Dissolution kinetics stages

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. 

Measurements of the specific surface area, SSA, of the products grown at various times indicate that the initial formation of a microcrystalline or amorphous precursor leads to a rapid increase in SSA. The development of these phases is also observed by scanning electron microscopy, and dissolution kinetic studies of the grown material have indicated the formation of OCP as a precursor phase ( , 7). The overall precipitation reaction appears to involve, therefore, not only the formation of different calcium phosphate phases, but also the concomitant dissolution of the thermodynamically unstable OCP formed rapidly in the initial stages of the reaction. In the presence of magnesium ion the overall rate of crystallization is reduced and lower Ca P ratios are observed for the first formed phases (51). 

This mechanism is consistent with the hypothesis that in the second stage dissolution kinetics is dependent on diffusion within the concentration boundary layer. It is conceivable that in the first stage field assisted dissolution may be the controlling step. In this stage formation of Ti(OH)4 or of hydroxy-cations, e.g. Ti(OH)3, has different effects on titanium transport. While Ti(OH)4 does not react with organic molecules, Ti(OH)3 can form organometallic complexes which may be transported systemically. 

The overall extraction process is sometimes subdivided into two general categories according to the main mechanisms responsible for the dissolution stage (/) those operations that occur because of the solubiHty of the solute in or its miscibility with the solvent, eg, oilseed extraction, and (2) extractions where the solvent must react with a constituent of the soHd material in order to produce a compound soluble in the solvent, eg, the extraction of metals from metalliferous ores. In the former case the rate of extraction is most likely to be controUed by diffusion phenomena, but in the latter the kinetics of the reaction producing the solute may play a dominant role. 

When a metal is in contact with its metal ion in solution, an equilibrium potential is established commonly referred to as Nernst potential (Er). Metal deposition occurs at potentials negative of Er, and dissolution for E > Er. However, when a metal is deposited onto a foreign metal substrate, which will always be the case for the initial stages of deposition, it is frequently observed that the first monolayer on the metal is deposited at potentials which are positive of the respective Nernst potential [37, 38]. This apparent violation for Nernst s law simply arises from the fact that the interaction between deposit metal and substrate is stronger than that between the atoms of the deposit. This effect has been termed underpotential deposition (upd), to contrast deposition processes at overpotentials. (One should keep in mind, however, that despite the symmetrical technical terms the physical origins of both effects are quite different. While the reason for an overpotential is solely due to kinetic hindrance of the deposition process, is that for underpotential deposition found in the energetics of the adatom-substrate interaction.)... 

Herrero and Abruna [25] have also studied the kinetics and mechanism of Hg UPD on Au(lll) electrodes in the presence and absence of bisulfate, chloride, and acetate ions. In the absence of the interacting anions (in perchloric acid), the Hg UPD was significantly controlled by gold-mercury surface interactions. In sulfuric acid solutions, the kinetics of the initial and final stages of mercury deposi-tion/dissolution was altered. The presence of two well-ordered structures at potentials below and above mercury deposition led to the formation of two pairs of sharp spikes in cyclic voltammograms. In the chloride medium, the voltammetric profile exhibited two sharp peaks and thus it was very similar to that obtained in sulfuric acid solution. Neither nucleation, nor growth kinetics mechanism was found to be linked to the process of formation/disruption of the mercury chloride adlayer. The transients obviously deviated from the ideal Langmuir behavior. 

The proposed NSPS can be met by hydrotreating the coal liquids obtained by filtering the product from the coal dissolution stage. The desulfurization kinetics can be presented by two parallel first-order rate expression, and hydrogen consumption kinetics can be presented by a first-order rate expression. A linear relationship exists between total sulfur content and SRC sulfur content of the hydrotreated product. For the Western Kentucky bituminous 9/14 coal studied here, the maximum selectivity and lowest SRC conversion to oil for a fixed SRC sulfur content are obtained using the highest reaction temperature (435°C) and the shortest reaction time 7 min.). ... 

Phase dissolution in polymer blends. The reverse process of phase separation is phase dissolution. Without loss of general validity, one may assume again that blends display LCST behavior. The primary objective is to study the kinetics of isothermal phase dissolution of phase-separated structures after a rapid temperature-jump from the two-phase region into the one-phase region below the lower critical solution temperature. Hence, phase-separated structures are dissolved by a continuous descent of the thermodynamic driving force responsible for the phase separation. The theory of phase separation may also be used to discuss the dynamics of phase dissolution. However, unlike the case of phase separation, the linearized theory now describes the late stage of phase dissolution where concentration gradients are sufficiently small. In the context of the Cahn theory, it follows for the decay rate R(q) of Eq. (29) [74]... 

Equation (6.94) illustrates that zero-order release kinetics are obtained if drug dissolution controls the release kinetics. However, as soon as the last particle in the matrix dissolves, the controlling mechanism of drug release shifts to Fickian diffusion. Figure 6.19 shows the dissolution-controlled release of KC1 at the early stage of release and the diffusion-controlled release at the later stage of release from an ethyl cellulose tablet. 

Factors which cause deviations from standard transport-controlled kinetics are discussed. Some of these are Surface roughness of the metal samples adsorption of reaction products a slow intermediate stage in the dissolution and conditions which cause the metal to assume a passive potential. 

The kinetics and the variables affecting initial separation of calcium sulfate from phosphoric acid as the hemihydrate have been carefully examined [35]. The value of separating the phosphate dissolution stage (rapid stirring) [36], from the calcium sulfate crystallization step via the hemihydrate (slow agitation) has been tested on a laboratory scale [37]. 

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