Uniform dissolution

Viscose Aging, Filtration, and Deaeration. After the dissolution step, the viscose cannot be spun into fibers because it contains many small air bubbles and particles. Furthermore, the degree of xanthation is too high, with too many of the xanthate groups in positions dictated by their accessibihty and not in the ideal positions for uniform dissolution. 

Reactions with aqueous solutions. Uniform dissolution or corrosion of metals in acid, alkaline or neutral solutions (e.g. dissolution of zinc in hydrochloric acid or in caustic soda solution general corrosion of zinc in water or during atmospheric exposure). Reactions with non-aqueous solution (e.g. dissolution of copper in a solution of ammonium acetate and bromine in alcohol). 

Dr. Pryor considers that in certain cases of uniform dissolution of metals in acids (e.g. AI in hydrochloric or sulphuric acid) or alkalis a thin him of oxide is present on the metal surface — the him is not rate-determining but its presence would indicate that reactions of this type should be classified under 2 (c). 

The unequal attack which occurs in tap water, condensate and other mild electrolytes may lead to perforations of thin-gauge sheet and even to deep pitting of castings. In stronger electrolytes the effect is variable. In chloride solutions such as sea-water, attack on the metal usually results in the pitting of some areas only, but where the metal surface has been rendered reactive, as by shot blasting, attack may be so rapid that uniform dissolution over the whole surface may occur. In either case magnesium-base alloys are not usually suitable for use in aqueous liquids since they are not intrinsically resistant to these electrolytes. 

Both titanium and boron can be added as grain refiners to ensure small grain size and hence high surface area grain boundaries. This reduces the risk of preferential attack at grain boundaries and promotes more uniform dissolution. 

The electrochemical characterisation studies, discussed in the previous section, showed that a 40 at.% Ru electrode, when subjected to extended electrolysis or potential or current cycling in NaCl solutions and when the chlorine overpotential reaches 300-400mV, behaves like a fresh, low at.% Ru (about 5 at.%) electrode. This strongly suggests that Ru losses from the Ru/Ti oxide coating occur during electrolysis. To determine whether or not the Ru losses in failed anodes take place by uniform dissolution across the entire coating or whether only localised surface... 

If the Ru loss in the deactivated anode is a result of uniform dissolution across the entire coating layer, resulting in a Ru loading of less than 2 g m-2, the anode has to be recoated to regain its electrocatalytic activity for the chlorine evolution reaction. Under these conditions, the existing anode coating must be stripped prior to recoating. However, if surface depletion of Ru is the cause for increased anode potential, then replenishment of these surface sites should result in the rejuvenation of the deactivated anodes. 

A certain amount of uniform dissolution may occur prior to and during the initiation of pores. Such an etched layer prior to the... 

Physical The original physical properties, including appearance, palatability, uniformity, dissolution, and suspendability are retained. 

It is therefore believed that at pH 6 and greater the corrosion process is localised and large local concentrations of ferrous iron are achieved. At pH 6 the oxidation to ferric iron is very rapid ( ) and precipitation of Fe(0H)j occurs to exhibit localised corrosion or "flash-rust" spots. At pH 5 and below a small but finite uniform dissolution of the iron substrate occurs. However, in this pH range the oxidation of the ferrous dissolution product to ferric ion is considerably slower, by almost 1000 times, and hence "flash rusting" is not observed. 

FIGURE 13.5 Two models for the structure of micelles, (a) Uniform dissolution of I in B. (b) Stabilization of a microdroplet of I by putting Aon the interface. (Reproduced from Xing, L. and W L. Mattice. liBWjtjmuir 14 4074-4080. With permission from American Chemical Society.)... 

Solubilization initially takes place by displacing the solvent molecules from the micellar core. Unlike with conventional surfactant micelles, a substantial amount of solvent is present in the micellar core of the small BAB tri block copolymers. This is supported by Hurterret al. (1993) and Linse and Malmsten (1992). At this stage, the micellarstructure complies with the model drawn in Figure 13.5a, which is the uniform dissolution of solute in B. 

Attributes Mixing Drying Flowability Mechanical Uniformity Dissolution Wettability Stability... 

In order to determine the corrosion state of an active-passive system, the position of the corrosion potential relative to pp must be determined. According to Fig. 4, if Econ is below Ew, the material will undergo uniform dissolution under film-free conditions. If EC0II is above Epp but below Et, the material will be passive and will dissolve at its passive current density, which is often on the order of 0.01 mpy. Corrosion-resistant alloys are designed to operate under such conditions. For situations in which Ec0II is above Et, the material will dissolve transpas-sively, i.e., uniformly. 

Inspection of the specimen surface at the points labeled a and b on curves 2 and 3 would show the difference in the physical processes causing the identical current densities to be measured. In the case of curve 2, the surface would have undergone uniform dissolution in some cases, the dissolution is so slight that it is imperceptible without magnification. In the case of curve 3, the... 

This is the place to start, since most often, analytical chemists are trying to help solve someone else s problem. We need to define the solute and its matrix as well as the nature of the analytical problem. For example, in the world of pharmaceuticals, there are raw material identification and purity determinations, in-process testing, dosage-form determinations, content uniformity, dissolution testing, stability studies, bioavailability, pharmacokinetics, and drug metabolism, to name a few. Each of these analytical problems has its own specific requirements. The matrix can be a raw material, granulation, tablet, capsule, solution, lotion, cream, syrup, dissolution medium, blood serum, urine, or various body tissues and fluids. Similar definitions can be described for virtually any industrial area and problem set. These definitions will help select sample preparation, separation, and detection techniques. 

Quantitative tests for the assay of major components (e.g., drug substance and preservatives) in samples of drug substance or drug product (assay, content uniformity, dissolution rate, etc.)... 

Identity Weight Percent/Assay/Content Uniformity/Dissolution Impurity Testing Quantitative TesP... 

Other finished product attributes (e.g., content uniformity, dissolution)... 

Although most rotary tablet presses operate by maintaining fixed roller positions during compression, some designs incorporate a compression compensator system in which the counterforce for compression is air pressure. This system compresses to a constant force and allows roller movement when the preset force is achieved. Under these conditions, potential exists to increase the time that the force is maintained near its peak value (approximately 90% of maximum). Compression to a constant force should theoretically provide a more uniform tablet hardness and more uniform dissolution profiles while allowing a greater variation in tablet thickness. 

A certain amount of uniform dissolution may occur prior to and during the initiation of pores. For example, as reported in one study, before the formation of the macropores on lowly doped p-Si( 100) in anhydrous HF-MeCN solutions the entire surface is etched forming (111) facets of about l xm. Pores then start to grow at the base of these facets. Once macropores are developed the surface etch rate is greatly reduced, by a factor of 4. 

The smaller the size, the larger is the interfacial area between the solid and liquid and, therefore, the higher is the rate of transfer of material moreover, the shorter is the distance the solute must diffuse with the solid. Nevertheless, fine particles may impede the subsequent particle-liquid separation and liquid circulation. To achieve a uniform dissolution and facilitate the separation, it is generally desirable to choose particles of a narrow size range. 

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