Dissolution Kinetics

Jarosite [KFe 3 (SO 4 ) 2 (OH) 6 ] is a mineral that is common in acidic, sulphate-rich environments, such as acid sulphate soils derived from pyrite-bearing sediments, weathering zones of sulphide ore deposits and acid mine or acid rock drainage (ARD/AMD) sites. ...

The rate of gold dissolution was measured in clear solution using a gold rotating disk electrode. The data, obtained at various cyanide concentrations, are in agreement with literature observations. The dissolution rates were independent of electrode rotation speed for air-saturated solutions and cyanide concentrations above 5 mol m−3 and were well below external mass transfer-limited rates for cyanide and oxygen. An activator molecule, NMI, was shown to increase the rate of dissolution and served to illustrate the importance of surface reactions in the dissolution mechanism. With increasing activator concentration, the dissolution rate first increased to a maximum value and then decreased. The optimum activator concentration at the maximum dissolution rate was found to be a function of the cyanide level. The results suggest competitive adsorption between the activator and cyanide species.A dissolution mechanism in which the active site contains two (or more) gold atoms is proposed. Dissolution kinetics are controlled by reactions on the crystalline gold surface in regions independent of potential and identified by measured potentials when both cyanide and oxygen are present. The data are explained by crystalline surface mass transfer away from the active surface site followed by charge transfer and dissolution. Model predictions are in reasonable agreement with observations. The proposed mechanism is based solely on kinetics measurements and calls attention to the need for further mechanistic research.

The in-vivo absorbability of drugs categorized into the biopharmaceutics classification system (BCS) class II is very difficult to be predicted because of the large variability in the absorption and/or dissolution kinetics and the lack of an adequate in-vitro system for evaluating the dissolution behavior. We tried to predict the in-vivo absorption kinetics of griseofulvin, categorized into BCS class II, orally administrated as powders into rats, based on Gastrointestinal-Transit-Absorption model (GITA model), consisting of the absorption, dissolution and GI-transit processes. Using the dissolution rate constants (kdis) of griseofulvin obtained with JP 1st solution, JP 2nd solution, FaSSIF, FeSSIF and modified SIBLM as a medium, simulation lines were not able to describe the observed mean plasma profile at all. On the other hand, a calculated line provided by employing kdis obtained with MREVID 2 (medium reflecting in-vivo dissolution 2), a new medium, was in better agreement with the observed mean plasma profile than existing media, indicating that the utilization of adequate kdis value made it possible to predict the in-vivo absorption kinetics of drugs classified into BCS class II based on GITA model and that MREVID 2 could be a useful medium for describing the in-vivo dissolution kinetics.

A thermodynamic approach was used to investigate solder alloy systems containing Sn, Ag, Sb, and Pb, during both equilibrium and Scheil cooling conditions. The modeled microstructure was used to explore recent experimental results and to establish the microstructure-property relationships in microsolder joints. This approach is shown to be very useful in the transition from Pb-Sn to lead-free solders by enabling

The dissolution rate of natural barite cleavage fragments was measured in deionized water and NaCl brines over the temperature range 44-85°C. Dissolution followed first-order kinetics, with an activation energy of 24.9 ± 10.1 kJ/mol. The observed insensitivity to stirring rate and low absolute value of the rate constant (extrapolated k at 25° C = 3.30 ± 1.31 × 10 -3L/ m2s) indicated desorption rather than volume diffusion as the rate-determining step. The NaCl concentration had no effect on k up to 0.1 M. Barite growth from supersaturated aqueous solutions was also studied. Precipitation followed a second order rate law, k25° C = 162 ± 65 L2/ sm2mol, Ea = 22.0 ± 14.3 kJ/mol, and was not sensitive to pH variation. A different rate law is likely to apply at high supersaturations, where a change in secondary growth morphology was observed.