Selective crystal dissolution

Selective Crystal Dissolution. An effective method for distinguishing bulk (lattice trapped) versus surface impurities involves the selective dissolution of a crystal sample while testing the liquid and/or crystalline phases for relative purity. In this technique, a small sample of crystals of a narrow size fraction is washed with successive small amounts of clean solvent until most of the crystalline phase is dissolved. The filtrate and/or crystalline phase are analyzed after each washing to discern whether impurities reside predominantly at the surface, or are more evenly distributed throughout the crystalline phase. The general approach is described by Narang and Sherwood (1978) for quantifying caproic acid incorporation in adipic crystals, and by Addadi et al. (1982) for amino acid separations. 

Ng, K.M., 1991. Systematic separation of a multicomponent mixture of solids based on selective crystallization and dissolution. Separations Technology, 1, 108. 

Analysis of the volumetric effects indicates that as a result of such mechanical activation, iron and manganese are concentrated in the extended part of the crystal, while tantalum and niobium are predominantly collected in the compressed part of the distorted crystal structure. It is interesting to note that this effect is more pronounced in the case of tantalite than it is for columbite, due to the higher rigidity of the former. Akimov and Chernyak [452] concluded that the effect of redistribution of the ions might cause the selective predominant dissolution of iron and manganese during the interaction with sulfuric acid and other acids. 

The existence of two polymorphs was reported for a NO-releasing derivative of acetyl-salicylic acid [28]. Selection crystallization of one form or the other was achieved from a number of solvent systems (14 solvents and 3 preparative methods), but several systems were identified that yielded mixtures of the two forms. The single-crystal structure of Form I was reported, but the habit of the Form II crystals precluded their characterization. The transition point of the two forms was calculated from intrinsic dissolution data to be higher than the melting points of both polymorphs and thus the two forms bear a monotropic relationship. 

EDS study at location B, at the bottom of a pit showed that location was mainly composed of Cu and Ni with a small amount of Fe, which could be attributed to contamination since Fe was not detected in some other pits. The EDS result indicates no denickelification inside the pit since both Ni and Cu were found, and the crystals appear to be compact with no evidence of any copper crystal deposit or selective nickel dissolution leaving a porous structure. It should also be noted that there was no corrosion product at the bottom of the pit, and there was clear evidence of copper redeposit at the edge of the pit, as indicated in EDS of the copper and oxygen peaks. 

Pure sodium tetraborates are produced from crushed raw sodium borate minerals (tincal, kernite) by dissolution with heating in a weak borax-containing mother liquor, separating off the impurities (clays) and selective crystallization. Either the penta- or deca-hydrate is formed during vacuum crystallization, depending upon the temperature (above or below 60.8°C). 

The contour plots from 2D-LC differ from those obtained by TREF-SEC because the separation principles are different. While TREE is based on crystallization-dissolution, interactive HPLC is based on the selective adsorption and desorption of the macromolecules. As a result, high-temperature 2D-LC enables selective separation of both semi-crystalline and amorphous polyolefins, whereas TREF-SEC cannot distinguish amorphous components. 

Porosity through thin dielectric films on metallic substrates may be measured by corrosion (liquid gas), selective chemical dissolution (electrographic printing - solution analysis), electrochemical decoration, anodic current measurement, gas bubble generation (electrolytic), liquid crystal (electric field) effects, and absorption (dyes - liquid or gaseous radioactive material). 

Selection of the most suitable chemical form of the active principle for a tablet, while not strictly within our terms of reference here, must be considered. For example, some chloramphenicol esters produce little clinical response [13], There is also a significant difference in the bioavailability of anhydrous and hydrated forms of ampicillin [14], Furthermore, different polymorphic forms, and even crystal habits, may have a pronounced influence on the bioavailability of some drugs due to the different dissolution rates they exhibit. Such changes can also give rise to manufacturing problems. Polymorphism is, of course, not restricted to active ingredients, as shown, for example, in an evaluation of the tableting characteristics of five forms or sorbitol [15]. 

Films deposited from mixed Cd/Pb solutions complexed with ammonia (for Cd) and hydroxide (for Pb), both in a minimum amount to effect dissolution, at pH values between 10 and 13 and deposition temperatures between 60 and 80°C were concluded, from consideration of the XRD, TEM (which showed two different crystal sizes), and optical spectra, to be mixtures of the two sulphides rather than a solid solution [21]. A study of these films for solar-selective surfaces was carried out. 

The bisphosphonates are analogs of pyrophosphate in which the P-O-P bond has been replaced with a nonhydrolyzable P-C-P bond (Figure 42-4). Etidronate, pamidronate, and alendronate have now been joined by risedronate, tiludronate, ibandronate, and zoledronate for clinical use. The bisphosphonates owe at least part of their clinical usefulness and toxicity to their ability to retard formation and dissolution of hydroxyapatite crystals within and outside the skeletal system. They localize to regions of bone resorption and so exert their greatest effects on osteoclasts. However, the exact mechanism by which they selectively inhibit bone resorption is not clear. 

POMs form by a self-assembly process, typically in an acidic aqueous solution and can be isolated as powder or crystals with counter-cations. Appropriate selection of counter-cations can control the solubility of POMs in various reaction media. For homogeneous system, alkylammonium cations, generally TBA, are selected as counter-cations of POM anions for the dissolution in organic solvents such as acetonitrile, DMF, DMSO and 1,2-dichloroethane. POMs with metal counter-cations such asNa +, K+,Rb+,Cs + and Ag + are not soluble in common organic solvents. 

Dissolution of a drug substance is controlled by several physicochemical properties, including solubility, surface area, and wetting properties. For insoluble compounds, dissolution is often the rate-limiting step in the absorption process. Knowledge ofthe dissolution rate of a drug substance is therefore very useful for formulation development. The appropriate dissolution experiments can help to identify factors that contribute to bioavailability problems, and also assist in the selection of the appropriate crystal form and/or salt form. Dissolution tests are also used for other purposes such as quality control and assisting with the determination of bioequivalence (Dressman et al., 1998). 

---