Reciprocal salt mixtures

Systems of reciprocal salt mixtures. The fused salt system of the ions Na" ", K+, Cl , and Br has the following restriction due to electro-neutrality... 

Reciprocal salt mixtures. In a binary system there are three kinds of ions, e.g. A+, B+/X . However, only the amount of two of these three constituents can be independently changed because of the constraint given by the condition of electro-neutrality... 

Figure 9. Reciprocal values of internal mobilities vs. molar volume in some binary systems (Li, K)Br, (Li, K)CI, (Na,K)OH, (Li, K)N03, and (Li, K)(S04)i/2 (Reprinted Rom M. Chemla and I. Okada, Ionic Mobilities of Monovalent Cations in Molten Salt Mixtures, Electrochim. Acta 35 1761-1776,Fig. 10, Copyright 1990 with permission from Elsevier Science.)...

Figure 10. Reciprocal salt solubilities in the ternary systems water/acetone or ethanol/salts at 23°C as a function of the solvent mixture ratio (Q. %) NaCI, KCl, CA, A) KiCrO open symbols represent acetone systems, closed symbols represent ethanol systems.

Figure 6-21. Interaction parameter, x, as a function of the volume fraction, 2, of rigid rods with an axis ratio of 150 in a solvent. The phases isotropic solutions (I), anisotropic mesophases (A), heterogeneous phases (H), and mixed phases of two mesophases (Ai + A2) are formed. The interaction parameter can be replaced by experimentally directly measurable quantities such as the reciprocal temperature [see Equation (6-94)] or the salt concentration in solvent/salt mixtures.

The four salts in each of the above systems form what is known as a reciprocal salt pair . Although all four may be present in aqueous solution, the composition of any mixture can be expressed in terms of three salts and water. Thus, from the phase rule point of view, an aqueous reciprocal salt pair system is considered to be a four-component system. 

The square-prism space model Figure 4.30b) illustrates another way in which a quaternary system of the reciprocal salt pair type may be represented. The vertical axis stands for the water content, and the points on the diagram are the same as those marked on Figure 4.30a. In both diagrams all surfaces formed between the internal curves represent solutions of three salts in water saturated with one salt, all internal curves solutions of three salts in water saturated with two salts, and the two points P and Q solutions of three salts in water saturated with the three salts. The section above the internal curved surfaces denotes unsaturated solutions, the section below them mixtures of liquid and solid. 

Molar or ionic bases must be used in this type of diagram for reciprocal salt pairs. The four corners of the square represent 100 mol of the pure salts AX, BX, BY and AY. Any point inside the square denotes 100mol of a mixture of these salts its composition can always be expressed in terms of three salts. The... 

Ca, K, Li, Na //NO3 with a minimum melting temperature of 109 °C [34], Recently, Bradshaw et al. published data on the quaternary reciprocal system K, Li, Na // NO2, NO3 with a minimum melting temperature of about 75 °C with the composition 33.5 mol% Li+, 47.9 mol% K" ", 18.6mol% Na" ", and a nitrate/nitriteratio of 0.56 [33]. Our own work presented in this paper focuses on the phase diagram determination of the latter system and stability measurements of different single salts and salt mixtures. 

This paper presents an overview of relevant alkali nitrate and nitrite salt mixtures for TES technology in the field of CSP. This overview includes well-known salt systems in the field (KN03-NaN03, KN03-NaN02-NaN03 - Hitec), as well as a recently published system with a low liquidus temperature. This system is based on the quaternary reciprocal systems K, Li, Na//N02, NO3. Compositions with a liquidus temperature as low as 72 °C have been reported. A phase diagram with a fixed ratio N03 /N02 = 0.5 has also been published. The stability of this mixture in air at 450 °C over 120 days is reported to be poor [33]. 

As a simple illustration of the special problems posed by these reciprocal molten salt mixtures we shall consider the mixed system which contains four singly charged ions. A , B , X, Y . This is a three-component system in the sense of thermodynamics, i.e. a mixture of any composition may be prepared by bringing together suitable amounts of three of the four p ire molten salts, AX, AY, BX, and BY. 

Reciprocal molten salt systems are those containing at least two cations and two anions. We shall deal with the simplest member of this class, that containing the ions A+, B+, X-, and Y-. The four constituents of the solution, AX, BX, AY, and BY, will be designated by 1, 2, 3, and 4 respectively. There are four ions in the system and one restriction of electroneutrality. Consequently, of the four constituents, there are only three which are independent components. In order to calculate the Helmholtz free energy of mixing conveniently, we must (arbitrarily) choose the three components. Here we choose BX, AY, and BY. This choice requires that in order to make mixtures of some compositions a negative quantity of BY must be used. This presents no difficulty in the theory and is thermodynamically self-consistent. One mole of some arbitrary composition (XA, XB, Yx, XY) can be made by mixing Arx moles of BX (component 2), XA moles of AY (component 3), and (XY — XA) moles of BY (component 4). ... 

The solubility is a function of the reciprocal radius of the cation of solvent salt. For a mixture, the equivalent radius of the solvent salt is the weighted mean of the radii of the cations of the components. The dissolution of TiCU is followed by formation of the anionic complex, TiClg . 

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