Oxoacidic properties

In the literature, there are reports of work concerned with the measurement of oxoacidic properties of ionic melts by gravimetric measurements of the solubility of acidic gases in these media [76, 77], The solubility of sulfur(VI) oxide in molten sodium phosphates was determined by the gravimetric method [76], A correlation was obtained between the melt basicities and the solubility of gaseous acid S03 in them. Iwamoto reported the estimation of the basic properties of molten salt by measurements of acidic gas solubilities (carbon dioxide and water) in them [77], However, similar methods cannot be used widely, owing to two factors. The first of these consists in the fact that the solubility of any gas in a liquid phase obeys Henry s law. Let us consider the following system of reactions ... 

The above description allows us to conclude that the kinetic method can be used only to determine thermodynamic parameters of strong acids, and only in strongly acidic media. Also, an available decomposition reaction, whose rate is limited by the stage of the reaction product breakdown, can be chosen to detect the oxoacidic properties of the studied ionic melt. 

Potentiometric investigations of oxoacidic properties of molten salts are performed using various kinds of oxygen electrodes metal-oxide, gas, and membrane ones. As a rule, these electrodes have been examined extensively, and their features are well described in the literature on molten salts electrochemistry. 

Oxoacidic properties of oxo-compounds of chromium molybdenum in molten equimolar KN03-NaN03 mixture were studied by Kust [115], who used the standard potentiometric cell with a liquid junction. The measurement routine consisted in certain additions of neutral chromate or molybdate to the solution of the corresponding salt of composition Na2R207. Determination of the equilibrium O2- concentrations in these buffer solutions allowed the authors to estimate the equilibrium constants as... 

The first study of the oxoacidic properties of molten sulfates was performed by Lux [17] on an equimolar K2S04-Na2S04 mixture at 950 °C. The measurements of activity of oxide ions in the melt were made by the potentiometric method with the use of an Au(C>2) gas oxygen electrode. 

Kaneko and Kojima studied the oxoacidic properties of Na2S207 and Na202 in the molten eutectic mixture K2S04-Li2S04-Na2S04 at 550 °C [124, 125]. The difference of pO values in their 0.01 mol kg-1 solutions is equal to 10 pO units. This value allows us to determine that the width of the acid-base range for the standard solutions in this melt is equal to 14 pO units. The acid-base range of stable existence of V02+ ion is found to lie within the interval of pO = 8.5-10.6. 

The silicate slag systems are involved in most pyro-metallurgical processes which result in the formation of a number of ferrous and non-ferrous metals (Cu, Ag, Zn, Cd Sn, Pb, Sb, Bi). These systems are oxide- and sulfide-oxide melts, and, therefore, the processes of their interaction with raw materials are dependent on the melt properties (oxidation ability, solubility of metals), and on their oxoacidic properties. Any metallurgical slag contains MgO, CaO, FeO and Si02 as one of main components. The oxidation ability of the slags increases with a rise of their basicity (increase of equilibrium O2-concentration), caused by the following electrochemical process ... 

The use of a membrane oxygen electrode for monitoring the oxoacidic properties of molten silicates is described by Perkins [132], Itoh and Yokokawa [133] examined the basicity of silicate melts of the Na20-Al203-Si02 system by the e.m.f. method in cells with the liquid junction, the oxoacidic properties of A1203 were shown to be weaker than those of Si02. 

Simultaneously, there takes place the fixation of the formed oxide ions by the excess of Cr03, resulting in the appearance of dichromate ions in the melt. This process leads to a decrease of the melt s acidity and stabilization of Cr(VI) oxo-compounds. Partial neutralization makes some difficulties for investigations of its oxoacidic properties, because of uncontrolled changes of the initial acid concentration. Therefore, potassium dichromate is considered to be a more convenient Lux acid of chromium, although there is no explicit possibility of determining directly the equilibrium constant of the following reaction ... 

