Rates of Oxides and Hydroxides

Several studies have been conducted on the rates of dissolution of oxides. The work of Stumm and coworkers is noteworthy in this area (Stumm et al., 1983, 1985 Zutic and Stumm, 1984 Stumm, 1986). They have studied the effects of H+ and various complex-forming anions on oxide dissolution rates and found that dissolution rate (v) depends strongly on the relative concentrations of proton surface groups -OH2 and ligand surface complexes -L such that (Stumm et al., 1985) 

the dissolution rate of oxides and hydroxides is composed of two additive rates such that 

Stumm (1986) defines the probability of finding two neighboring protonated groups as being proportional to 0h, where 0H is the degree of surface protonation such that 

For a trivalent metal oxide, three neighboring surface sites must be protonated and the probability of finding them is °°0h  

The rate-determining step for oxide dissolution in a slightly acidic solution (reaction c in Fig. 7.8) is given in Eq. (7.4) and occurs at kink or step sites on the oxide or hydroxide surface. It can be expressed as 

---

Rate-Limiting Steps in Mineral Dissolution 146 Feldspar, Amphibole, and Pyroxene Dissolution Kinetics 148 Parabolic Kinetics 149 Dissolution Mechanism 155 Dissolution Rates of Oxides and Hydroxides 156 Supplementary Reading 161... 

The dissolution of oxide and hydroxide minerals is pH dependent. In acidic solutions, the dependence of the rate of reaction on proton concentration, [H L can be described by... 

At sufficiently low pH, the rate of dissolution of oxides and hydroxides is approximately first-order with respect to H. The data of Surana and Warren (1969) for the dissolution of goethite suggest a first-order dependence on concentrations of H2SO4 and HCIO4, and on activity in HCl at concentrations >0.5 M. Part of the difficulty in determining the difference between dependence on concentration and H activity may be due to the specific effects of anions and the effects of ionic strength that will be discussed later. 

The low fractional orders found for the influence of [H ] and complexing ligands on the dissolution of oxides and hydroxides can be explained by the surface complexation theory (Wieland et at, 1988). For conditions where the detachment of product ions from the surface is rate limiting, the rate is given by the general rate equation... 

This approach seems to fit the data of Stumm and coworkers for dissolution of oxides and hydroxides in dilute acids, but Carroll-Webb and Walther (1988) suggest for corundum that 7 = 1, and that the required number of protons adjacent to an Al to promote detachment from the surface need not be equal to the number required to convert the Al in corundum to aP . At sufficiently low pH, the dissolution of Al(0H)3 is first-order with respect to protons in solution (Pulfer et al., 1984 Bloom and Erich, 1987) and dissolution cannot be explained by the surface complexation model. For this reaction, protonation rather than detachment of surface sites appears to be rate controlling. 

Silicate mineral weathering is more complex than the dissolution of oxides and hydroxides. The primary silicates, those silicates of most interest in weathering studies, are not stable in aqueous environments at earth surface temperatures. For these minerals, solution equilibrium with respect to soluble mineral components is not a factor in determining stability, and only the forward dissolution rate needs to be considered. 

A reaction mixture in this equilibrium was prepared, in which H3ASO3 was labeled with As. The exchange was quenched by dilution with water and addition of an excess of ammonium hydroxide. The As04 ions were precipitated as ammonium magnesium arsenate and subjected to activity measurement. The rates of oxidation and reduction measured in this way were in agreement with the kinetic expressions and rate constants, which hold for the same reaction remote from equilibrium. 

The mechanism and rate of hydrogen peroxide decomposition depend on many factors, including temperature, pH, presence or absence of a catalyst (7—10), such as metal ions, oxides, and hydroxides etc. Some common metal ions that actively support homogeneous catalysis of the decomposition include ferrous, ferric, cuprous, cupric, chromate, dichromate, molybdate, tungstate, and vanadate. For combinations, such as iron and... 

The dissolution of passive films is, in the main, controlled by a chemical activation step in contrast to film-free conditions at. Many protective anodic films are oxides and hydroxides whose dissolution depends upon the hydrogen ion concentration, and the rate follows a Freundlich adsorption equation ... 

