Rate determining step, in dissolution

THE RATE-DETERMINING STEP IN DISSOLUTION KINETICS THE SURFACE SPECIES AS THE PRECURSOR OF THE ACTIVATED COMPLEX... 

In extraction from a polymer/additive solid matrix the rate-determining step in the extraction process is governed by the interaction of the solvent of sufficient dissolution power with the matrix and the removal of the analyte (cf. Section 3.4.1.3). There appears to exist a direct relationship between degree of swelling and efficiency of extraction. The amount of C02 absorbed depends on temperature, pressure and the polymer concerned. Crystalline polymers are-not surprisingly-plasticised less... 

Note that in the mechanistic schemes presented, the dissolution steps of reactant and products have been omitted for the sake of brevity. These include, for example CO (g) <-> CO (1), C02 (1) <-> C02 (g), and H2 (1) <-> H2 (g). From the standpoint of thermodynamics, when the equilibrium lies far to the right, reactions are deemed to be irreversible and may be denoted with a forward arrow - symbol. In cases where the reaction is considered to be reversible (i.e., equilibrium lies somewhere in the middle), the forward and backward arrows (e.g., <-> ) are employed. In some cases, however, we do not specify reversible/irreversible steps, and therefore arrows (e.g., or <-> ) might be used in a general sense. From a kinetic standpoint, in some cases a step will be defined that is considerably slower than the others (i.e., the rate determining step) in those cases, the remaining steps may be considered to be pseudo-equilibrated. The reader must therefore use discretion in interpreting the mechanistic schemes. The context of the discussion should clue the reader into how to interpret the arrows. 

If cation diffusion 1s Indeed Involved 1n the rate determining step, the dissolution rates at temperatures above Tg are expected to obey a WLF-type relationship (17). In the present context, the ratio of the dissolution rate at temperature T to that at Tg replaces the shift factor,... 

Fig. 5.13 displays the dissolution scenario of the gibbsite surface and of the edge surface. The dissolution reaction can be interpreted as a coupled release of Al and Si. The detachment of the Al center is the rate determining step. In a fast subsequent step Si is released from the same surface site. The AI(III) H+ stoichiometry of the precursor group (the group to be detached) is 1 3 at the gibbsite surface and 1 1 at the edge surface. 

An examination of the theoretical models proposed for metal dissolution and for the general Impedance behavior of electrodes enables the rate-determining step of the corrosion reaction to be Identified. It Is then possible to separately study the rate determining step In order to find a suitable Inhibitor or a suitable surface coating. 

The amount of liquid in the rectum has been reported to be about 3 ml. This is small compared to the volume of fluid available throughout the GI tract when a drug is administered orally. Such a small volume of fluid can limit dissolution of drugs, particularly those with low aqueous solubility. It also may be a constraint to the rapid dissolution and release of compounds from water-soluble vehicles where dissolution of the vehicle is considered to be the rate-determining step in drug release from the vehicle. 

In this chapter we discuss the rates of adsorption, paying special attention to those few cases where information on the rate of specific adsorption (reaction of an adsorbate in the adsorption layer) is available. Furthermore, we elaborate on the chemical processes involved in the dissolution of minerals and concentrate on the dissolution of oxides, silicates, and carbonates, which play an enormous rx)le in the chemical weathering and erosion. We try to demonstrate that in most cases the rate-determining step in the dissolution is a chemical reaction at the surface of the mineral. Thus we have here an excellent example of the relationship between surface stracture and reactivity. Surface chemistry plays an equally important role in the formation of the solid phase (precipitation, nucleation, and crystal growth). Nature s selectivity is reflected in the creation of a crystal and its growth. 

The same discussion with the same equations holds for anodic reactions with diffusion of the species Red as the rate-determining step. In the case of intense metal corrosion, the diffusion of cations from the electrode surface to the bulk may become the rate-determining step, with their aecumula-tion at the electrode surface and the final precipitation of a salt film. These processes are important for intense active metal dissolution and localized corrosion, as will be discussed in Sec. 1.5.4. At small current densities, a superposition of charge transfer and diffusion control is obtained. Therefore the current density increases exponentially in the vicinity of the Nernst potential ac-... 

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