Combes and Tremillon [154] studied the oxoacidic properties of tungsten(VI) oxide and the solubility of calcium tungstate in a molten equimolar KCl-NaCl mixture at 1000 K. A potentiometric cell with the membrane indicator electrode Ni,NiOlYSZ was used for the detection of the equilibrium oxide ion concentration. Investigation of the equilibria taking place in CaW04 solutions in KCl-NaCl allowed them to determine the solubility of CaO in the said melt at 1000 K as 0.084 mol%. The solubility of Scheelite (CaW04) was determined to be 10-3 5 mol kg-1, and the equilibrium constant of reaction (1.2.38) was estimated as 1010. 

It should be noted that a small pO drop characteristic of this reaction can be observed for the titration curves obtained with the use of carbonate as a titrant. This demonstrates that the oxoacidic properties of V2O7- are stronger than those of carbon dioxide at low CO2 pressures (pco2 0)—that is, the equilibrium of the reaction... 

Moreover, investigation of the oxoacidic properties of the V2O5 derivatives by the potentiometric titration method with the use of Ni,NiOlYSZ oxygen electrode is reported in Ref. [158]. The data obtained show that V2O5 is a dibasic acid. The potentiometric determination was performed from the base to the acid. The equilibrium constants ipK) corresponding to reactions (1.2.86) and (1.2.87) were equal to —9.3 0.3 and — 8.3 0.3, respectively (in molar fractions). 

Potentiometric investigation of the oxoacidic properties of the oxides mentioned above was reported in Refs. [160,161]. A priori, before performing this study it was assumed that niobium(V) oxide would be the complete analogue of V2O5 and that its acidic properties would be even stronger. The basis for such an assumption may be found in Group VIb, where there is an appreciable increase of the acidic properties in the sequence Cr-Mo-W. 

Investigations of the oxoacidic properties of boron(III) compounds in molten Nal and KCl-NaCl at 700 °C were reported by Cherginets and... 

The pO drop at the equivalence point corresponds to the ligand number 1, which confirms the running of the interaction (1.2.91) in molten Nal. This interaction is characterized by a pK value of —5.02 0.6. It should be emphasized that with the excess of the Lux acid the equilibrium is achieved slowly enough. This seems to be caused by the fact that sodium tetraborate is practically completely polymerized, although the polymerizations effects on the titration process are less pronounced than in the case of B203. After the equivalence point, with the pO drop of 3-3.5 pO units, the equilibrium conditions are achieved in a shorter period, since under these conditions all the polymerized particles are destroyed. The excess of the base over that necessary for the formation of B02 results in the fixation of O2- by the formed metaborate, which demonstrates the oxoacidic properties. The dependence of the ligand number on the initial titrant molality allows us to assume that the final product is sodium orthoborate, B03- (equation (1.2.92)), whose pK value is close to 2. All that we have said above shows that the oxoacidic properties of tetraborate ions in molten sodium iodide are stronger... 

Rybkin and Seredenko reported the construction of empirical scales of oxoacidity (acidity rows) in molten KC1 at 800 °C and Csl at 650 °C [62, 63]. Estimation of the oxoacidic properties was performed for buffer solutions obtained by the addition of equimolar quantities of conjugated acid and base in the melt. E.m.f. (pO) measurements were performed in the potentiometric cell with the use of a membrane oxygen electrode Pt(02)lYSZ. 

THE OXOBASICITY INDEX AS A MEASURE OF RELATIVE OXOACIDIC PROPERTIES OF HIGH-TEMPERATURE IONIC SOLVENTS... 

The oxobasicity index as a measure of relative oxoacidic properties... 

In the general case, the basicity of the standard solution (a()2 = 1) of a strong base in a solvent of the second kind is dependent on the absolute concentration of free oxide ions in this solution, and it is an important fundamental characteristic of the oxoacidic properties of the said ionic solvent. 

