Molten KCl-NaCl

Cherginets and Rebrova studied oxide solubilities in this melt at 700 C. Solubility products of CoO (pP=4.43+0.11) NiO (pP=5.34 0.2) and MgO (pP=5.87 0.05) were higher than those in molten eutectic KCl-NaCl. The shift of the solubilities in molar fraction scale was 3.36 log units and allowed to estimate solubilities of other MeO oxides on the base of the known values in KCl-NaCl. It has been shown that acidic properties of Pb and Cd cations were suppressed by those of LC, therefore, these cations and Ba and Sr did not change oxoacidic properties of the molten KCl-LiCl. 

One of the first works devoted to oxide solubilities determination in this melt has been performed by Naumann and Reinhardt CaO, SrO and BaO solubilities have been detennined by isothermal saturation technique with isotopic control of the saturation in temperature range from melting point to 900 C. KCl-NaCl and individual molten chlorides, KCl and NaCl, have been used as solvents. Oxide solubilities have been stated to increase in the sequence CaO SrO BaO and solubility product values in KCl-NaCl and KCl were similar, and flie values for NaCl were higher. A comparison of experimental and thermodynamic data for MeO and Me +O solutions have shown that the solubility values are between values calculated for solutions of completely dissoeiated and eompletely moleeular oxide. It means ftiat ftiese eonstituents were present simultaneously. 

Similar investigations have been done in the temperature range 700-800 C by Volkovich. Saturated solutions were prepared by placing pressed oxide tablets into the melt and holding them for 2-3 h. The analysis of solution samples was performed by eomplexometrie titration. 

Potentiometrie studies of eations Ca, LC and Ba were eondueted to determine acidie properties at 700 C the ealeulated dissoeiation eonstants (in mol%) were 2.8x 10 for 

2x10 for BaO, these values showed that all studied cations had considerable acidic properties. -  

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Water with dissolved salt was more likely to lead to explosions, and the water temperature was less significant. (Related tests with molten KCl-NaCl mixtures poured into water led to explosions even for deep water pools or when soluble oils were present.)... 

Fig. 1.2.5. Results of potentiometric investigations of acidic properties of Cr202- in the molten KCl-NaCl equimolar mixture at 700 °C (a) potentiometric curves 1, 0.101 mol kg-1 of Cr202- + O2- (F, the dependence of the ligand number n against the initial concentration of the Lux base) 2, 0.080 mol kg-1 Cr202- + CO2- 3, 0.043 mol kg-1 Cr202- + 02 (b) fractions of oxocompounds of CrVI in diluted solutions (<0.05 mol kg-1) at different pO.

Despite the possibility of the simultaneous realization of these processes, the potentiometric curves of acid-base titration of Mo03 have no distinctive features as compared with the conventional ones [145]. However, the reverse titration curve differs appreciably from the direct titration results. Using the latter data the equilibrium constant of the formation of dimolybdate in the molten KCl-NaCl eutectic was calculated in Ref. [145] as... 

Shapoval et al. reported an examination of the acidic properties of metaphosphate ions in the molten KCl-NaCl eutectic at 700 °C [141]. This investigation was performed by the potentiometric method with the use of the gas platinum-oxygen electrode as an indicator. The potentiometric titration with weights of a strong base resulted in the neutralization process occurring in two stages, listed below ... 

To determine the acidic properties of vanadium oxocompounds, we performed potentiometric titration of V2O5 in an equimolar mixture of molten KCl-NaCl at 700 °C with known weights of Na2C03 and NaOH (see Fig. 1.2.9a) [157]. The addition of initial weights of V205 to the molten... 

Fig. 1.2.11. Results of potentiometric investigation of the acidic properties of Nb205 (1, 0.01 mol kg-1) and Ge02 (2, 0.051 mol kg-1) and the corresponding dependences of the ligand number n (F and 2, respectively) in the molten KCl-NaCl eutectic at 700 °C.

The correct quantitative data can be obtained only for molten alkali metal halides and, mainly for the molten KCl-NaCl eutectic as the reference melt for oxoacidity scales. These data are collected in Table 1.2.3. 

These chloride melts are referred to as ionic media with low-acidic properties. As seen from the previous Part, they can be used as a zero (reference melts) for the construction of the general acidity scale of high-temperature ionic melts. The behaviour of most oxygen electrodes was studied in the molten KCl-NaCl, which is generally accepted as the classic molten medium. 

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. 

The potentiometric investigations of the Pt(02)lYSZ membrane oxygen electrode at 700 and 800 °C by the calibration using various Lux bases have been reported in some of our papers. The potentiometric cell used for the studies in the molten KCl-NaCl eutectic consisted of the oxygen electrode of the type under study and the silver reference one [233]. The oxide-ion donors... 

The data represented by dependence 4 (Fig. 2.4.9a) show that carbonate ions are incompletely dissociated according to equation (2.3), even in diluted solutions, since the corresponding calibration E-pO plot is located above the similar plot obtained with the use of strong bases (KOH or NaOH). The calibration -pO plot for the Pt(02)lYSZ electrode, obtained at 800 °C, has no appreciable distinction from the plots constructed at 700 °C it consists of two linear sections with the inflection point located at pO 3. The parameters of the calibration E-pO plots obtained in the molten KCl-NaCl eutectic melt are contained in Table 2.4.3. 

