Dissolution rate temperatures

Rhovanil Pine Mesh, a specially caUbrated extra pure vanillin that avoids demixing with other very fine dry ingredients such as sucrose, flour, and dextrose, provides a faster dissolution rate at lower stirring, at lower temperature, in low acidity medium, or in viscous Hquids. 

Fig. 7.8. High temperature conversion of a-silicon nitride with an MgO additive to the p-pha.se is thought to be a consequence of dissolution of the a phase in a magnesium silicate with subsequent recrystallization from the melt. Enhanced dissolution rate should then strongly influence a. p conversion [84B01].

In a detailed study the dissolution kinetics of shock-modified rutile in hydrofluoric acid were carefully studied by Casey and co-workers [88C01], Based on the defect studies of the previous sections in which quantitative measures of point and line defects were obtained, dissolution rates were measured on the as-shocked as well as on shocked and subsequently annealed powders. At each of the annealing temperatures of 200, 245, 330, 475, 675, 850, and 1000 °C, the defects were characterized. It was observed that the dissolution rates varied by only a factor of 2 in the most extreme case. Such a small effect was surprising given the very large dislocation densities in the samples. It was concluded that the dissolution rates were not controlled by the dislocations as had been previously proposed. 

Kassner used a rotating disc, for which the hydrodynamic conditions are well defined, to study the dissolution kinetics of Type 304 stainless steel in liquid Bi-Sn eutectic. He established a temperature and velocity dependence of the dissolution rate that was consistent with liquid diffusion control with a transition to reaction control at 860 C when the speed of the disc was increased. The rotating disc technique has also been used to investigate the corrosion stability of both alloy and stainless steels in molten iron sulphide and a copper/65% calcium melt at 1220 C . The dissolution rate of the steels tested was two orders of magnitude higher in the molten sulphide than in the metal melt. 

Based on an analysis of the initial dissolution rate in different solutions at different temperatures, several very useful conclusions and recommendations were made. It was found that the apparent activation energies for the dissolution of niobium and tantalum in 10 mol/1 HF solution are 56.5 and 65.5 kJ/mol, respectively for columbite, and 42.7 and 61.1, respectively, in the case of tantalite. It was also concluded that the mechanism of dissolution is the same for both columbite and tantalite. In addition, the initial dissolution rate of niobiuth (RNb) from columbite is controlled primarily by the activities of the... 

This aromatic alcohol has been an effective preservative and still is used in several ophthalmic products. Over the years it has proved to be a relatively safe preservative for ophthalmic products [138] and has produced minimal effects in various tests [99,136,139]. In addition to its relatively slower rate of activity, it imposes a number of limitations on the formulation and packaging. It possesses adequate stability when stored at room temperature in an acidic solution, usually about pH 5 or below. If autoclaved for 20-30 minutes at a pH of 5, it will decompose about 30%. The hydrolytic decomposition of chlorobutanol produces hydrochloric acid (HC1), resulting in a decreasing pH as a function of time. As a result, the hydrolysis rate also decreases. Chlorobutanol is generally used at a concentration of 0.5%. Its maximum water solubility is only about 0.7% at room temperature, which may be lowered by active or excipients, and is slow to dissolve. Heat can be used to increase dissolution rate but will also cause some decomposition and loss from sublimation. Concentrations as low as 0.125% have shown antimicrobial activity under the proper conditions. 

The surface area of a solid material is important in that it provides information on the available void spaces on the surfaces of a powdered solid [48]. In addition, the dissolution rate of a solid is partially determined by its surface area. The most reproducible measurements of the surface area of a solid are obtained by adsorbing a monolayer of inert gas onto the solid surface at reduced temperature and subsequently desorbing this gas at room temperature. The sorption isotherms obtained in this technique are interpreted using the equations developed by Brunauer, Emmett, and Teller, and therefore the technique is referred to as the B.E.T. method [49]. The surface area is obtained in units of square meters of surface per gram of material. 

Sulfathiazole has been found to crystallize in three distinct polymorphic forms, all of which are kinetically stable in the solid state but two of which are unstable in contact with water [130]. As evident in Fig. 20, the initial intrinsic dissolution rates are different, but as forms I and II convert into form III, the dissolved concentrations converge. Only the dissolution rate of form III was constant during the studies, indicating it to be the thermodynamically stable form at room temperature. Aqueous suspensions of forms I or II were all found to convert into form III over time, supporting the finding of the dissolution studies. Interestingly, around the melting points of the three polymorphs, form I exhibited... 

Wolff-Boenisch, D., Gislason, S.R., Oelkers, E.H. and Putnis, C.V. (2004). The dissolution rates of natural glasses as a function of their composition at pH 4 and 10.6, and temperatures from 25 to 74 degrees C. Geochimica et Cosmochimica Acta 68 4843-4858. 

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. 

This paper presents new data on dissolution kinetics. The effects of alkali concentration, size of the cation, and salt addition were studied. The influence of segmental mobility on dissolution was elucidated by measuring the temperature coefficients of the dissolution rates. Experiments were also carried out to study the relation between the chemical structure of a polymeric Inhibitor and Its effectiveness 1n retarding dissolution. Based on these results,... 

If cation diffusion 1s Indeed Involved 1n the rate determining step, the dissolution rates at temperatures above Tg are expected to obey a WLF-type relationship (17). In the present context, the ratio of the dissolution rate at temperature T to that at Tg replaces the shift factor,... 

The significance of polymorphism in the pharmaceutical industry lies in the fact that polymorphs can exhibit differential solubility, dissolution rate, chemical reactivity, melting point, chemical stability or bioavailability, among others. Such differences can have considerable impact on a drag s effectiveness. Usually, only one polymorph is stable at a given temperature, the others being metastable and evolving to the stable phase... 

Measurements were undertaken of the solubility of each phase in acid solutions, of the growth rate of gypsum crystals and the dissolution rate of hemihydrate. The growth rate depends on the square of the supersaturation and on temperature with an activation energy of 64 kJ/mol. The nucleation rate appears to vary linearly with supersaturation. 

As the shape of the needle-like hemihydrate crystals changes as they dissolve, it is necessary to convert to the crystal width as a measure of size. In terms of this measure, the dissolution rate is first order with undersaturation and shows only a small temperature effect (activation energy of 10 kJ/mol). 

A plot of the dissolution rate against driving force, AC, is shown in Figure 13. This shows a linear dependence on undersaturation and a slight dependence on temperature (activation energy of 10 kJ/mole). This indicates that the dissolution is mass transfer controlled. The results can be correlated by... 

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