Both zinc and zinc alloys have excellent resistance to corrosion in the atmosphere and in most natural waters. The property which gives zinc this valuable corrosion resistance is its ability to form a protective layer consisting of zinc oxide and hydroxide, or of various basic salts, depending on the nature of the environment. When the protective layers have formed and completely cover the surface of the metal, the corrosion proceeds at a greatly reduced rate. 

The anaerobic oxidation of phosphines to their oxides by hydroxide ion has been shown to involve the liberation of hydrogen, possibly from the intermediate (21). These oxidations were studied with water-soluble phosphines, since solubility was found to be the main factor controlling the rate of oxidation. The preparation, and detailed n.m.r. spectrum, of PP-dimethyl-P P -diphenyldiphosphine disulphide (22) is a relatively rare example of a study of a mixed disulphide. Many examples of routine oxidation of phosphines to their oxides have appeared. These include the preparation of polyhalogenoarylphosphine oxides using dichromate... 

Sodium hydroxide was introduced into a reactor containing 90 kg of ethylene oxide. The reactor detonated eight hours later. A study was carried out to try to understand what caused the accident. It showed that in similar conditions the temperature of the medium reaches 100°C when 13% of oxide is polymerised, then 160°C at 28% of conversion. At this stage, it takes sixteen seconds for the medium to reach a conversion rate of 100% and a temperature that reaches 700°C. 

The open circuit potential data for the B210/NVP system mirrors the behavior of the rust ratings over the temperature range examined. A plausible explanation of the change of the open circuit potential is as follows. As temperature is increased, the composition of the various oxides and hydroxides which make up the zinc phosphate conversion layer and the base iron oxide layer undergo changes. 

The rate-limiting step in CD for the first two mechanisms is almost always formation of the chalcogenide ion. This reaction should be slow otherwise fast, homogeneous precipitation of the metal chalcogenide will occur with little fihn formation. (Even rapid precipitation can lead to a film however, this film will be extremely thin and in most cases not visible.) Almost all the literature on CD is limited to sulfides (mostly), selenides, and oxides (including hydrated oxides and hydroxides). Anion-forming reactions are described in this section. 

Droplets entering the flame evaporate then the remaining solid vaporizes and decomposes into atoms. Many elements form oxides and hydroxides in the outer cone. Molecules do not have the same spectra as atoms, so the atomic signal is lowered. Molecules also emit broad radiation that must be subtracted from the sharp atomic signals. If the flame is relatively rich in fuel (a rich flame), excess carbon tends to reduce metal oxides and hydroxides and thereby increases sensitivity. A lean flame, with excess oxidant, is hotter. Different elements require either rich or lean flames for best analysis. The height in the flame at which maximum atomic absorption or emission is observed depends on the element being measured and the flow rates of sample, fuel, and oxidizer.6... 

Biiltemann 2 observed that vanadium ammonium alum separates out in blue crystals from a solution containing sulphuric acid, but from solutions containing a weak acid, or from neutral solutions, red crystals are obtained. (The chromium alums can also be prepared in differently coloured modifications.) The analytical data, melting-point, electrical conductivity, rate of efflorescence, and general behaviour of both kinds of crystals are identical, so that it is difficult to ascribe different constitutions to them. Meyer and Markowitz3 have shown that both forms separate out when the molecular proportion of sulphuric acid in the solution is less than that theoretically required, and attribute the red colour to the presence of traces of vanadous oxide, V203, or its hydroxide, V(OH)3. Vanadium rubidium and vanadium ciesium alums behave in the same way. A vanadium guanidine alum has also been prepared.4... 

In natural anoxic environments, the major alternative oxidants are iron(III) and manga-nese(IV) oxides and hydroxides. Both are common in natural systems, as crystalline or amorphous particles or coatings on other particles. In the absence of photocatalysis, however, iron and manganese oxides are weak oxidants. As a result, they appear to react at significant rates only with phenols and anilines (45, 59-64). 

---