Fig. 2.4.10. Results of investigation of Pt(02) and Pt(02) IYSZ oxygen electrode reversibility in the molten KCl-NaCl eutectic at 700 °C (a) potentiometric curves 1, Pt(02)lYSZ 2, Pt(02) (b) E-pd dependences 1, Pt(02)lYSZ 2, Pt(02).

Fig. 2.4.12. Dependence of e.m.f. vs the initial molality of the base-titrant in the titration of CrCl3 (—0.02 mol kg-1) with NaOH weights in the molten KCl-NaCl eutectic at 700 °C.

The dependence of the dissociation constant (pK) of carbonate ion in the molten KCl-NaCl equimolar mixture was found by Combes et al. [310] to be expressed by the following equation,... 

Combes et al. reported investigations of the equilibrium constant of reaction (1.2.4) in the molten KCl-NaCl equimolar mixture [119, 180, 240], The experimental routine consists in addition of a weight of O2- donor to the melt afterwards the equilibrium e.m.f. of the cell with the indicator oxygen electrode (NilNiOlZr02) is measured and the e.m.f. values are determined at partial pressures of water vapour from 0.0013 to 0.011 atm. The calculations of the equilibrium constant of reaction (1.2.4), performed using the obtained e.m.f. values, show that in the temperature range 737-817 °C the dependence of the pK of the equilibrium (1.2.4) upon the inverse temperature is expressed by the following equation ... 

Studies of the process of hydroxide ion dissociation performed over a wide range of initial concentrations of oxide ions and partial pressures of H20 demonstrate that in molten alkali metal halides this process does not comply with the generally accepted equation (1.2.4) [232, 321], The dissociation process of OH-, both in the KCl-LiCl eutectic and in the molten KCl-NaCl equimolar mixture, was found to proceed according to the scheme ... 

As to the problem of completeness of hydroxide ion dissociation in the atmosphere of inert gas, which is not purified from water traces, the situation is as follows. By taking into account the value of the equilibrium constant of reaction (2.5.68), which is equal to 0.006 at 700 °C in the molten KCl-NaCl equimolar mixture, one can estimate the degree of dissociation of hydroxide ion as 0.995 ( 1) at the partial pressure of water vapour near 10-5 atm. (this is the usual concentration for gaseous extra-purity A), 0.857 at 0.001 atm. (for pure Ar) and 0.376 at 0.01 atm. (which is appropriate for nitrogen of technical quality). This means that only extra-pure argon can be used without preliminary purification for the creation of an inert atmosphere over the melts for oxoacidity studies, where the degree of hydroxide-ion dissociation approaches 1 (unity). 

The isothermal saturation method implies the carrying out of a number of experiments to obtain a set of data suitable for statistical treatment. This makes the consumption of reagents and time higher than needed with the method of potentiometric titration. Some information about the duration of the experiments and the frequency of sample testing may be obtained from Fig. 3.6.1, which presents the results of investigations of SrO and BaO solubilities in the molten KCl-NaCl eutectic at different temperatures. It is seen that after keeping the solid oxide in the melt for 2-3 h the dependences achieve a plateau which corresponds to the formation of the saturated solution of oxide. 

The experimental results and those derived from the potentiometric titration of lead cations with additions of NaOH in the molten KCl-NaCl equimolar mixture at 700 °C are typical examples of the division of the potentiometric titration curves into sections for unsaturated and saturated solutions. We shall now consider the data in Table 3.6.1 and Fig. 3.6.3. 

Experimental and derived results of potentiometric titration of PbCl2 (0.050 mol kg-1) in the molten KCl-NaCl equimolar mixture at 700 °C... 

All the routines described for the determination of the thermodynamic (concentration) parameters in metal oxide solutions include some indirectly obtained values. For example, the equilibrium concentration of metal cations is calculated proceeding from the quantity of the oxide-ion donor consumed for titration (precipitation). Direct determination of the concentration of metal cations in the melt (if it is possible) allows one to obtain more correctly the obtained solubility product values. Our paper [332] reports a method for correction of the solubility product values for oxides on the basis of the potentiometric titration data. The modification of the standard routine consists of the simultaneous use of two indicator electrodes, one of which is the membrane oxygen electrode and the other is a metal electrode, reversible to the cations the oxide consists of. This routine was used to estimate the solubility products of copper(I) and nickel(II) oxides in the molten KCl-NaCl equimolar mixture at 700 °C. Investigation of Cu20 by the proposed method is of considerable importance since, as will be shown further, the process of dissociation/dissolution of copper(I) oxide in molten alkali-metal halides differs from the generally accepted one which was considered, e.g. in Ref. [119]. 

The silver electrode, which is a silver wire with a platinum top immersed in a 0.2 mol kg-1 solution of AgCl in the molten KCl-NaCl equimolar mixture is used as reference electrode [332]. The metal electrode is made as a plate of the metal (2-3 X 1 cm), whose oxide solubility is determined with the platinum top immersed in the solution studied. 

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