend on several factors, so let s examine some actual cases. 

The acidity and the oxidation/reduction properties of molten salts can be adapted to the required values with the composition of mixtures of molten salts. 

Results of Mixtures of 1 1 Molten Salts. We are not aware of any MSA calculation for mixtures of molten salts. One of us has recently completed(24) calculations of the partial structure functions S j(k) for various LiBr-KBr mixtures, for which molecular dynamics simulations were available(25). 

Similarly, what if a mixture of molten salts contained more than one anion For example, in a mixture of NaBr and NaCl, which of the two anions is oxidized at the cathode The answer is similar the anion that is more easily oxidized (the one with the more negative electrode potential). 

THERMODYNAMIC PROPERTIES OF MOLTEN SALT SOLUTIONS Mixtures of Molten Salts... 

Ce-Mo process. Treatment in the Ce-Mo process resulted in a very corrosion resistant snrface with a high value. Further illustration of the marked improvements in corrosion resistance of Al 6061 is shown in Fig. 2.8, where the impedance only show little decrease after the treated sample was exposed to 0.5 N NaCl for 30 days. The impedance spectra shown in Fig. 2.8 for Al 6061 are very similar to those commonly observed for stainless steels in NaCl solutions. Mansfeld and Perez also developed a process for the surface modification of AA6061-T6 by immersion in a mixture of molten salts such as a NaCl-SnClj-CeCl3 melt at 200 °C. Samples treated in molten salts for 2 hours were subsequently exposed to a NaCl solution for 30 days to evaluate resistance against pitting corrosion by EIS. The results showed that this coating technique enhanced the pitting corrosion resistance of alitmimtm alloys in contact with seawater. 

Reports in this volume by Chimishkyan (p. 155) and by Wertejuk et al. (p. 91) describe methods for destruction of Adamsite developed in Russia and Poland, respectively. For arsenical agents of all types, other possibilities include oxidation of the arsenical agent with sodium hypochlorite (NaOCl) in aqueous medium, and oxidation by a mixture of molten salts that includes oxidants such a sodium nitrate and sodium peroxide. 

Voigt W, Zheng D (2002) Solid-liquid equilibria in mixtures of molten salt hydrates for the design of heat storage materials. Pure Appl Chem 74 1909-1920... 

Mixtures of molten salts have the advantage of lower melting points, and comparative results can be obtained by using one salt as the main constituent—the solvent—to which smaller amounts of a second substance are added. For mixtures of the salts of the alkali metals, the conductances are high and approximately additive. When an alkali metal chloride is mixed with a poorly conducting chloride, such as those of Zn, Cd or Al, there is evidence that the latter takes up chloride ions to form a complex anion. This is in keeping with the tendencies observed in aqueous solutions, and complex-ion formation, like ion-pairing, can be expected to be common in ionic melts. 

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