Activation energy of dissolution

Some of the discrepancies among activation energies in the literature may be related to the fact that the apparent activation energy of dissolution may change as solution chemistry changes. For example, apparent activation energies for dissolution of silicates have been observed to decrease in... 

Where n varies with temperature, the apparent activation energy of dissolution will vary with pH of the system. As a consequence, in the literature, two kinds of are discussed apH-dependent and a pH-independent activation energy (Chen and Brantley, 1998). The pH-dependent (apparent) activation energy, E , is reported by investigators who plot In (rate) versus l/T, and is valid only at the pH of measurement. The pH-independent is determined from aplot of In(feH) versus 1/T. Where n is independent of temperature, = E. For phases where n increases with temperature, activation energies reported in Table 6 are larger... 

The influence of pH, temperature and stirring rates on die dissolution of famotidine (I) was studied. The dissociation constant, intrinsic solubility, diffusion coefficient, hydrodynamic layer diickness and activation energy of dissolution of I were determined.. .. intrinsic solubility is 0.278 mg.mU. "... 

Some films are termed "passive," for stainless steels or aluminum alloys, for instance. These films can play an important role in environment-sensitive crack initiation and fracture. Under thermodynamic equilibrium conditions, the film stability may be inferred from E =/(pH) diagrams, where E is the electrical potential related to the chemical free energy G by G = -nEF, and F is Faraday s number. At equilibrium, one can define the electrode potential (related to AG) and the current density I (I e here AG is the activation energy of dissolution). 

Film-free conditions It has been observed for many metals that the magnitude of / i, (see Section 1.4) increases with temperature and that the activation energy for dissolution is low, suggestive of a diffusion-limited anode process when the migration of corrosion products away from the surface is rate controlling. Some examples of the value of the activation energy for this process are given in Table 2.4. 

The speed of agitation governs the rate. The boundary layer thickness decreases at high speeds. A physical process such as the dissolution of an ionic salt in plain water is associated with an activation energy of the order of less than about 5 kcal mol-1. The influence of temperature on physical processes is less pronounced than that from agitation. 

Figure 8. Activation energies of p-Cl-PHMP dissolution in different alkalis ( ) and ionic conductivities in cellulose acetate ( ).

Measurements were undertaken of the solubility of each phase in acid solutions, of the growth rate of gypsum crystals and the dissolution rate of hemihydrate. The growth rate depends on the square of the supersaturation and on temperature with an activation energy of 64 kJ/mol. The nucleation rate appears to vary linearly with supersaturation. 

As the shape of the needle-like hemihydrate crystals changes as they dissolve, it is necessary to convert to the crystal width as a measure of size. In terms of this measure, the dissolution rate is first order with undersaturation and shows only a small temperature effect (activation energy of 10 kJ/mol). 

A plot of the dissolution rate against driving force, AC, is shown in Figure 13. This shows a linear dependence on undersaturation and a slight dependence on temperature (activation energy of 10 kJ/mole). This indicates that the dissolution is mass transfer controlled. The results can be correlated by... 

Kabai, J. (1973) Determination of specific activation energies of metal oxides and metal oxide hydrates by measurement of the rate of dissolution. Acta Chem. Acad. Sci. Hung. 78 57-... 

Apparent activation energy of the anodic dissolution of nickel in sulfuric acid solutions in the presence of Cl and CNS ions was determined in [58]. 

Apparent activation energies of acid dissolution of calcite and dolomite and severa. samples of agricultural limestone indicate that the activation energy of dolomite varies... 

Sajwan, K. S., and V. P. Evangelou. 1991. Apparent activation energies of acid dissolution of carbonates by an isothermal automanometric apparatus. Soil Sci. 152 243-249. 

In many respects the action of NaOH on aluminum and silicon is similar. Streicher (52) reported an activation energy of 13.7 kcal/mole for the dissolution of aluminum in 2% NaOH. Orem (53) studied the action of 15% NaOH on spheres of aluminum and on holes drilled into plates whose surfaces were oriented in the three main crystallographic planes. He found that the relationship between the etch rates was ill < 110 < 100. ... 

Generally the act ivation energy for iron dissolution in dilute depolarized solutions at cylinder peripheral speeds of 10,000 cm/min is 5000-6000 cal/inole. hi the same solutions the rates reach a maximum at higher speeds and may even decrease. At 50,000-60,000 cm/hiin activation energies of... 

Stone and Tiemann (S28) have reported the specific rates of silica extraction from taconite ores as a function of time, temperature, and sodium hydroxide concentration. The microcrystalline varieties of quartz associated with the taconites are easily disintegrated by sodium hydroxide under pressure. The rate of silica dissolution has an activation energy of 15,000-19,000 cal/mole. 

Figure 9 Estimates of the pH-independent activation energy for dissolution at low pH for selected minerals plotted versus connectedness for the dissolving phase. The maximum and minimum values of the Ea are plotted for quartz, alkali feldspars, and forsterite. Connectedness of anorthite is assumed during dissolution to equal 0 because A1 is leached first, leaving Si tetrahedra without any bridging oxygens. Similarly, alkali feldspars are plotted at a connectedness of 3. Estimates of activation energy calculated by Pelmenschikov et al. (2000, 2001) are included (see text). Experimental data are summarized in Table 6.

Rate equations for the dissolution of feldspar in the presence of organic ligands have been proposed and have been related to surface complexation at the feldspar surface (Stillings et al., 1996 Welch and Ullman, 2000). These latter workers also report that the apparent activation energy of silica release from bytownite decreases from —lOkcalmol to 7kcalmol in the presence of oxalate and gluconate in neutral solutions (Welch and UUman, 2000). 

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