Lux-flood concept

The concept of acid-base reactions with respect to the oxide ion was first introduced by Lux (1929) and supported by Flood (1947). According to the Lux-Flood concept— an acid is the oxide-ion acceptor while a base is the oxide donor . Examples ... 

An acidity scale has been proposed in which the difference in the acidity parameters. (aB — aA), of a metal oxide and a nonmetal oxide is the square root of the enthalpy of reaction of the acid and base.4 Thus for reaction 9.5, the enthalpy of reaction is —8fikJ mol-1 and so the a values of CaO and SiO, differ by about 9 units. Selected values are fisted in Table 9.1. Although based on the Lux-Flood concept, the values are obviously of more general interest. The most basic oxide, as expected, is cesium oxide, amphoteric oxides have values near zero (water was chosen to calibrate the scale at a value of 0.0). and the most acidic oxide is CLO,. the anhydride of perchloric acid. 

Thus, in the modified Lewis (or Lux-Flood) concept, pure alkali halides represent the highest degree of basicity as the solvent composition changes from alkali halide-rich to alkali halide-deficient melts, the solvent becomes acidic. Acid-base properties of molten halides may be used to explain stabilization of unusually low (or high) oxidation states, the differences in stability of the same oxidation state in related melts, and the effects on coordination observed spectrally for certain metal ions. Or, restating the idea in other terms, the redox potentials depend on melt basicity. Thus, the systematic variation of melt composition is a useful technique in the arsenal of the molten salt electrochemist who is interested in the chemistry of solute species in molten salt solvents. In this respect, it is important to note that variation of temperature may be used to serve the same purpose for example, it has been shown that in neutral chloro-aluminates C1- decreases with temperature. 

Since Arrhenius, definitions have extended the scope of what we mean by acids and bases. These theories include the proton transfer definition of Bronsted-Lowry (Bronsted, 1923 Lowry, 1923a,b), the solvent system concept (Day Selbin, 1969), the Lux-Flood theory for oxide melts, the electron pair donor and acceptor definition of Lewis (1923, 1938) and the broad theory of Usanovich (1939). These theories are described in more detail below. 

The Lux-Flood theory relates to oxide melts. Geologists have often used acid-base concepts for the empirical classification of igneous silicate rocks (Read, 1948). Silica is implicitly assumed to be responsible for acidity, and the silica content of a rock is used as a measure of its acid-base balance ... 

Lux—Flood Definition In contrast to the BrpnstedLowry theory, which emphasizes the proton as the principal species in acid-base reactions, the definition proposed by Lux and extended by Flood describes add-base behavior in terms of the oxide ion. This add-base concept was advanced to treat non protonic systems which were not amenable to the Bipnsted-Lowry definition. For example, in high-temperature inorganic melts, reactions such as the following take place ... 

In these melts, the concept of acidity of Lux-Flood [343] (L-F) is applied. A Lux-Flood acid is defined as an oxide ion acceptor leading to the formation of the conjugate base ... 

There were some attempts to estimate acid-base properties for oxide compounds, both solids and melts. The most popular of them is Lux-Flood s acid-base theory.2,3 This concept seems to be more effective for estimating the acid-base characteristics of anhydrous borates and of some promising solvents for the flux growth of borate crystals. According to Lewis-Lux s equation,... 

Hence, on the basis of the generalized solvosystem concept it is possible to formulate the main distinctive features of different kinds of high-temperature ionic melts as media for acid-base interactions by the Lux-Flood method. 

Acid-base concepts share some features with seemingly different types of interactions complexing, precipitation, and even redox. Comparison of these through theories of Bronsted, Lewis, Usanovich, and Lux-Flood extends our use of the electronic interpretations of... 

The reversibility of such conversion systems was explained by an acido-basic concept which involves 0 anion transfer (Lux-Flood model) and is based ontheacidity or basicity of oxides (Smith scale). All oxides that react reversibly with Li are basic as is the Li20 itself. ... 

The concept of acidic and basic oxides is familiar to geologists and dates back to Berzelius. A more exact definition, however, was proposed by Flood and Forland (19 7) following the work of Lux (1935). In aqueous systems, acidic or basic behavioior is conveniently treated using the conjugate acid-base formalism of Bronsted and Lowry ... 

The acid-base concept of Lux and Flood/ acid + = base... 

---