Dissolution rate, definition

By the above definition, b is positive for crystal dissolution, and negative for crystal growth. During convective crystal dissolution, the dissolution rate u is directly proportional to b. During diffusive crystal dissolution, the dissolution rate is proportional to parameter a, which is positively related to b. Hence, for the dissolution of a given mineral in a melt, the size of parameter b is important. The numerator of b is proportional to the degree of undersaturation. If the initial melt is saturated, b = 0 and there is no crystal dissolution or growth. The denominator characterizes the concentration difference between the crystal and the saturated... 

When the rate of dissolution of a solid is rapid, the process is controlled by the rates of chemical and thermal diffusion in the solvent. As the dissolution rate (or the undersaturation of the solvent) becomes smaller, surface imperfections of the solid play a role of increasing importance in dissolution until they completely dominate the process. These general features of the behavior are well known (i). However there is some controversy about the mechanisms by which imperfections affect the dissolution process. This paper attempts a better definition of some of the mechanisms on the basis of simple cause-and-effect arguments derived directly from experiments. 

The advantage of the definitions proposed by the FDA and USP is that they state that the parameters to be correlated, namely the in vitro dissolution rate and in vivo input rate of active material. 

The etch rate of the (111) surface, although much smaller than those of the (100) and (110) planes, shows definite values, in the range of 2-10 A/s in KOH solutions. It is still much larger than the dissolution current density on a passivated surface in KOH (a dissolution rate of 2-10 A/s is equivalent to a current density of several milliamperes per square centimeter). In alkaline solutions, the dissolution rate of silicon oxide is less than 0.01 A/s (see Chapter 4), which is several orders of magnitude smaller than the etch rates of a (111) surface. Thus, it is unlikely that the silicon surface of any orientation is covered by Si02 during etching. 

In wet granulation, the quantity of solvent, usually water, has a definite influence on the tablet properties. Increasing the amount of water as granulation liquid gave naproxen tablets with a significantly higher dissolution rate [534], ace-... 

Dose numbers of >20 are by definition related to low soluble and dissolution rate limited substances (see Amidon et al. ). In such cases special formulation approaches are to be considered from the scratch in the formulation strategy. 

Polymorphic form also influences dissolution rate and solubility. By definition, metastable polymorphs should have higher solubility and faster dissolution rates than those of their more stable crystalline counterparts because they possess a higher Gibbs free energy. Generally, only a moderate enhancement of solubility and dissolution rate can be achieved through polymorphic modification, although... 

Observations from studies of surface sediments have allowed definition of regionally varying levels in the ocean at which pronounced changes in the presence or preservation of calcium carbonate result from the depth-dependent increase of dissolution on the seafloor. The first such level to be identified was simply the depth boundary in the ocean separating carbonate-rich sediments above from carbonate-free sediments below. This level is termed the calcite (or carbonate) compensation depth (CCD) and represents the depth at which the rate of carbonate dissolution on the seafloor exactly balances the rate of carbonate supply from the overlying surface waters. Because the supply and dissolution rates of carbonate differ from place to place in the ocean, the depth of the CCD is variable. In the Pacific, the CCD is typically found at depths between about 3500 and 4500 m. In the North Atlantic and parts of the South Atlantic, it is found... 

For polycrystalline metals, the dissolution rate of the various crystallographic orientations is different. Grain boundaries or precipitations will also show a different corrosion rate. There is no accepted definition for what constitutes uniform corrosion. A possible definition could be that the variation of thickness loss all over the surface should not he greater than 5%. 

The second holds that metals passive by Definition 1 are covered by a chemisorbed film—for example, of oxygen. Such a layer displaces the normally adsorbed H2O molecules and decreases the anodic dissolution rate involving hydration of metal ions. Expressed another way, adsorbed oxygen decreases the exchange current density (increases anodic overvoltage) corresponding to the overall reaction M -1- ze. Even less than a monolayer on the surface is... 

Experimental measurements of solubility are influenced by many different factors, including the purity of the solute and solvent, presence of cosolvents, presence of salts, temperature, physical form of the undissolved solute, ionization state, and solution pH [18]. Consequently many different definitions of solubility are in common use in the published literature. Here we discuss the intrinsic aqueous solubility, Sg, which is defined as the concentration of the neutral form of the molecule in saturated aqueous solution at thermodynamic equilibrium at a given tanperature [18-20]. Intrinsic aqueous solubility is used to calculate dissolution rate and pH-dependent solubility in models such as the Noyes-Whimey equation [21] and the Henderson-Hasselbalch equation [22, 23], respectively. Prediction of the intrinsic aqueous solubility of bioactive molecules is of great importance in the biochemical sciences because it is a key determinant in the bioavailability of novel pharmaceuticals [1, 3, 24-26] and the environmental fate of potential pollutants [27, 28],... 

As the p-Cl-BHA was the most effective, it was subjected to a thorough study. The results show that the p-Cl-BHA definitely reduced the copper dissolution rate in 0.5 M NaCl solution (Fig. 9-15). The mass of the electrode increased due to the adsorption of the inhibitor and formation of an inhibitor-corrosion-products complex on the electrode surface. This protective film accounts for the inhibition efficiency of p-Cl-BHA against localized attack on the copper electrode in NaCl (Shaban et al., 1998 a, b). 

The reaction proces.ses can be described by a combination of results from leach tests and SIMS. The nominal composition and leach rates (g in - d" ) of a glass formulation, PNL 76-68 (Pacific North-West Laboratories), used as the. standard high-level nuclear waste glass matrix [4], are shown in Table 4. The definition of leach rate u.sed normalizes the loss (in g) to the proportion of that element in the nominal bulk composition. It will be seen that the leach rates for Cs, Na, Mo, Si and B exceeded the overall rate of mass loss from the gla.ss surface, whereas the elements Fe, Zn and Ti did not show significant loss to solution under these conditions. There were also several elements, e.g.. Ca. Ba, Cd and Sr, that were neither rapidly leached nor apparently retained in the surface layers. Studies described in the above-mentioned reviews, using XPS, FTIR, SIMS, SEM and dissolution rates, have established clearly that the primary reaction occurring in solution is the bond-breaking attack by OH at Si (or Al and... 

---