From the above-said it follows that the difference in the experimentally determined constants (pK) of the same equilibrium in different ionic solvents should reflect the difference in their acidic properties. In order to deduce a parameter which describes the relative oxoacidic properties of ionic melts, let... 

Finally, the suitability of the oxide solubility data for estimations of the oxoacidic properties of molten media should be considered. The solubility product of a metal oxide, MeO, PmcO, can be presented as... 

As follows from Part 1, the ionic melts based on molten alkali metal halides are referred to the solvents of the Second Kind (Kind II), and, therefore, the acid-base ranges for these media are half-open (see Fig. 1.1.1, scheme N3). Therefore, to form an idea of the relative oxoacidic properties of the studied chloride melts it is enough to know their oxobasicity indices. The necessary experimental parameters obtained at 600 °C are presented in Table 1.3.1. The data in this Table show that the KCl-LiCl eutectic melt possesses appreciable acidic properties, the corresponding oxobasicity index being equal to 3.2. 

Hence, the oxoacidic properties of the ionic melts based on alkali metal chlorides at 600 °C strengthen in the following sequence CsCl-KCl-NaCl — KCl-LiCl — CaCl2-BaCl2-NaCl — CaCl2-NaCl. 

A number of our works are devoted to the investigations of different kinds of acid-base equilibria in the ionic melts based on alkali metal halides in order to determine their oxobasicity indices at 700 and 800 °C. Unfortunately, none of the necessary experimental data have been published by other investigators. The equimolar KCl-NaCl mixture has been chosen to be the reference melt at these temperatures, although its oxoacidic properties differ by less than 0.1 from the CsCl-KCl-NaCl eutectic (see below), i.e. they are practically coincident. The solubilities of 11 metal oxides in the equimolar KCl-NaCl mixture are reported in Ref. [175]. Similar investigations in the molten CsCl-KCl-NaCl eutectic [188] allow us to conclude that the solubility products of the same oxide (in molar fraction scale) in both melts are close. This leads to the conclusion that both melts are suitable as reference ones, not only at 700 °C but also at other temperatures at which these media exist in the liquid state. 

For the KCl-LiCl eutectic, some of our investigations give an excellent illustration of the levelling of acidic properties by high-acidic melt cations. For this purpose, we now consider Fig. 1.3.3, where the results of Pb2+ and Cd2+ titration with KOH are presented. The behaviour of the KCl-LiCl eutectic melt doped with a 0.05 mol kg-1 addition of these cations does not differ from that of the pure melt. That is, the changes of the oxoacidic properties of the solutions, as compared with the pure eutectic, cannot be detected. This is because the acidic properties of Pb2+ and Cd2+ cations are not stronger than those of Li+, and the following equilibrium ... 

Assuming that the activities of C03 and C02 are independent of the anion composition of the ionic melt, there may be formulated a non-thermodynamic hypothesis that the changes of the equilibrium constant of equation (1.3.29) in various melts are caused only by the difference in their oxoacidic properties. 

To estimate the relative oxoacidic properties of molten alkali-metal halides, the carbonate-ion dissociation reaction was studied for the following sets of melts at 800 °C KCl-KBr-KI [183], CsCl-CsBr-CsI [197], NaCl-NaBr-Nal (830 °C) [198]. Also, in order to arrange all the melts in the oxoacidity scale at 800 °C, the solubility of MgO was determined for the following melts CsCl, KC1, NaCl, KCl-NaCl, KCl-LiCl, CsCl-KCl-NaCl, which allowed the estimation of the oxobasicity indices by the solubility method. 

The oxoacidity scale of molten salts at 800 °C is presented in Fig. 1.3.5. The oxoacidic properties of the chloride melts are stated to be close, whereas those of the bromide ones differ appreciably. As a rule, the bromides are more acidic than the corresponding chlorides. Similar to the single chloride melts, the molten iodides are of the same acidities, which are weaker that those of the chloride melts. Molten bromides are found to possess higher acidities in the chloride-bromide-iodide sequences for potassium and... 

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