General Dissolution

Mechanisms of dissolution kinetics of crystals have been intensively studied in the pharmaceutical domain, because the rate of dissolution affects the bioavailability of drug crystals. Many efforts have been made to describe the crystal dissolution behavior. A variety of empirical or semi-empirical models have been used to describe drug dissolution or release from formulations [1-6]. Noyes and Whitney published the first quantitative study of the dissolution process in 1897 [7]. They found that the dissolution process is diffusion controlled and involves no chemical reaction. The Noyes-Whitney equation simply states that the dissolution rate is directly proportional to the difference between the solubility and the solution concentration  

Although the Noyes-Whitney equation has been used widely, it has been shown to be inadequate in modeling either S-shape experimental data or data with a steeper initial slope. Therefore, a more general function, based on the Weibull distribution [8], was proposed [9] and applied empirically and successfully to all types of dissolution curves [10]  

The velocity of liquid flow around suspended solid particles is reduced by frictional resistance and results in a region characterized by a velocity gradient between the surface of the solid particle and the bulk fluid. This region is termed the hydrodynamic boundary layer and the stagnant layer within it that is diffusion-controlled is often known as the effective diffusion boundary layer. The thickness of this stagnant layer has been suggested to be about 10 times smaller than the thickness of the hydrodynamic boundary layer [13]. 

Bisrat et al. concluded that for sparingly soluble, suspended drugs, diffusional transport plays a major role in the dissolution kinetics [19]. They studied the effect of particle size and viscosity on dissolution rate and apparent diffusional distance (.h-App) of oxazepam and griseofulvin. The term apparent diffusional distance was used as a simplified measure of the distance over which diffusion dominates and was calculated as follows  

Combining the above expressions with equation (4) and setting h equal to ro for small partides results in an expression for dissolution rate in terms of Do and Dn  

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Compounds which dissolve in concentrated sulphuric acid may be further subdivided into those which are soluble in syrupy phosphoric acid (A) and those which are insoluble in this solvent (B) in general, dissolution takes place without the production of appreciable heat or colour. Those in class A include alcohols, esters, aldehydes, methyl ketones and cyclic ketones provided that they contain less than nine carbon atoms. The solubility limit is somewhat lower than this for ethers thus re-propyl ether dissolves in 85 per cent, phosphoric acid but re-butyl ether and anisole do not. Ethyl benzoate and ethyl malonate are insoluble. 

Alkaline solutions above about pH 11 due to general dissolution and caustic cracking ... 

Reactivation Ratio EPR Test (Fig. 19.20c) This is a simpler and more rapid method than the single or double loop tests, and depends on the fact that the value of determined during the anodic scan of a double loop test (which produces general dissolution without intergranular attack on sensitised material) is essentially the same for all AlSl Type 304 and 304L steels. 

Fig. 5.16 Dissolution rate of 30% Ru02 + 70% Ti02 at 2 kA nrT2 in 300 g I-1 NaCI at 70°C arising from erosion and general dissolution (plotted from the data in Ref. [24]). 

Sufficient bioavailability must be achieved by the chosen formulation with an appropriate selection of excipients. In general, dissolution rate tests and pharmacokinetic studies are used to assess the bioavailability of the drug product. 

The simplest activity term expressions of rate laws are those involving the acidity of the system. Because, generally, dissolution of a silicate can be schematically depicted as... 

See cuirent United States Pharmacopeia/National Formulary, section <711 >, for general dissolution specifications. All profiles should be conducted on at least 12 individual dosage units. 

In nonreactive molten salts, on the other hand, flux components are not incorporated into the product phase. Here, the molten salt acts more in the classical sense as a reagent to promote the reaction at a lower temperature than would be required by the ceramic, or direct, route (Section 5.2). This is accomplished by two attributes of molten salts an acid-base equilibrium that enables the general dissolution-recrystallization of metal oxides and a highly electropositive (oxidizing) environment that stabilizes the highest oxidation state of many transition metals (Gopalakrishnan, 1995), which can lead to mixed valency. A plethora of complex transition metal oxides have been synthesized in nonreactive molten alkali metal hydroxides, carbonates, and hypochlorites. Examples of such molten salt routes to mixed transition metal oxides include (Rao and Raveau, 1998) ... 

Uptake of the parabens probably proceeds by a general dissolution of an ester into the cell with no specific binding sites at the cell surface. In the deep rough mutants, the increased sensitivity to the parabens probably results from the appearance of phospholipid patches at the cell surface which aid in penetration of the parabens and especially of the most hydrophobic one (the Bu ester). A similar mechanism is likely to account for the uptake of phenolics across the outer membrane into the cells. 

The specific heats of aqueous ions of several metals are not readily available in the literature. As a result, it is difficult to determine the temperature dependence of the thermodynamic quantities and, hence, that of the solubility product constants for these metal oxides. This dependence can be roughly determined from the general dissolution equation... 

Consider Eq. 7.3, for example. It describes a general dissolution reaction for an oxide in an acidic medium. Similar to this equation, the dissolution reaction involving a redox reaction is... 

Since the alumina crucibles were put into use, metallic iron has only rarely been observed in the slag. On the other hand, varying degrees of attack on the alumina crucible by the slag have been observed. Generally, dissolution of AI2O3 by the slag has been... 

Generally, dissolution results are reported as cumulated percent drug release vs. time. Presently, most of the tolerances are based on a single time point, such as not less than 85% dissolved or released in 30 min. However, more reports are appearing with percent drug release values at multiple time points, resulting in a drug release pattern, commonly known as a profile. ... 

In general, dissolution rates of other silicate and aluminosilicate minerals also increase under acid and alkaline conditions and are relatively independent of pH in the near-neutral pH range. Drever (1994) summarizes this behavior with a general equation for the forward rate far from equilibrium... 

Figure 2.11 Generalized dissolution rates (R, mol/m s) of carbonate and silicate minerals in water near 25°C as a function of pH based on laboratory measurements. After A. Ler-man, Transport and kinetics in surficial processes. In Aquatic chemical kinetics, ed.

The Tafel constant was b = 0.20 V decade-1 for iron electrodes [55] and b = 0.20 V decade-1 for austenitic stainless steels [54] in acid solution. It is noticed that these Tafel constants are greater than those (0.03-0.1 V) usually observed with general dissolution of metals in acid solution. The other mode of localized corrosion is the active mode of corrosion that prevails in the potential range less positive (more cathodic) than the passivation potential, EP, in which potential range the localized corrosion is mainly controlled by the acidity of the occluded pit solution. In the potential range of active metal dissolution, the anodic dissolution current density is also an exponential function of the electrode potential, except for diffusion-controlled dissolution. 

FIGURE 4.5.15. Ruthenium losses arising from erosion and general dissolution with 30% RUO2 + 70% Ti02 electrode at 2 kA m in 300 gpl NaCl at 70°C (plotted from the data in ref. [75]). 

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