Dissolution reactions and mechanisms

The principles of dissolution have been reviewed by Bloom and Nater (1991), Blesa et al. (1994) and Casey (1995). The driving force for dissolution is the extent of undersaturation with respect to the oxide. Undersaturation is thus a necessity for dissolution as is supersaturation for precipitation. Other factors being equal, the rate of reaction will increase as degree of undersaturation rises. The extent of undersaturation varies from one system to the next. Dissolution of anodic Fe oxide films often takes place in nearly saturated solutions, whereas extraction of iron from its ores requires markedly undersaturated solutions in order to be efficient. In most natural systems (soils and waters) the aqueous phase is fairly dose to saturation with respect to the iron oxides and dissolution may, therefore, be extremely slow. The dissolution process can be accelerated by the presence of higher levels of electrons or chelating ligands. 

Most of the data in this chapter was obtained from laboratory experiments in which the dissolution kinetics were followed by monitoring the change in the level of iron released into solution. The dissolution rate and mechanism are often established on the basis of data corresponding to the first few percent of the reaction, (e.g. Stumm et ak, 1985). To insure that the initial stages are in fact representative of the behaviour of the bulk oxide ( and not an impurity, for example), a complete dissolution curve should be obtained in any investigation. 

The factors which influence the rate of dissolution of iron oxides are the properties of the overall system (e. g. temperature, UV light), the composition of the solution phase (e.g. pH, redox potential, concentration of acids, reductants and complexing agents) and the properties of the oxide (e. g. specific surface area, stoichiometry, crystal chemistry, crystal habit and presence of defects or guest ions). Models which take all of these factors into account are not available. In general, only the specific surface area, the composition of the solution and in some cases the tendency of ions in solution to form surface complexes are considered. 

Most dissolution studies concentrate on establishing the mechanism of dissolution. There are few studies in which different oxides have been compared to provide 

Additives to the solution in which dissolution takes place are of great importance. 

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The basic mechanism for the instability of ultrapure metals was suggested by Wagner and Traud in a classic paper in 1938.1 The essence of their view is that for corrosion to occur, there need not exist spatially separated electron-sink and -source areas on the corroding metal. Hence, impurities or other heterogeneities on the surface are not essential for the occurrence of corrosion. The necessary and sufficient condition for corrosion is that the metal dissolution reaction and some electronation reaction proceed simultaneously at the metal/environment interface. For these two processes to take place simultaneously, it is necessary and sufficient that the corrosion potential be more positive than the equilibrium potential of the M, + + ne M reaction and more negative than the equilibrium potential of the electronation (cathodic) reaction A + ne — D involving electron acceptors contained in the electrolyte (Fig. 12.8). 

The two fundamental issues related to porous layer formation are the mechanism of the dissolution reaction and the processes controlling the morphology of the porous layer. Although the overall reaction is well characterized, many aspects of the reaction mechanism remain unresolved. 

There are two concepts concerning the role of kink atoms in the iron dissolution kinetics and mechanism. One of them is that kink atoms are simply the atoms that dissolve according to the reaction... 

Since the hydroxyl anion is involved in the mechanism given before, the implication is that other anions may also take part in the dissolution process, and that the effect of various chemicals may be interpreted in the light of the effect of each anion species. Most studies have been in solutions of sulphuric and hydrochloric acids and typically the reaction postulated for active dissolution in the presence of sulphuric acid is ... 

Tantalum is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

A most striking result from the work described above is that the composition of the bottoms product and residues from the dissolution reaction did not depend on the chemical structure of the original coal material only their relative quantities differed. This supports the view of a mechanism involving the stabilisation of reactive fragments rather than an asphaltene-intermediate mechanism. The formation of a carbon-rich condensed material as a residue of the reaction and the fact that hydrogen transfer occurred largely to specific parts of the coal further supports this view. 

The fundamental reason for the uneven distribution of reactions is that the rate of electrochemical reactions on a semiconductor is sensitive to the radius of curvature of the surface. This sensitivity can either be associated with the thickness of the space charge layer or the resistance of the substrate. Thus, when the rate of the dissolution reactions depends on the thickness of the space charge layer, formation of pores can in principle occur on a semiconductor electrode. The specific porous structures are determined by the spatial and temporal distributions of reactions and their rates which are affected by the geometric elements in the system. Because of the intricate relations among the kinetic factors and geometric elements, the detail features of PS morphology and the mechanisms for their formation are complex and greatly vary with experimental conditions. 

Wollast, R. (1990), "Rate and Mechanism of Dissolution of Carbonates in the System CaC03-MgC03", in W. Stumm, Ed., Aquatic Chemical Kinetics, Reaction Rates of Processes in Natural Waters, Wiley-Interscience, New York, pp. 431-445. 

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