Solid occlusion

Occlusions, which are a second type of coprecipitated impurity, occur when physically adsorbed interfering ions become trapped within the growing precipitate. Occlusions form in two ways. The most common mechanism occurs when physically adsorbed ions are surrounded by additional precipitate before they can be desorbed or displaced (see Figure 8.4a). In this case the precipitate s mass is always greater than expected. Occlusions also form when rapid precipitation traps a pocket of solution within the growing precipitate (Figure 8.4b). Since the trapped solution contains dissolved solids, the precipitate s mass normally increases. The mass of the precipitate may be less than expected, however, if the occluded material consists primarily of the analyte in a lower-molecular-weight form from that of the precipitate. 

Inclusions, occlusions, and surface adsorbates are called coprecipitates because they represent soluble species that are brought into solid form along with the desired precipitate. Another source of impurities occurs when other species in solution precipitate under the conditions of the analysis. Solution conditions necessary to minimize the solubility of a desired precipitate may lead to the formation of an additional precipitate that interferes in the analysis. For example, the precipitation of nickel dimethylgloxime requires a plT that is slightly basic. Under these conditions, however, any Fe + that might be present precipitates as Fe(01T)3. Finally, since most precipitants are not selective toward a single analyte, there is always a risk that the precipitant will react, sequentially, with more than one species. 

Bound moisture This is moisture retained within the solid such that it exerts a vapour pressure less than that of free solvent (Figure 4.24). Such solvent may be adsorbed on the surface, retained in capillaries or within cells or occlusions of liquor. The latter can be difficult to remove without resorting to high temperatures, which may damage the crystals. 

There are several techniques available for mechanical thrombolysis. The most common is probing the thrombus with a microguidewire. This technique appears to be useful in facilitating chemical thrombolysis. Alternatively, a snare (e.g., Amplatz Goose-Neck Microsnare, Microvena, White Bear Lake, MN) can be used for multiple passes through the occlusion to disrupt the thrombus. " A snare can also be used for clot retrieval, mostly in simations in which the clot has a firm consistency or contains solid material. ... 

Induced precipitation is a collective name for processes accompanying the formation of solid phase, such as occlusion, adsorption, compound formation, formation of isomorphous mixtures, mixed crystals, colloidal solutions, etc. In... 

B. The physical nature of solids must be carefully considered both before testing and in interpreting results. Shape (sharp edges), size (small particles may abrade the skin by being rubbed back and forth under the occlusive wrap), and rigidity (stiff fibers or very hard particles will be physically irritating) of solids may all enhance an irritation response. 

These schemes have been frequently suggested [105-107] as possible mechanisms to achieve the chirally pure starting point for prebiotic molecular evolution toward our present homochiral biopolymers. Demonstrably successftd amplification mechanisms are the spontaneous resolution of enantiomeric mixtures under race-mizing conditions, [509 lattice-controlled solid-state asymmetric reactions, [108] and other autocatalytic processes. [103, 104] Other experimentally successful mechanisms that have been proposed for chirality amplification are those involving kinetic resolutions [109] enantioselective occlusions of enantiomers on opposite crystal faces, [110] and lyotropic liquid crystals. [Ill] These systems are interesting in themselves but are not of direct prebiotic relevance because of their limited scope and the specialized experimental conditions needed for their implementation. 

Jenning, V., et al., Vitamin A loaded solid lipid nanoparticles for topical use occlusive properties and drug targeting to the upper skin. Eur. J. Pharm. Biopharm., 49, 211-18, 2000. 

Oil is to be leached from granulated halibut livers with pure ether as solvent. Content of oil in the feed is 0.321b/lb dry (oil-free) solids and 95% is to be recovered. The economic upper limit to extract concentration is 70% oil. Ravenscroft [Jnd. Eng. Chem. 28, 851 (1934)] measured the relation between the concentration of oil in the solution, y, and the entrainment or occlusion of solution by the solid phase, K lb solution/lb dry solid, which is represented by the equation... 

