KCl-NaCl melt

The synthesis and ion-exchange properties of ceric tungstate have been investigated.521 The acidic properties of WOa in a KCl-NaCl melt have been studied.522" and the reduction characteristics of Mo042- in fused LiCl-KCl reported.5226... 

Since the concentrations of all these admixtures depend on the purity of the salts used for preparation of the melts, they are not constant. This means that at each specific melt-solvent pO, the values of the initial solvent fluctuate about a certain magnitude. For example, pure Csl and KCl-NaCl melts are characterized by pO 4, whereas for Nal and KC1-LiCl melts these values are lower and vary between 3 and 3.5. For comparison, at 298 K pure water as a solvent is characterized by the pH value equal to 7, and this value remains constant after achieving a certain degree of purity of H20. 

The reaction (1.2.70) leads to a reduction of the equilibrium concentration of the acid (MoO +) in the KCl-NaCl melt that, in its turn, causes a shift of equilibrium (1.2.69) to the right. As a result, the [Mo03]/[MoC>4 ] ratio decreases, which results in a reduction in pO, as follows from the equation... 

If the f) values obtained in Refs. [169, 170] may be attributed to the primary medium effect of O2-, they allow us to consider the above-mentioned melts as extremely aggressive acidic media, analogous to anhydrous sulfuric acid in comparison with water (H0 -log y0ff+ = -11.1) [174], In particular, such melts should dissolve appreciable quantities of solid oxides, e.g. MgO or NiO, up to concentrations of the order of 1 mol kg-1. For example, MgO is practically insoluble in the equimolar KCl-NaCl mixture (pP = 9.33 0.06) [175], but should be characterized by values of the order of pP 1 (since 9.33 — 8 1) in the KCl-LiCl eutectic that corresponds to the value of the solubility product of the order of 10 1. For the KCl-NaCl-CaCl2 melt, the solubility-product value should be close to 1. On the contrary, most metal oxides which are slightly soluble in the KCl-NaCl melt were shown by... 

The potentiometric investigation of the metal-oxide nickel electrode in the KCl-NaCl melt at 700 °C shows that this electrode is reversible to oxide ions (see Fig. 2.4.9b) and the slope in the pO range from 1 to 3 is estimated to be equal to 0.109 0.010 V. This is in a good accordance with the 1.15RT/F values characteristic of this kind of oxygen electrode (Table 2.4.3). 

The reversibility of the Pt(02)IZr02(Ca0) membrane oxygen electrode in molten KCl-NaCl has been studied in a number of works. Thus, the authors of Refs. [65, 236, 237] reported the application of the electrode with a Zr02(CaO) solid electrolyte membrane for the potentiometric investigation of some heterogeneous acid-base equilibria and the construction of the row of cation acidity in the KCl-NaCl melt. 

Barbin et al. performed investigations of carbonate-ion stability in ionic melts of a KCl-NaCl system containing 30, 50, 70, and 90 mol% of NaCl in the 752-852 °C temperature range [312], In order to study this equilibrium, the salt mixture with the addition of sodium carbonate in a concentration to provide a saturated solution of Na2C03 in KCl-NaCl melt, was melted and... 

Although the oxoacidic properties of the CsCl-KCl-NaCl and KCl-NaCl melts are practically the same, the pK values of equilibrium (1.2.3) in the former melt are considerably higher than in the latter, at the same temperature. The reason for this is as follows the dissociation constant of carbonate ion in any melt is mainly defined by the stability of the most unstable individual carbonate formed by the constituent cations of the melt the greater... 

An example of the SAM data treatment for zinc oxide is presented in Table 3.7.6. The quantities of ZnO corresponding to points 1-6 do not result in the formation of a saturated solution the e.m.f. (pO) data make it possible to estimate the dissociation constant of ZnO in the KCl-NaCl melt as pKZnQ = 6.37 0.08. Point 7 corresponds to the weight of ZnO which provides saturation of the melt by zinc oxide, thereat the added powder of oxide is dissolved in the melt only partially. Subsequent additions of ZnO to the saturated melt result in an appreciable reduction in the saturated solution s concentration (decrease in the solubility product of ZnO) owing to the reduction in the averaged molar surface area of the deposit and, in turn, the oxide solubility. This reduction ends after the true plateau is achieved, when the changes of molar surface-area are negligible. 

For all the metal cations studied, the potentiometric titration curves at different temperatures show no essential distinctions, and resemble the corresponding dependences for the KCl-NaCl melt. The solubility product calculations demonstrate that an increase in the melt temperature causes the increase in the oxide solubility (pTWo) by 0.3-0.4. The obtained data show... 

