Dissolution of an oxide

Promotion of the dissolution of an oxide by a ligand. The ligand illustrated here, in a short hand notation, is a bidentate ligand with two oxygen donor atoms (such as in oxalate, salicylate, citrate or diphenols). 

Electron injection has been observed during the chemical dissolution of an oxide film in HF [Mai, Ozl, Bi5]. The injected electrons are easily detected if the anodized electrode is n-type and kept in the dark. Independently of oxide thickness and whether the oxide is thermally grown or formed by anodization, injected electrons are only observed during the dissolution of the last few monolayers adjacent to the silicon interface. The electron injection current transient depends on dissolution rate respectively HF concentration, however, the exchanged charge per area is always in the order of 0.6 mC cm-2. This is shown in Fig. 4.14 for an n-type silicon electrode illuminated with chopped light. The transient injection current is clearly visible in the dark phases. 

The origin of the electron injection peak at the end of the dissolution of an oxide film is not understood in detail. Silicon interface atoms with three Si-O bonds and a single Si-Si bond are proposed to be responsible for the effect [Mai]. On the other hand, during the dissolution process silicon interface atoms with one Si-O bond and three Si-Si bonds lead to a configuration identical to the one for which electron injection is observed during divalent dissolution (Fig. 4.3, step 2). In any case, the injected charge exceeds by a factor of 3 to 5 the charge expected... 

This discussion should not be seen as explaining the acid/base character of oxides, i.e. their solubilities in water at various pH values. We are emphasising the close relationship between the acid/base behaviour of oxides and the nature of aqueous species. The dissolution of an oxide (other than a neutral oxide) in water, or in acids/alkalies, is an acid-base process, a chemical reaction rather than a mere separation of ions. The relative acid/base strengths of oxides are further discussed in Section 9.2. 

Reaction 5.1 forms the basis for dissolution of an oxide for CBPC formation. It represents dissociation of metal oxides in which cations and anions are formed in an aqueous solution. In general, divalent metal oxides dissociate more easily than trivalent oxides, and quadrivalent oxides dissolve less easily than trivalent oxides, though some exceptions may be found to this general trend. The actual rate of dissolution will be discussed in detail as we develop a thermodynamic basis for these transformations. 

The thermodynamics for the dissolution of an oxide in water are described by chemical equilibria of the form... 

Reductive Dissolution. The reductive dissolution of an oxide such as Fe(III) (hydr)oxide can be accounted for by the following sequence involving the reductant R (24). 

Figure 1. Schematic diagram of oxalate-promoted dissolution of an oxide mineral for M = Al(III) or Fe(III) based on the surface-controlled dissolution model. (Reproduced with permission from reference 22. Copyright 1986 Per-...

Binding of complex-forming ligands to oxide and hydroxide surfaces increases dissolution rates. Stumm and Furrer (1987) suggest that in acidic solutions the measured rate of dissolution of an oxide or hydroxide can be treated as the sum of the rate of the proton-promoted reaction (/ h) plus the rate of the ligand-promoted reaction (7 l )... 

The rate-controlling step for dissolution of an oxide or primary silicate mineral generally involves a surface reaction. For surface-controlled dissolution, the rate-controlling step is either the detachment of silica or a metal ion from the surface or the attack of the surface to form precursor sites for detachment. Surface detachment controlled kinetics can be modelled using the surface complexation rate model (Wieland et al., 1988) that models rates as a function of the surface concentration of surface complexation sites that are precursors for dissolution. In this model, the formation of precursor sites is rapid compared to the rate of detachment and the concentration of sites can be described by surface complexation theory (Sposito, 1983). 

In eontrast, the stoiehiometries and eoneentrations of different reaetive eom-plexes at the mineral surfaee are unknown so we eannot fully evaluate rate laws sueh as that of Eq. (8). There exist numerous simplifying rate laws, however, that relate rates of dissolution of an oxide mineral to total adsorbed eoneentrations, and do not distinguish among the stoiehiometrieally distinet adsorbates, as we diseuss below. 

Dissolution of an oxide or hydroxide in an alkaline soiution, in which OH ions are the possible complexing ions, may be treated simiiarly. However, in this case the reverse reaction for the crystal anion cannot neglected and the equations become complicated. The dissolution of compounds other than oxides and hydroxides may also be treated in a similar way. 

Considering the case of pH > 9, the fonnation of an oxide film is favoured compared with Fe dissolution. 

The protective quality of the passive film is detennined by the ion transfer tlirough the film as well as the stability of the film with respect to dissolution. The dissolution of passive oxide films can occur either chemically or electrochemically. The latter case takes place if an oxidized or reduced component of the passive film is more soluble in the electrolyte than the original component. An example of this is the oxidative dissolution of CrjO ... 

Complex-Forming Solutions. The solubHity of a metal can be enhanced by complexation using a suitable ligand. The dissolution of copper oxide in ammoniacal solutions is an example ... 

Ura.nium, The hydrometallurgical treatment of uranium ores is a concentration and purification process. Typical ore grade is 0.1—0.5% U Og, and pregnant solutions contain ca 1 kg/m of U Og. The dissolution requires the presence of an oxidant, either oxygen or a ferric salt. 

Tin anodes dissolve by etching corrosion in acid baths based on stannous salts, but in the alkaline stannate bath they undergo transpassive dissolution via an oxide film. In the latter the OH" ion is responsible for both film dissolution and for complexing the tin. Anodes must not be left idle because the film dissolves and thereafter corrosion produces the detrimental divalent stannite oxyanion. Anodes are introduced live at the start of deposition, and transpassive corrosion is established by observing the colour of the film... 

In general, corrosion of metal is always accompanied by dissolution of a metal and reduction of an oxidant such as a proton in acidic solution and dissolved oxygen in a neutral solution. That is, metal corrosion is not a single electrode reaction, but a complex reaction composed of the oxidation of metal atoms and the reduction of oxidants. 

Corrosion (from Latin corrodere, gnaw to pieces ) of metals is the spontaneous chemical (oxidative) destruction of metals under the elfect of their environment. Most often it follows an electrochemical mechanism, where anodic dissolution (oxidation) of the metal and cathodic reduction of an oxidizing agent occur as coupled reactions. Sometimes a chemical mechanism is observed. 

Often the dissolution of a metal leads to the formation of an oxide film on the electrode surface. These films are usually nonconducting and hinder the further dissolution of the metal, a phenomenon known... 

When the surface is completely covered by an oxide film, dissolution becomes independent of the geometric factors such as surface curvature and orientation, which are responsible for the formation and directional growth of pores. Fundamentally, unlike silicon, which does not have an atomic structure identical in different directions, anodic silicon oxides are amorphous in nature and thus have intrinsically identical structure in all orientations. Also, on the oxide covered surface the rate determining step is no longer electrochemical but the chemical dissolution of the oxide.1... 

When the pore bottom is covered by an oxide, the change of applied potential occurs almost completely in the oxide due to the very high resistance of the oxide. The rate of reactions is now limited by the chemical dissolution of the oxide on the oxide covered area. When the entire pore bottom is covered with an oxide the rate of reaction is the same on the entire surface of the pore bottom. As a result, the bottom flattens and the condition for PS formation disappears. The change of oxide coverage on the pore bottom can also occur when diffusion of the electrolyte inside deep pores becomes the rate limiting process. Since the current at which formation of an oxide occurs increases with HF concentration, a decreased HF concentration at pore bottom due to the diffusion effect can result in the formation of an oxide on the pore bottom of a deep pore at a condition that does not occur in shallow pores. 

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