Microcrystalline form

A variation of this procedure is simply to precipitate the material in a microcrystalline form from solution in one solvent at room temperature, by adding a little more of the second solvent, filtering off the crystals, adding a little more of the second solvent and repeating the process. This ensures, at least in the first or last precipitation, a material which contains as little as possible of the impurities, which may also be precipitated in this way. With salts, the first solvent is commonly water, and the second solvent is alcohol or acetone. 

Crystalline free silica (Si02, including microcrystalline forms)... 

A microcrystalline form of silica. An impure form of flint used in abrasives. 

A microcrystalline form of native quartz, more opaque and granular than chalcedony. Used as an abrasive and in ceramics. 

A 1-liter flask is equipped with a magnetic stirrer, a thermometer immersed in the reaction mixture, and a drying tube. In the flask is placed 100 ml of anhydrous pyridine, and the flask is cooled in an ice-water bath to 15-20° (lower temperatures impede the complex formation). Chromium trioxide (80 g) is added in small portions to the stirred solvent at a rate so as to keep the temperature below 30°. After about one-third of the chromium trioxide has been added, the yellow complex begins to precipitate. At the end of the addition (about 1 hour), a slurry of the yellow complex in pyridine remains. (This form of the complex is apparently a microcrystalline form and is very difficult to handle.)... 

Pure elemental silicon is a hard, dark gray solid with a metallic luster and with a crystalline structure the same as that of the diamond form of carbon. For this reason, silicon shows many chemical and physical similarities. There is also a brown, powdery form of silicon having a microcrystalline form. The element is prepared commercially by reducing the oxide by reacting it with carbon (as coke) in electric furnaces. On a small scale, silicon has been obtained from the oxide by reduction with aluminum meted. 

The effect of particle size reduction on the bioavailability of nitrofurantoin was shown in Fig. 4. The microcrystalline form (< 10 pm) is more rapidly and completely absorbed from the tablet dosage form than is the macrocrystalline form (74-177 pm) from the capsule dosage form. This is not a completely satisfactory illustration of the effect of particle size on the rate and extent of availability, since other manufacturing variables have not been held constant. Nevertheless, it does suggest some correlation between particle size, dissolution rate, and rate of availability. 

While the cuprous cyanide solution is warmed gently (to 60°-70°) on the water bath, a solution of p-tolyldiazonium chloride is prepared as follows Heat 20 g. of p-toluidine with a mixture of 50 g. of concentrated hydrochloric acid and 150 c.c. of water until dissolution is complete. Immerse the solution in ice-water and stir vigorously with a glass rod so that the toluidine hydrochloride separates as far as possible in a microcrystalline form. Then cool the mixture in ice and diazotise with a solution of 16 g. of sodium nitrite in 80 c.c. of water, added until the nitrous acid test with potassium iodide-starch paper persists. The diazonium chloride solution so obtained is poured during the course of about ten minutes into the warm cuprous cyanide solution, which is meanwhile shaken frequently. After the diazo-solution has been added the reaction mixture is heated under an air condenser on the water bath fox a further quarter of an hour, and then the toluic nitrile is separated by distillation with steam (fume chamber, HCN ). The nitrile (which passes over as a yellowish oil) is extracted from the distillate with ether, the p-cresol produced as a by-product is removed by shaking the ethereal extract twice with 2 A-sodium hydroxide solution, the ether is evaporated,... 

Lemberger, A.P., Higuchi T., Busse, L.W., Swintowsky, J.V., and Wurster, D.E., Preparation of some steroids in microcrystalline form by rapid freeze-sublimation technique, /. Am. Pharm. 

The same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards bond formation between cations and anions has been shown by precise determinations of the electron density distribution (/)]. Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp hybridization of silicon in silica. Here, too, transitions from crystalline to amorphous do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. 

The many forms of so-called amorphous (non-crystalline) carbon such as charcoals and lampblack are all actually microcrystalline forms of graphite. The latter has a covalently bonded layer structure comprising a network of joined flat hexagonal Ce rings where the separation of the layers is reported to be 3.35A. This is about equal to the sum of the Van der Waals (intermolecular) radii, indicating that the forces between layers should be relatively slight, as is evidenced by the observed softness and lubricity of the material. 

