Dissolution rate, continuous-flow

The wide variety of methods for determining the dissolution rates of solids may be categorized either as batch methods (Fig. 13A) or as continuous-flow methods (Fig. 13B). The common batch-type dissolution methods are derived from the beaker-stirrer method of Levy and Hayes [89] and include a number of thoroughly standardized procedures, especially those defined by the U.S. Pharmacopoeia [90]. 

Continuous-flow (i.e., column dissolution) methods, depicted schematically in Fig. 13B, have limited, but growing, application [92-94]. The volume flow rate, dVIdt, of the dissolution medium must remain constant to achieve the steady state shown in Fig. 14B, so that... 

Volume, V, of the dissolution medium in a batch-type apparatus (Eq. 31), or volume flow rate, dV/dt, in a continuous-flow apparatus (Eq. 

Rates of dissolution of U02(s) obtained in a thin layer continuous flow-through reactor as a function of pH. 

Fig. 5.15b shows a thin-film continuous flow reactor used by Bruno et al. (1991) for determining the dissolution rate of U02 under reducing conditions. A known weight of U02(s) was enclosed into the reactor between two membrane filters (0.22 jum). The reducing conditions of the feed solution were obtained by bubbling H2(g) in the presence of a palladium catalyst. The dissolution rates determined using continu-... 

Continuous flow-type reactors to measure dissolution rates... 

Dissolution rate of carbonates as a function of pH. These experiments were carried out with a continuous flow reactor in open systems with controlled pco2-... 

The flow-through cell is applicable not only for the determination of the dissolution rate of tablets and sugar-coated tablets, but has also been applied to suppositories, soft-gelatin capsules, semisolids, powders, granules, and implants. A small volume cell containing the sample solution is subjected to a continuous stream of dissolution media. The dissolution... 

Weathering rates in the field are as much as one to two orders of magnitude slower than dissolution rates measured in the laboratory (Benedetti et al., 1994 see Chapter 5.05). The difference is due to a number of factors (i) there are differences in surface area between laboratory minerals and natural minerals (ii) secondary precipitates may protect primary mineral surfaces in the field (iii) in soils, most flow is through macropores and not all mineral surfaces are continually exposed to flowing solutions as they are in laboratory experiments and (iv) most... 

Tingstad, J.E. Riegelman, S.J. Dissolution rate studies I design and evaluation of a continuous flow apparatus. J. Pharm. Sci. 1970, 59, 692-696. 

Figure 13.12. Continuous flow-type reactors to measure dissolution rates, (a) Experimental scheme of the thin-film continuous flow reactor used for example by Bruno et al. (1991) to determine dissolution rate of UO2 under reducing conditions, (b) Schematic diagram of the fluidized-bed reactor by Chou and Wollast (1984), and developed further by Mast and Drever (1987).

Additive dissolution rates vary considerably with the type, origin, preparation, and concentration of the additive. At Combustion Engineering, a prototype scrubber system, pilot plant scrubber system, continuous flow stirred tank reactors, and batch reactors have been used to determine the dissolution rates for individual additives. 

Most investigators have observed a decrease in the rate of dissolution as weathering reactions proceed (Fig. 7-12). Commonly, the initial rate of weathering decreases by an order of magnitude within the 1st d of reaction, and may decrease by an additional one to three orders of magnitude within 2 wk to 1 yr or more. This decrease is not due to the presence of a reverse reaction, as it can be readily observed in continuous-flow reactor systems where dissolution product concentrations are carefully maintained at very low levels (Chou and Wollast, 1984 Holdren and Speyer, 1985). The dissolution rate may be nonlinear even after extensive weathering curvilinear rates have been observed after more than 400 d of reaction for labradorite at pH 4 (Erich and Bloom, 1987, unpublished data). This phenomenon has been related to several mechanisms ... 

The overall dissolution rate in a continuous dissolver is controlled by the metal feed rate, the temperature, the concentration of feed solution, and its flow rate. The metal composition of product solution at steady-state operation is just the ratio of the metal mass feed rate to the solution volume feed rate. 

Continuous flow methods have the advantages that sink conditions can be easily achieved, and that a change in dissolution rate is reflected in a change in [9]. At the same time, they require a significant flow rate that may require relatively large volumes of dissolution medium. Should the solid be characterized by a low solubility and a slow dissolution rate, will be small, and a very sensitive analytical method would be required. 

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