Closed dissolution experiments

Another source of divergence is the use of different models for the aqueous carbonate systems. Precipitation and dissolution experiments can be carried out in closed or open systems and various ways of pH-adjustments (see 8.2). 

In the paddle method, bulk Reynolds numbers range from Re = 2292 (25 rpm, 900 mL) up to Re = 31025 (200 rpm, 500 mL). In contrast, Reynolds numbers employing the basket apparatus range from Re = 231 to Re = 4541. These Reynolds numbers are derived from dissolution experiments in which oxygen was the solute [(10), Chapter 13.4.8] and illustrate that turbulent flow patterns may occur within the bulk medium, namely for flow close to the liquid surface of the dissolution medium. The numbers are valid provided that the whole liquid surface rotates. According to Levich (9), the onset of turbulent bulk flow under these conditions can then be assumed at Re 1500. 

Figure 3.14. Stabilities of calcite, and synthetic (closed squares) and biogenic (closed circles) magnesian calcites as a function of composition. Stabilities are expressed as -log IAPMg-Calcite- The curve is a hand-drawn "best" fit to the synthetic data. Also plotted are the results of precipitation experiments by Mucci and Morse (1984, open squares) and biogenic dissolution experiments by Walter and Morse (1984a, open circles). (After Bischoff et al., 1987.)...

Oelkers E. H., Schott J., and Devidal J.-L. (2001) On the interpretation of closed system mineral dissolution experiments Comment on Mechanism of kaolinite dissolution at room temperature and pressure Part II. Kinetic study by Huertas et al. (1999). Geochim. Cosmochim. Acta 65, 4429-4432. 

A closely related experiment attempted to determine the effect of hydroxide ion concentration on the rate of anodic processes in the absence of polarization (78). This was done with a series of Tafel plots (79) (l-V relationship near the region where the OCP) in varying hydroxide ion concentrations. The results indicated that the anodic current representative of dissolution rate in the absence of overpotential is independent of hydroxide concentration. 

Figures 6 and 7 are derived from laboratory experiments and illustrate that flow can become turbulent close to particle walls even when the bulk flow remains laminar. The turbulent vortices bore into the particle surface, magnifying cavitations and abrading protrusions, and hence accelerating the dissolution process [(10), Chapter 4.3.5]. However, irregulari-...

For the dissolution test to be used as an effective drug product characterization and quality control tool, the method must be developed with the various end uses in mind. In some cases, the method used in the early phase of product and formulation development could be different from the final test procedure utilized for control of the product quality. Methods used for formulation screening or BA and/or bioequivalency evaluations may simply be impractical for a quality control environment. It is essential that with the accumulation of experience, the early method be critically re-evaluated and potentially simplified, giving preference to compendial apparatus and media. Hence, the final dissolution method submitted for product registration may not necessarily closely imitate the in vivo environment but should still test the key performance indicators of the formulation. 

Prior to 1988, dissolution studies had only been reported on natural samples and the results were complicated by the presence of inclusions (Ringwood et al. 1988). Nevertheless, the release rate of Ca from fully amorphous zirconolite from Sri Lanka, averaged over temperatures of 95 and 200 °C, appeared to decrease from about 10-1 g/m2/d to 3 x 10-3 g/m2/d after about 2 weeks. McGlinn et al. (1995) studied the pH dependence of single-phase zirconolite in pure flowing water at 90 °C. Results showed that after 43 days the release rates for Ca decrease with increasing pH for all samples, although there is a lot of scatter, especially where the data are close to the detection limit. Over the duration of the experiments,... 

When working with capillary columns, the splitless mode is used for very dilute samples. In this mode, the injection is made very slowly, leaving valve no. 2 in the closed position (Fig. 2.5) for approximately 0.5 to 1 min. This allows vaporisation of the compounds and solvent in the first decimetre of the column by a complex mechanism of dissolution in the stationary phase, which is saturated with solvent. Compound discrimination is very weak using this method. The proper use of this injection mode, which demands some experience, requires a temperature program that starts with a colder temperature so that the solvent can precede the analytes in the column. This mode is typically used for trace analyses. The opening of valve no. 2 eliminates, from the injector, compounds which are less volatile and that can interfere with the analyses. 

Y-intercept and the slope, respectively, and are listed in Table I. The results of the dissolution rate determination method (method 2) are presented in Figure 3. As can be seen, the maximum removable mineral (P0) by dilute acid is independent of the size of the shale particles. However, the carbonate fraction in the shale mineral matrix is very close to this figure. This could mean that the accessibility of the leaching agent to the leachable materials in shale is complete in the size ranges studied in this experiment—but at different rates. This could also indicate that the carbonate deposit sites are not isolated but can, perhaps, be thought of as interconnected by minerals built of the dilute acid-resistant minerals. 

It is also important to keep in mind that the relation between the saturation state of seawater and carbonate dissolution kinetics is not a simple first order dependency. Instead it is an exponential of about third to fourth order (e.g., Berner and Morse, 1974). Thus dissolution rates are very sensitive to saturation state. This type of behavior has not only been demonstrated in the laboratory (see Chapter 2), but also has been observed in numerous in situ experiments in which carbonate materials and tests have been suspended in the oceanic water column. The depth at which a rapid increase in dissolution rate with increasing water depth is observed usually has been referred to as the chemical or hydrographic lysocline. In some areas of the ocean it is close to coincident with the FL (e.g., Morse and Berner, 1972). 

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