The investigations performed in the chloride melts demonstrate that, for melts of the same anion composition containing cations of close acidities (and recall that the oxobasicity indices of CaCl-KCl-NaCl and KCl-NaCl melts are practically the same), the metal-oxide solubilities have very close values. This means that, at the same anion composition, the oxide solubility products depend on the acidity of the constituent cation of the melt... 

Both coefficients coincide with those for the KCl-NaCl melt (1.8 0.9, and 0.053 0.01, respectively), which means that the character of the interaction between the metal cations and chloride ions remains unchanged within the 100-200 °C temperature range. 

It can be seen that the slope of this dependence is close to that of the similar plot obtained in the molten KCl-NaCl equimolar mixture (0.053 0.01) the intersects of these plots differs by 1.84. Such a behaviour of the P MeO — /(/) 2) plot points to the fact that the differences in metal-oxide solubilities in chloride melts of various cation compositions are mainly caused by the difference in the constituent cation acidities. The increase in the metal-oxide solubilities in the Ba2+-based melt as compared with the KCl-NaCl melt (see Table 3.7.23) demonstrates that the oxoacidic properties of Ba2+ cation are considerably stronger than those of K+ or Na+ cations. It should be... 

Fig. 3.7.22. Dependence of solubility of Ce203 in the (CeCy KCl-NaCl) - melts from the concentration of CeCl3 plotted according to the data of Ref. [169].

V.I. Shapoval, V.F. Grischchenko and L.I. Zarubitskaya, Behaviour of Platinum-Oxygen Electrode in KCl-NaCl Melt Containing, Tungstate, Ukr. Khim. Zh. 38 (1972) 1088-1091. 

The new niobium phases with Nb(II,IV), K and F ions were obtained on cathode by molten salts electrolysis of the system K2NbOp5 +FLINAK. These phases never are produced by electrolysis in the chloride molten system. Niobium phase with similar crystal habit and K ions in composition has been reported by E.Christensen et al [1]. We initiated the present investigation in order to define some individual characteristics of the phases. The aims of our study are following (a) identification of the niobium phases (b) determination of their atomic structure (c) analysis of their transformations under washing out a cathodic product with acid water solution and under the action of the KCl+NaCl melt. 

The phases under discussion are decomposed in KCl+NaCl melt to form the phase with IKF net and/or niobium oxides. Products of decomposition depend upon the composition of initial phase and the valence of Nb in it. 

Similarly, the Gibbs energies of solution in salt melts can be determined by measuring the vapour pressures above the melts, for instance the vapour pressures of KCl and NaCl over KCl + NaCl melts. 

Chronovoltammetric studies [9] showed that a stable discharge of combined complexes of zirconium and boron can be achieved by maintaining the molar ratio at a level [Zr(IV)4-B(III)] [F ]> 1 4 at a ratio [Zr(IV)] [B(III)] = 1 2 in the KCl-NaCl melt. Under these conditions, the voltammograms demonstrated a wave of electroreduction of chloride-fluoride heterocenter complexes of zirconium and boron in place of the waves of partial electroreduction of zirconium and boron (Figure 4.8.3). 

It is obvious that the expression in the parentheses by this author is nothing but the primary medium effect of expressed by the difference in values of the equilibrium constants of [9.2.57] for the media compared the reference KCl-NaCl melt, with pKnci/HjO equal to 14 at 700°C and the studied melt. 

Potentiometric studies of cations Ca, Li and Ba were conducted to determine their acidic properties at 700°C. " The calculated dissociation constants (in mol%) were 2.8x10 for Li20, and 1.2x10 for BaO. These values showed that all studied cations had considerable acidic properties in KCl-NaCl melt. 

The thermal dependence of Zt02 solubility (molar fractions) in molten KCl-NaCl was evaluated in the temperature range of 973-1174K x = (-5.7+3.1)10 + (7+3)10. The calculations made on the basis of this dependence show that Z1O2 solubility in KCl-NaCl melt at T 973K is negligible. 

In addition to the oxide solubility data, there are also data on solubility products of alkaline earth carbonates in KCl-NaCl melt. The dissolution process can be described... 

The data collected in this table show that the order of arrangement of oxides according to increase of their solubility corresponds to the similar dependences in the KCl-NaCl and CsCl-KCl-NaCl melts at 600 and 700°C. The performed study makes it possible to estimate the dissociation constants of PbO and CdO, which are close, therefore, it can assumed that the dissociation constants of oxides, having close solubility product values, are practically the same. The solubility parameters are close to those in the KCl-NaCl melt at 700°C. The dependences of the oxide solubility on the polarizing action of cation by Goldschmidt can be approximated by the following equation ... 

Comparison of this data with the results for Si -based melts shows that the oxobasicity indices of SrCl2-KCl-NaCl and BaCl2-KCl-NaCl melts are practically the same ... 

---