Mechanism of Action An antibacterial UTI agent that inhibits the synthesis of bacterial DNA, RNA, proteins, and cell walls by altering or inactivating ribosomal proteins. Therapeutic Effect Bacteriostatic (bactericidal at high concentrations). Pharmacokinetics Microcrystalline form rapidly and completely absorbed macrocrystalline form more slowly absorbed. Food increases absorption. Protein binding 40%. Primarily concentrated in urine and kidneys. Metabolized in most body tissues. Primarily excreted in urine. Removed by hemodialysis. Half-life 20-60 min. 

Improvements in theophylline preparations have come from alterations in the physical state of the drugs rather than from new chemical formulations. For example, the increased surface area of anhydrous theophylline in a microcrystalline form facilitates solubilization for complete and rapid absorption after oral administration. Numerous sustained-release preparations (see Preparations Available) are available and can produce therapeutic blood levels for 12 hours or more. These preparations offer the advantages of less frequent drug administration, less fluctuation of theophylline blood levels, and, in many cases, more effective treatment of nocturnal bronchospasm. 

Magnetic susceptibilities of 10a and 10b were measured on a SQUID suscep-tometer in microcrystalline form. %T-T plots are shown in Fig. 9.5. The data were analyzed in terms of a modified singlet-triplet two-spin model (the Blea-ney-Bowers-type), in which two spins (S = V2) couple antiferromagnetically within a biradical molecule by exchange interaction J. The best-fit parameters obtained by means of a least-squares method were 2J/kB = -2.2 + 0.04 K for 10a and -11.6 + 0.4 K for 10b. Although the interaction (2J/kB = -2.2 K) between the two spins in the open-ring isomer 10a was weak, the spins of 10b showed a remarkable antiferromagnetic interaction (2J/kB = -11.6 K). 

Amorphous carbon refers to charcoal, soot, coal, and carbon black. These materials are mostly microcrystalline forms of graphite. They are characterized by small particle sizes and large surface areas with partially saturated valences. These small particles readily absorb gases and solutes from solution, and they form strong, stable dispersions in polymers, such as the dispersion of carbon black in tires. 

Mixed RRR-Tocopherols Concentrate occurs as a brown-red to light yellow, clear, viscous oil. It may show a slight separation of waxlike constituents in microcrystalline form. It oxidizes and darkens slowly in air and on exposure to light, particularly when in alkaline media. It is insoluble in water soluble in alcohol and miscible with acetone, with chloroform, with ether, and with vegetable oils. 

Prandtl and Mohr15 probably were the first to prepare rare earth hexacyanofer-rates Ln[Fe(CN)6] H20 in microcrystalline form. Some of these cyanides... 

Zirconium hydroxide is precipitated by bases at lower pH than the hafnium compound. Zr and Hf are obviously unable to form true hydroxides, and these compounds are more correctly formulated as MO2 XH2O. Amorphous hydrous zirconia and hafhia (a-phase) transform to microcrystalline forms (/f-phase) with noticeable heat evolution. They lose water up to the composition MO2 H2O at 140 °C (Zr) or 155 °C (Hf). Hydrous zirconia has excellent absorptive capacity, particularly for oxygen-containing anions. For example, the concentration of S04 anions over hydrous zirconia is so low that no precipitate forms on the addition of barium salts to the filtrate. While the hydroxides of composition M(OH)4 are not stable, in alkaline solutions, M(OH)s are present and even M(0H)6 anions have been reported in very concentrated alkalis. Salts of these anions, such as Na2Hf(OH)6, can be isolated. 

If the acid is added too rapidly, the resulting precipitate is difficult to filter, but, if the acid is added slowly, the diphenylphosphinic acid precipitates in a filterable microcrystalline form. After acidification, the mixture is cooled, the solid is recovered by filtration, washed thoroughly with dilute hydrochloric acid (10 ml. of concentrated hydrochloric acid per liter), and then with cold water. The wet filter cake is then washed with 250 ml. of benzene to remove the biphenyl which occurs as a by-product. After drying at 110°, the crude product, off-white in color, is obtained in a yield of 90 to 96%. Recrystallization from hot 95% ethanol produces a white crystalline product. Anal. Calcd. for (C6H5)2P(0)0H C, 66.05 H, 5.08 P, 14.20. Found C, 66.02 0.10 H, 5.08 0.23 P, 14.16 0.23. Calculated formula weight 218.19. Neutral equivalent determined by titration with standard base in methanol 219.2 1.4. 

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