Equilibrium measurements of the solid hydrate phase have been previously avoided due to experimental difficulties such as water occlusion, solid phase inhomogeneity, and measurements of solid phase concentrations. Instead, researchers have traditionally measured fluid phase properties (i.e., pressure, temperature, gas phase composition, and aqueous inhibitor concentrations) and predicted hydrate formation conditions of the solid phase using a modified van der Waals and Platteeuw (1959) theory, specified in Chapter 5. 

SrC03, as well as occlusions or solid solutions in the SrO. Contamination of the crucible during the heat cycle should also be included. Such small contamination errors are estimated to add up to not more than a contributing relative standard uncertainty of 2x10 4. 

In his monograph (9), Rabo reported the first studies of solid-state reactions between zeolites, mainly Y zeolite, and some salts. These studies revealed either ion exchange or more or less reversible occlusion of the salt. In cases of occlusion, the salt anion (halide, nitrate, or oxygenated chlorine anions) was usually located in sodalite cavities. 

A dose of 0.5 mL of liquid or 0.5 g of solid or semisolid test chemical is applied to the site. Separate animals are not required for an untreated control group, since adjacent areas of untreated skin serve as controls for the test. The test chemical should be applied to a small area (approximately 6 cm2) of skin and covered with a gauze patch, held in place with nonirritating tape. In the case of liquids or some pastes, it may be necessary to apply the test chemical to the guaze patch and apply that to the skin. The patch should be loosely held in contact with the skin by means of a suitable occlusive dressing for the duration of the exposure period. The animals access to the patch and resultant ingestion of the test chemical by licking or by inhalation should be prevented. 

Cutaneous exposure may be selected to simulate the main route of human exposure and as a model for induction of skin lesions. During skin absorption, the chemical applied is transferred from the outer surface of the skin through the homy layer, the epidermis, the comeum, and into the systemic circulation. Absorption of chemicals through the skin is time-dependent, and this can be demonstrated by the application of occlusive bandages to prevent loss of the test material from the application site. Solid chemicals and chemicals soluble in secretions of the skin may dissolve in the secretions to a variable extent. 

Endothermic occlusion takes place by diffusion of hydrogen into a metal lattice which is very little changed by the process. In exothermic occlusion by palladium, however, the face-centred cubic lattice (a phase) of palladium, with lattice constant 3.88 A, will accomodate, below 100°C, no more than about 5 at. % hydrogen, and then undergoes a transition to an expanded phase ( 3 phase), with lattice constant 4.02 A and H/Pd = 0.5—0.6. The H—Pd system thus splits into a and 3 phases in the manner familiar for two partially miscible liquids. The consolute temperature (rarely observable for solid phases) is about 310°C at H/Pd = 0.22. The phase diagram is, however, not well established because formation of the... 

Berkeley Scarifier as described by Haley et al. (1974). Materials are tested undiluted, and 0.5 ml liquid, or 0.5 g solid or semisolid material is applied. Each test site is covered with two layers of 1-in square surgical gauze secured in place with tape. The entire trunk of the animal is then wrapped with rubberized cloth or other occlusive impervious material to retard evaporation of the substances and hold the patches in one position. The wrappings are removed 24 h after application and the test sites are evaluated for erythema and edema using a prescribed scale. Evaluations of abraded and intact sites are recorded separately. Test sites are evaluated again 48 h later (72 h after application) using the same scale. 

The test article is administered as solution or suspension to the back of six albino rabbits. The test material is administered to two sites onto the rabbits skin where the skin has to be clipped. On one site the skin remains intact, on the other the skin is mechanically abraded in order to remove the outer layer of the skin, the stratum comeum. The clipped areas should have a surface of about 1 square inch onto which the test material is administered. The dose level to be administered to the skin is 0,5 ml in case of liquids or 0,5 g in case of solid materials. The treated skin areas are covered with surgical gaze and fixed with a non-irritation tape. A second layer consisting of an occlusive material is wrapped over the entire trunk of the rabbits in order to avoid or retard the evaporation of the test material from the treated skin areas. After 24 hours, the wrapping is removed from the animals and the treated skin sites are evaluated for erythema and oedema, the classical signs of inflammation. 

---