Types of Dissolution

As mentioned above, the formation of activated complexes Is a prerequisite for a chemical reaction. In order to attain the activated state, the system requires some source of energy. There are three main sources by which the molecules can acquire the necessary activation energy  

Thermal energy associated with translation of molecules and their internal vibrations and rotations. 

Taking the above three sources as the basis, we can distinguish between the traditional terminology of dissolution as follows  

The subdivision of chemical etching stems from the types of reactions taking place on a dissolving surface. In solvation or solution etching the ions/molecules on a surface are severed by solvation, in true chemical etching by double displacement reactions, e.g., 

Finally, it should be pointed out that since photoetching and electrolytic etching are carried out at T 0, there is always a thermal contribution to the activation enegy. 

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The anode is usually soluble, and is made from a high purity form of the metal being deposited, or occasionally from an alloy. A soluble anode is often the cheapest and most convenient means of replacing the metal reduced at the cathode. Effective anode corrosion is important, and different examples present a variety of types of dissolution. 

As examples of some water-soluble salts, mention may be made of potassium chloride, copper sulfate, and sodium vanadate. As examples of some water-insoluble salts, mention may be made of some typical ones such as lead chloride, silver chloride, lead sulfate, and calcium sulfate. The solubilities of most salts increases with increasing temperature. Some salts possess solubilities that vary very little with temperature or even decline. An interesting example is provided by ferrous sulfate, the water solubility of which increases as temperature is raised from room temperature, remains fairly constant between 57 and 67 °C, and decreases at higher temperatures to below 12 g l-1 at 120 °C. Table 5.2 presents the different types of dissolution reactions in aqueous solutions, and Table 5.3 in an indicative way presents the wide and varied types of raw materials that different leaching systems treat. It will be relevant to have a look at Table 5.4 which captures some of the essential and desirable features for a successful leaching system. 

Table 5.2 Types of dissolution reactions in aqueous solutions.

Fig. 2 Two types of dissolution-controlled, pulsed delivery systems (A) single bead-type device with alternating drug and rate-controlling layers (B) beads containing drug with differing thickness of dissolving coats.

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] ... 

Comparison of the three different types of dissolution reactions... 

In conclusion, it appears necessary to study more extensively those properties of the various oxides, which determine their specific dissolution behaviour. As pointed out by Postma (1993), the variation in reactivity, a solid phase parameter, may, in some cases, be twice as high as the effect of the type of dissolution (protonation, complexation, reduction). 

It was shown by Soriaga et al. [115] that this unique type of dissolution can be used for the selective and quantitative anodic... 

Apparatus. The nature of the dosage form will determine the type of dissolution apparatus that will be used for method development and validation. The following questions must be asked when selecting the dissolution apparatus. Is it a capsule Will a sinker be required How stable is the drug substance after dissolution in the medium Is the formulation an immediate release or an extended release formulation Is this a transdermal patch ... 

In addition to selection of a suitable type of dissolution apparatus, the dissolution media is another critical element in generating IVIVCs. Besides the compendial dissolution media, proper selection and use of biorelevant dissolution media to mimic Gl biological conditions can be of beneLt as has been reported, especially for dosage forms containing poorly water-soluble drugs [10-16], The commonly used dissolution media are ... 

In 1951, Weibull [116] described a more general function that can be applied to all common types of dissolution curves. This function was introduced in the pharmaceutical field by Langenbucher in 1972 [117] to describe the accumulated fraction of the drug in solution at time t, and it has the following form 1... 

Ueberreiter (1968) demonstrated that the state of a polymer influences the type of dissolution to a great extent. 

The type of solvent action that fused nonmetallic oxides have on metallic oxides may be likened to the second type of dissolution process, i.e., proton-transfer reactions. The process may be pictured as follows. The oxygens cannot discriminate between the metal ions (of the metallic oxide), with which they have been associated in the lattice of a metal oxide before dissolution, and the oxygen atoms of the Si04 tetrahedra contained in the solvent—fused silica. The oxygen atoms sometimes therefore leave the metal ions and associate with those of the tetrahedra. Dissolution has occurred with a type of oxygen-transfer reaction (see Fig. 5.69). 

