Isothermal saturation method

This method is simple enough and includes some modifications  

placing baked sample of the oxide in the melt-solvent and regular tests of the metal concentration in the melt up to the saturation. Known test routines are either radiochemicaP or complexonometric analysis of cooled samples of the saturated solutions  

the addition of known weights of oxide to the melt up to the saturation detected by a potentiometric technique.  

The sum of molecular and ionic form concentrations in the saturated solution, 2sMeo is obtained as the main result of such investigations, this magnitude may be expressed by the following equation  

Since Ssmeo s the main result of this method, there exists a possibility to determine the dissociation constant according to routine 2 if an oxygen electrode was preliminary calibrated by known additions of the completely dissociated Lux base. But there is no information about such studies. 

The method of isothermal saturation consists of the addition of an excess quantity of oxide to the melt studied. Such an addition results in the formation of a saturated solution which is in equilibrium with the oxide precipitate. The fact that the equilibrium conditions have been achieved is detected in different ways among them we should mention potentiometric measurements using different oxygen electrodes [238, 326] and titrimetric determination of the concentration of metal ions in a sample of the melt [327, 328]. The sum of the concentrations of ionic and non-dissociated forms of the oxide according to equation (3.6.5) is the main result of these determinations. Taking into account the thermodynamic parameters which describe equilibria in the saturated solutions, equation (3.6.5) may be rewritten as 

Equilibria in solid oxide-ionic melt systems 

Naturally, if the potentiometric titration routine is used for controlling the saturation of the solution, the preliminary calibration of such a cell with known quantities of a strong Lux base allows us to divide the total magnitudes of oxide solubility into those connected with the ionic and molecular forms of the oxide. However, such determinations are not described in the literature. 

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 analysis of the melt samples with dissolved oxides is often used [327, 328] although it may give rise to considerable distortions of the data obtained, owing to inclusions of particles of the suspended oxide in the analysed sample. This fact must be especially taken into account, since the densities of the oxides studied are often 1.5-2 times higher than that of the ionic melt-solvent. In particular, such inclusions might be present in the 

---

The system Nd(I03)3-MI03-H20 (M = K, Rb) were studied by the isothermal saturation method, and 3Nd(I03)3-2KI03-2H20 was described and studied by TGA and IR.247... 

Naumann and Reinhardt [329] described the use of the radioactive marker method to determine the oxide solubilities. The advantages of this method concern only the procedure for the subsequent treatment of the obtained sample of the melt with the metal oxide, whereas its drawbacks are similar to those of the classic isothermal saturation methods. This radiochemical method has not been used extensively for solubility determinations in molten salts. 

There are many data on oxide solubility in the melts of the KCl-NaCl system, obtained by all the above-described methods. We shall begin our consideration with the isothermal saturation method. 

Amirova et al. [342] reported an investigation of V2O3 solubility in molten KC1, NaCl and the equimolar KCl-NaCl mixture at 700 °C (only for the last melt), 850 and 950 °C. The solubility of vanadium(III) oxide was determined by the isothermal saturation method. The concentration of V(III) in the melt was determined by two methods. The first consisted in the... 

Barbin and Nekrasov studied the solubility of lithium oxide in the molten KCl-NaCl equimolar mixture in the temperature range 973-1073 K by the isothermal saturation method [344,345]. The dependence of Li20 solubility in... 

The fact that the data obtained by the isothermal saturation method are lower than those from the potentiometric titration leads us to the conclusion that... 

As mentioned in Section 3.7.1.2, there is a considerable scatter of solubility product values obtained in the molten KCl-NaCl eutectic using different methods of solubility determination. This disagreement in the solubility parameters may be explained by differences in the sizes of oxide particles whose solubility is to be determined. The difference in size causes the scatter of the solubility data according to the Ostwald-Freundlich equation and the employment of the isothermal saturation method, which implies the use of commercial powders (often pressed and sintered), leads to values which are considerably lower than those obtained by the potentiometric titration technique where the metal-oxides are formed in situ. Owing to this fact, the regularities connected with the effect of physico-chemical parameters of the oxides or the oxide cations should be derived only from solubility data obtained under the same or similar experimental conditions. However, this does not concern the dissociation constants of the oxides, since homogeneous acid-base equilibria are not sensitive to the properties of the solid phase of... 

The system Nd(I03)3-MI03-H20 (M = K, Rb) were studied by the isothermal saturation method, and 3Nd(I03)3-2KI03-2H20 was described and studied by TGA and IR. Perrhenates and pertechnetates M(M 04)3 4H20 (M = Y, Ho-Lu M = Tc or Re) are shown to be isostructural by powder pattern with the single crystal X-ray structure which had been obtained for M — Yb, M = Re. IR studies were also made. ... 

Studies connected with the analysis of cooled samples allow to obtain thermal dependencies of oxide solubilities by analysis of the saturated melt heated to the definite temperature. Main disadvantages of isothermal saturation method are ... 

As for determination of solubility and dissociation parameters it should be noted that this method allows to determine values only if tho potentiometric curve contains the non-saturated solution region. Studies in the wide temperature range are impeded as compared with the isothermal saturation method. 

Frederics and Temple studied CuO, MgO, PbO and ZnO solubilities in molten equimolar mixture KN03-NaN03 in the range 290-320 C. The solubilities were determined by isothermal saturation method with the potentiometric control of solubilities. Solubility products were 2.24x10 for CuO, 2.2x10 for ZnO, 2.16x10 for MgO and 4.34x 10 for PbO. The increase of the temperature to 320 C led to the increase of the solubilities by 6-10 times. 

Individual Lux acids derived from B2O3 (borax) and P2O5 (sodium metaphosphate) are interesting since they are often used as acidic components of different fluxes. Delimarsky and Andreeva determined PbO solubility in molten NaPOj at 720 C by isothermal saturation method with the potentiometric control of saturation. The concentration cell with oxygen electrodes Pt(02) was used. PbO solubility was estimated as 31.6 mol%. 

Equihbrium concentration of Mg and in the melt at 800°C equals to 2.1x10 mol kg. Similar investigation performed by the isothermal saturation method gave the... 

---