Isothermal saturation

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... 

Fig. 6.2. The kinetic curves of isothermal saturation of PrOH with various samples of AKOPr j at 20°C m.p. 140°C (I), 118°C (11, HI), supercooled melt (IV) [1642],...

Here, 6 is the relative coverage and KL is a constant, called the Langmuir constant . Tmon is the maximum amount adsorbed which, in the case of Langmuir adsorption, is a mono-layer. Type C adsorption isotherms are characterized by a saturation at high concentrations. A possible reason is that the surface is completely filled with adsorbed molecules. Langmuir adsorption is often observed for the adsorption from solution but only rarely for the adsorption of gases. This type of adsorption isotherm can also be observed for porous materials. Once all pores have been filled the isotherm saturates (see Section 9.4.3). 

The adsorption isotherm saturates at high partial pressure and the maximal amount adsorbed is that of a monolayer. 

Investigations of the solubilities of metal oxides in oxygen-containing and oxygen-free ionic melts are performed with saturated solutions obtained in different ways. During the past few decades two methods for the solubility studies have been examined and introduced into practice. They are the methods of isothermal saturation and of potentiometric titration. 

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... 

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. 

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. 

The potentiometric titration method possesses some advantages characteristic of potentiometry. The measured e.m.f. values are dependent on the logarithm of the activity (concentration) of the potential-defining ion and this considerably widens the range of concentrations which can be detected. The experimental techniques and routine are rather simple and the obtained results are well reproducible. Owing to the stable activity coefficients of metal cations the measurements can be performed in sufficiently concentrated solutions of the corresponding metal halides. Performing the measurement does not imply any interference in the acid-base processes in the melt studied and the experiment is faster than the isothermal saturation technique discussed above. 

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... 

Komarov and Krotov studied the solubility of Zr02 in the molten KCl-NaCl equimolar mixture at various temperatures, using the method of isothermal saturation [246]. According to their data, the thermal dependence of Zr02 solubility can be approximated by the following equation ... 

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... 

Fig. 3.7.6. Dependence of the solubility products of oxides (pP) of alkaline-earth metals in the molten KCl-NaCl equimolar mixture against the cation radius determined by the methods of potentiometric titration (1, 2, 5) and isothermal saturation (3, 4, 6).

Inyushkina et al. investigated the solubility of MgO in individual molten alkali-metal chlorides CsCl (700-850 °C), KC1 (820-1000 °C), NaCl (850-1100 °C), and RbCl (750-900 °C) [377]. The study was performed by the method of isothermal saturation MgO was added to the melt in the form of the ordinary commercial powder, or of pills pressed at p = 80 MPa and calcined at 1250 °C. Saturation of the solutions with respect to MgO solubility was shown in Ref. [377] to be achieved after keeping the oxide in contact with the melt for 4 h. 

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... 

Days between -180...0 d correspond to the isothermal saturation period between the construction and the starting of the heaters. Days between 0... 53 d correspond to the heater adjustment period, which is approximated by a linear increase of the heater temperature to the final value assessed by the measurements. The temperature boundary condition at the rock is of the Robin type < =H(T-T,) with a calibrated value of the heat transfer coefficient H [W/(m K)]. 

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. ... 

Sch] Isothermal saturation of melt with Fe and analyzing 0-22 mass% Cr, 0-22 mass% Ni, 1450- 1530°C... 

The mentioned method seems to have no wide usage for studying behavior of oxides in molten salts, solubility studies by isothermal saturation and potentiometric titration methods are more precise and informative. 

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 compared with isothermal saturation a potentiometric method possesses the following advantages ... 

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%. 

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 CaOsolubility 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. 

Barbin et al. determined thermal dependence of LijO solubility in equimolar mixture KCl-NaCl by isothermal saturation technique in temperature range of 973-1073 K N=0.107-5.221/T, in this range the solubility varied from 0.52 to 0.86 mol%. In earlier work by Kaneko and Kojima, the solubility at 973 K was lower (0.31 mol%). The solubility of lithium oxide was close to that for BaO. 

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