Display Dissolution

Table 5.1 displays dissolution reactions for oxides and hydroxides whose ceramics have potential practical applications. Also included is the pH range in which these reactions are valid. The pH range can be derived by the method described above, or as was done in Chapter 4, it can be calculated by equating the two subsequent equations at the transition boundary where these equations are equally valid. This table shows only the dissolution equations that are valid in acidic and neutral pH. For details covering the entire pH range and general discussions, the reader is referred to Ref. [5]. 

Tirrawarra Sandstone, either meteoric invasion or source-rock maturation probably accounts for the association of authigenic kaolinite patches with siderite spar displaying dissolution (Fig. 4B). 

The starting material and shock-activated powder were mixed with 5-wt% MgO and heated for various periods. At the end of each period the phase content of the samples was determined with x-ray diffraction. In this environment it is thought that the phase is formed by a dissolution-precipitation process as shown in Fig. 7.8. As indicated in Fig. 7.9, the shock-activated silicon nitride displays substantially enhanced dissolution rates that are strongly dependent on shock pressure between 22 and 27 GPa. 

Figure 4.50. Cumulative dissolution results. Two experimental tablet formulations were tested against each other in a dissolution test in which tablets are immersed in a stirred aqueous medium (number of tablets, constructional details and operation of apparatus, and amount of medium are givens). Eighty or more percent of the drug in either formulation is set free within 10 minutes. The slow terminal release displayed by formulation B could point towards an unwanted drug/excipient interaction. The vertical bars indicate ymean - with Sy 3%. A simple linear/exponential model was used to approximate the data for the strength 2 formulation. Strengths I and 3 are not depicted but look very similar.

Pancreatic enzyme replacement is the mainstay of gastrointestinal therapy. Most enzyme products are formulated as capsules containing enteric-coated microspheres or microtablets to avoid inactivation of enzymes in the acidic stomach instead, they dissolve in the more alkaline environment of the duodenum. Capsules may be opened and the microbeads swallowed with food, as long as they are not chewed. A powder form is available for patients unable to swallow the capsules or microbeads, but bioavailability is poor. While products may contain similar enzyme ratios, they are not bioequivalent and cannot be substituted. Generic enzyme products generally display poor dissolution and should not be used.5 Table 13-3 lists commonly used enzyme replacement products. 

Dependence on Base Concentration. The dissolution rates of substituted PHHPs at different alkali concentrations are displayed 1n Figure 1 for seven different novolac resins. In each case, there appears to be a limiting concentration C0 below which the rate of dissolution 1s too slow to be measured 1n the experimental time scale. The ascending portion of the curve can be represented by a power law dependence of the rate on concentration C, eq.(1),... 

Fig. 5.13 displays the dissolution scenario of the gibbsite surface and of the edge surface. The dissolution reaction can be interpreted as a coupled release of Al and Si. The detachment of the Al center is the rate determining step. In a fast subsequent step Si is released from the same surface site. The AI(III) H+ stoichiometry of the precursor group (the group to be detached) is 1 3 at the gibbsite surface and 1 1 at the edge surface. 

The calculations are similar and the result is displayed in Fig. 5.14b for pH = 4.4. Obviously Pb-adsorption is accompanied by an increase in net charge and a marked decrease in surface protonation [sFeOHg]. Plausibly, this reduction in Cji, can decrease the dissolution rate. 

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