As shown above, if the initial dissolutional parameters are known (mean annual temperature, type of dissolution system), then the [Ca ] of the cave dripwaters can be estimated Drake and Wigley (1975) demonstrated an empirical correlation between regional temperatures and spring water /7CO2, and thus CO2. The next section summarises the controls on precipitation and translates them into growth rates. 

One important aspect of this type of dissolution approach is that a good contact between the crystal surface and the sample is essential, not only for the imaging of the tablet or pharmaceutical formulation but also to ensure that water penetrates into the sample only from the side of the tablet. This is especially important when mathematical modeling of the dissolution process is compared with experimental data. When using the diamond ATR approach, the tablet is pressed onto the diamond, and hence the leakage of water between the diamond surface and the tablet is not expected [66]. 

In studies of the (100) face, two distinct types of dissolution behavior were observed. In the center of the face, dislocation etching revealed that the average dislocation spacing was considerably less than the size of the UME probe (50), i.e., in the SECM configuration the electrode would sit over a constant source of dissolution sites once the process had been initiated. This was reflected in the shape of experimental SECM chronoamper-ometric transients (3), such as those depicted in Figure 17 at different tip-substrate separations. In all cases, the current rapidly attained a steady state, indicative of a uniform, constant rate of dissolution. As the UME was moved closer to the crystal surface, the limiting current increased, consistent with an increase in the flux of Cu2+ from the crystal surface to the tip electrode. 

Another approach to obtain a parameter that describes the dissolution rate is to use statistical moments to determine the mean dissolution time (MDT) (von Hattingberg 1984). This method has the advantage of being applicable to all types of dissolution profiles, and it does not require fitting to any model. The only prerequisite is that data points are available close to the final plateau level. The MDT can be interpreted as the most likely time for a molecule to be dissolved from a solid dosage form. In the case of zero- and first-order dissolution processes, the MDT corresponds to the time when 50 and 63.2 percent have been released, respectively. The MDT is determined from ... 

Dissolution-precipitation models. Dubinina and Lakshtanov (1997) developed a kinetic model that describes isotopic fractionation between a mineral and fluid involved in one of three types of dissolution-precipitation processes (Fig. 11). Type I (mineral synthesis) considers successive dissolution of an unstable phase, A, of uniform isotopic composition and precipitation (crystallization) of phase B. Type II (Ostwald ripening) involves the partial dissolution of phase B which has a non-uniform isotopic composition... 

He also proposed two types of dissolution. If the polymer permitted the existence of all four layers described above, the dissolution was termed normal . The main feature of this type of dissolution was the presence of a gel layer. The other type of dissolution was one without a gel layer. In this case, cracks were observed running into the polymer matrix and these caused small blocks of the polymer to erode and leave the surface. 

By lowering the experimental temperature, transition from normal dissolution to the type of dissolution without a gel layer was observed in the dissolution of polyfmethyl methacrylate) (PMMA) [27]. The temperature at which the gel layer disappeared was termed as the gel temperature. Ueberreiter and Asmussen [28] observed that polystyrene (PS) undergoes normal dissolution in most solvents owing to its low gel temperature. 

From inspection of these pictures the different stability of steps with even the same orientation is remarkable. Since STM does not show ad-atoms, this method is of no help in confirming one or the other type of dissolution mechanism. 

Another type of dissolution reagents is represented by concentrated chaotropic salt solutions like hthium bromide (LiBr) (Ambrose et al., 1951), lithium thiocyanate (LiSCN) (Sridhara et al., 1973), calcium chloride (CaCl2) (Ajisawa, 1998) based on either only water or alcohol—aqueous solutions such as CaCb—ethanol—H2O (Ajisawa, 1998 Chen et al.,... 

Simultaneous dissolution, where at steady state die alloy elements go into the solution at a rate proportional to their atomic concentration in the alloy. Well-known examples pertain to the iron-base stainless steels and especially ferritic Fe-Cr, on which this type of dissolution is repeatedly found [173,174]. 

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