Two-stage dissolution

Delayed-release dosage forms (enteric coated) require a two-stage dissolution test, first in simulated gastric fluid to ensure drug is not released in the stomach, and then in simulated intestinal fluid to ensure drug release in the intestine. 

Fig. 15.5 Two-stage dissolution of tablet formulations made with crystaUine drag substance and amorphous solid dispersion in media simulating normal gastric pH...

Production of A1 metal involves two stages (a) the extraction, purification and dehydration of bauxite, and (b) the electrolysis of AI2O3 dissolved in molten cryolite Na3AlF6. Bauxite is now almost universally treated by the Bayer process this involves dissolution in aqueous NaOH, separation from insoluble impurities (red muds), partial precipitation of the trihydrate... 

Dissolution of polymers is a very slow process it can take days or even weeks for particularly high relative molar mass substances. Two stages are discernible during the process of dissolution. Firstly, a swollen gel is produced by solvent molecules gradually diffusing into the polymer. Secondly, this gel gradually disintegrates as yet more solvent enters the... 

It should be pointed out that the diglyceryl silane is a solid compound [113,138,139] thus sol-gel processing with its participation is performed through dissolution of the precursor in an aqueous solution. This leads, as shown experimentally in Ref. [140], to the hydrolysis and subsequent sol formation in accordance with Equations (2)-(4). The important consequence is that the diglyceryl silane can be involved only in the two-stage immobilization procedure shown schematically in Figure 3.3. 

The rate of reduction of a vat dye depends partly on the intrinsic chemical properties of the dye and partly on the size and physical form of the dispersed particles undergoing this reaction. The physical factors are much less important than the chemical aspects [26]. The vatting process entails conversion of the insoluble keto form into the soluble sodium enolate (section 1.6.1). The reaction takes place in two stages at ambient temperature. Extremely rapid reduction to the hydroquinone is followed by slower dissolution in the alkaline solution. At higher temperatures, however, the dissolution rate approximates more closely to the rate of reduction. Temperature and dithionite concentration are the important variables and the rate of reduction is much less dependent on dye or alkali concentration. 

Dissolution of collodion cotton. The collodion cotton used for the manufacture of dynamite gives solutions of very high viscosity. Since it dissolves very slowly, to avoid delays in production, dissolution is usually divided into two stages, viz. initial dissolution and mixing. 

The most important property of sodium and potassium silicate glasses and hydrated amorphous powders is their solubility in water. The dissolution of vitreous alkali is a two-stage process. In an ion-exchange process between the alkali-metal ions in the glass and the hydrogen ions in the aqueous phase, the aqueous phase becomes alkaline, due to the excess of hydroxyl ions produced while a protective layer of silanol groups is formed in the surface of the glass. In the second phase, a nucleophilic depolymerization similar to the base-catalyzed depolymerization of silicate micelles in water takes place. 

For the coal studied here, a bituminous Western Kentucky 9/14 coal, dissolution of the coal has been shown to occur very rapidly, requiring less than 30 minutes to liquefy most of the coal ( 90 percent). However, a relatively long reaction time (120 min.) is required to reduce its sulfur level low enough to meet even the current standards (1). A new short residence time two-stage SRC type process has been suggested by Auburn University (2 ) for solvent refining this coal. This process has been shown to have the potential of producing a low-sulfur solid SRC product that meets the proposed NSPS. It involves the dissolution of the coal... 

While this is not by any means the end of the Work, yet it betokens that the end will be successful. The great sage, Alze, gives us some very clear and satisfactory directions. He says "By one operation and way, by one substance, and by one mixing, the whole work is accomplished, while its purity also is one and it is perfected in two stages, each consisting of a dissolution and a coction, with the repetition of these. 

Fig. 7-4. Selectivity of lignin dissolution during (1) kraft, (2) acid sulfite, and (3) two-stage sulfite pulping of softwood (see Sjostrom, 1964). Note that the delignification proceeds very unselectively at the beginning and at the end of the cook.

The first approach to separation of the two stages was the nonintegrated two-stage liquefaction (NTSL) process,55 in which a second-stage upgrading reactor was placed downstream of an SRC-I type dissolution reactor. Nonintegrated means that the second-stage catalytic hydrotreater did not contribute... 

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