Introduction

Victor L. Cherginets

Theoretical studies and the application of chemical reactions within the matrix of various molecular and ionic solvents are among the most important trends in modem chemistry and engineering since the nature of the solvent significantly affects the technological processes running in it [1, 2]. Modern solution technologies are mainly based on the use of molecular liquids as solvents at room temperature. However, an increase in the application of high-temperature ionic melts as liquid media has been evident in recent decades. This is a consequence of a number of unique features which are characteristic of this class of solvents.

Molten salts are now used widely in science and engineering as convenient media (solvents) for performing a range of technological processes, including surface treatments (etching, fluxes), the electrolytic obtaining of metals and alloys, refining and the electrochemical deposition of a number of galvanic and diffusion coatings [3]. Ionic melts serve as solvents or starting materials for growing a number of widely used scintillation [CsI (Tl), NaI (Tl)] and acoustical (KCl) single crystals. For some decades, high-temperature ionic media have been attracting considerable attention as solvents for the treatment of nuclear materials and, especially importantly, nuclear wastes without producing sewage [4]. We would also note the recent growth of interest in room-temperature ionic liquids, owing to their potential use as reaction media for green chemical reactions [5, 6], electrolytes for high-current-density batteries, etc.

Ionic melts possess a number of features which compare favourably with the conventional room-temperature molecular solvents. The practically complete dissociation of ionic media into the constituent ions allows one to create high (up to 10 A cm− 2) current densities in electrolysis. The absence of oxidants similar to H+ makes it possible to obtain final products which cannot be extracted from aqueous solvents (i.e. alkali- and alkaline-earth metals, sub-ions, etc.). From an ecological standpoint, molten ionic media are especially applicable as technological solvents because their employment does not cause accumulation of liquid wastes since cooling to room temperature transforms the ionic liquids into their solid state.

Processes which take place in ionic melts–solvents are considerably affected by impurities contained in the initial components of the melt or formed during the preparation (mainly by melting) of solid components of the solvents owing to high-temperature hydrolysis of the melts or their interactions with container materials (A12O3, SiO2, etc.) as well as active components of the atmosphere (O2, CO2, H2O, etc.). The list of these impurities is long, and includes multivalent cations of transition metals and different complex anions (oxo- or halide anions). The effect of the mentioned admixtures on the processes in ionic melts is mainly dependent on the degree of their donor–acceptor interactions with constituent parts of ionic melts.

Oxide-containing impurities are the most widespread in molten salts and their effect on technological processes and the parameters of the products fabricated using ionic solvents is often negative. It consists in the fixation of reagents of acidic character (cations, polynuclear anions, acidic gases) that favours the formation of insoluble (precipitates or suspensions) or slightly dissociated products in the ionic melt. All these reasons cause retardation of the desired process owing to a decrease in the equilibrium concentrations of the starting acidic reagents, the appearance of inclusions of oxide particles in electrochemically deposited metals, additional absorption bands in optical single crystals, a reduction of their radiation resistance [7, 8], substantial corrosion of container materials, etc.

Quantitative investigations of the reactions of oxide ions and oxo- compounds in high-temperature ionic solvents are, therefore, of considerable scientific and applied importance. The interactions of such kinds are referred to as acid-base ones, according to Lewis. Since 1939, when Lux proposed a definition of acids as oxide ion acceptors and bases as donors of O2 −, such acid-base interactions came to be called “oxoacidity”. The most general scheme of a Lux acid-base interaction is presented by the following equation:

+:O....:2–⇄B≡A:O....:2−,

and the “pO” index introduced by Lux serves as a measure of the basic (acidic) properties of a melt

≡–logaO2−≈−logmO2−

This index is similar to the pOH scale in aqueous solutions. Measurements of the pO index during the running of acid–base reactions of various kinds make it possible to calculate their thermodynamic characteristics and, to a first approximation, the equilibrium constants. Because there are no reliable experimental data on the activity coefficients of most substances in ionic solutions or theoretical substantiations of their estimations, the equilibrium constants are often obtained as concentration, but not thermodynamic parameters. Therefore, the pO indices discussed in the following sections of this book are the negative logarithms of O2 − concentrations. Also, in most cases, the studies concern diluted solutions of ionic solids or liquids in ionic liquids, i.e. both ingredients are substances with ionic bonding. The same nature of the bonding allows us to consider the formed solutions as close to ideal, but not infinitely diluted ones, as takes place in solutions of ionic substances in water or other molecular solvents at room-temperature.

The ability of high-temperature ionic solvents to dissolve various substances (e.g. metals or their oxides) is considered among their most important characteristics. In the case of scintillation, single crystals based on alkali-metal halides, some halides-dopants (TlI, CeX3, etc.), are available as admixtures since they create levels in the forbidden zone which are responsible for the scintillation. In contrast, oxygen-containing admixtures are harmful since their existence in the crystals causes a considerable reduction in their light-yield, radiation resistance, etc.

Therefore, investigations of oxide solubilities in molten salts are of great interest for the following reasons. For some purposes (fluxes, etching, growth of oxide crystals and composites from ionic melts), it is necessary to choose a melt where the solubility of oxide materials should be as high as possible. If an ionic melt is used as a material for the growth of an optical single crystal (alkali- and alkaline-earth metal halides) then the requirements should be quite the opposite: the solubility of the oxygen-containing admixtures should be very low to provide a low quantity of additional absorption bands and disseminating centres in the crystals.

Investigations of oxide solubilities in high-temperature ionic solvents, especially those based on alkali metal halides, represent an important branch of oxoacidity studies, although they have not developed considerably during the past few decades. One of the most serious causes of the absence of essential progress in the studies of oxide solubility is the seemingly bad reproducibility of the reliable data on the solubility-product values of oxides obtained by different investigators—their scatter covers some orders of pPMeo depending on the experimental routine used. Other reasons, such as the synthesis of the initial metal halides, the thorough purification of ionic solvents from traces of oxide ions and the design of the apparatus for the experiments, complicate determination of the solubility characteristics in high-temperature ionic melts.

Examinations of Lux acid–base reactions of various kinds were very intensive in the 1960s and 1970s, but then this intensity decreased. The experimental routines existing at that time essentially limited the range of ionic solvents and reagents which could be studied successfully. For example, molten bromides and iodides as media for oxoacidity processes were practically unstudied because of the impossibility of using gas–oxygen electrodes that provided the supply of gaseous oxygen in the said media possessing strong reducing properties. However, for the next two decades, various kinds of membrane oxygen electrodes were thoroughly tested and introduced into wide practice and the methods of treatment of the results, and their interpretation, were extensively revised. This necessitates newer studies of oxoacidity, taking into account the modified scientific basis. Extensive basic and applied research on oxoacidity may result in the creation of principally new solution technologies based on the employment of high- temperature liquids.

Up to now, attempts have not been made to summarize the results of oxoacidity investigations and to acquaint a wide range of readers with achievements in oxoacidity and subsequent studies required for scientific and applied purposes. Where attempts have been made in books [3, 9], the body of data considered was scant and without deep scientific interpretation.

The present book is devoted to a systematization of investigations of oxoacidity and oxide solubility studies in ionic melts published up to 1 July 2004. The book brings together material that is quite widely dispersed, including work published in journals of the former Soviet Union—which remains rather inaccessible and is...