Contact recrystallization

It is no wonder that the particles are spherical but crystalline, if one considers the formation mechanism. The rather smooth surface of the spherical magnetite may be due to the rapid contact recrystallization of the constituent primary particles (5), forming the rigid polycrystalline structure. Flowever, it must be noted that polycrystalline spheres are also prepared by normal deposition of monomeric solute, as shown in the formation of the uniform spherical polycrystalline particles of metal sulfides in Chapters 3.1-3.3. Thus, while we may be able to predict the final particle shape and structure from the formation mechanism, it is risky to conclude the formation mechanism only from characterization of the product. As a rule, scrupulous analyses are needed for concluding the growth mechanism in a particle system. 

Synthetic Fe oxides are usually in the micron or submicron size range. Ultrafine particles with dimensions in the nanometer range frequently require more specialized techniques than do the larger crystals. The two basic requirements for a monodisperse system must be met and in addition, coagulation and/or contact recrystallization of the primary particles must be prevented. 

Tungsten has Htde effect on recrystallization temperature or the high temperature properties of molybdenum. However, the Mo—30% W alloy is recognized as a standard commercial alloy for stirrers, pipes, and other equipment that is required to be in contact with molten zinc during processing of the metal and in galvanizing and die casting operations. 

A solution of 1-piperazino ethyl acetate (Q2 mol) in benzene (300 ml) is treated with 3,4.5-trimethoxy cinnamoyl chloride (0,2 mol) in the presence of sodium bicarbonate (0.3 mol). After contacting for one hour at room temperature, the mixture is refluxed for a further hour. The benzene solution is then treated with an aqueous solution of sodium bicarbonate. After evaporation of the solvent, a solid product is obtained which is recrystallized from isopropyl ether. Melting point = 96°C. This base, when treated with hydrochloric acid, gives a hydrochloride having a melting point of 200°C with decomposition. By the action of malaic acid the acid maleate is obtained, having a melting point of 130°C. 

The dominant mechanism of purification for column crystallization of solid-solution systems is recrystallization. The rate of mass transfer resulting from recrystallization is related to the concentrations of the solid phase and free liquid which are in intimate contact. A model based on height-of-transfer-unit (HTU) concepts representing the composition profile in the purification section for the high-melting component of a binary solid-solution system has been reported by Powers et al. (in Zief and Wilcox, op. cit., p. 363) for total-reflux operation. Typical data for the purification of a solid-solution system, azobenzene-stilbene, are shown in Fig. 20-10. The column crystallizer was operated at total reflux. The solid line through the data was com-putecfby Powers et al. (op. cit., p. 364) by using an experimental HTU value of 3.3 cm. 

To a stirred suspension of l.Og of the above dye in some 50ml of warm acetone was added excess zinc dust and a few drops of concentrated hydrochloric acid. The mixture was stirred until the coloration discharged, and then filtered. The cake was extracted with hot acetone. The filtrate and extracts were combined, concentrated, and poured into water. The precipitate was filtered, washed repeatedly with water and cold methanol, and dried. Recrystallization from methanol gave 0.9g (90%) of the leuco dye 3-(4-carbethoxyanilino)phenothiazine as a white powder which gradually turned pinkish on contact with air. 

A solution of 10.5 g. (0.046 mol) of freshly distilled bis(tri-fluoromethyl)-l,2-dithiete (Note 2) in 200 ml. of n-pentane is cooled to —10° in a 1-1. round-bottomed flask equipped with an efficient reflux condenser and protected from moist air by a dry nitrogen blanket. A solution of 3.0 ml. (0.023 mol) of nickel carbonyl dissolved in 100 ml. of w-pentane is added down the condenser in one portion to this solution. The mixture is swirled to mix. An intense blue-violet color develops in about 15 to 20 seconds and after 1 to 2 minutes, vigorous evolution of carbon monoxide occurs. This evolution subsides in 10 minutes and the deep violet solution is allowed to warm to 0° during 2 hours to ensure complete reaction. Most of the pentane is removed by distillation at atmospheric pressure, the remaining 50 to 60 ml. is removed in vacuo (0.1 mm.), and the resultant crystalline mass is evacuated (0.1 mm.) at 50° for 4 hours. The crude product consists of shiny black-purple needles and weighs 11.8 g. (98%). Recrystallization from dry benzene (Note 3) gives shiny black crystals, m.p. 134 to 135° (sealed tube). The complex is air-stable but should be kept out of contact with moist air. 

With the long alkyl chain substitutions on the A-heterocyclic carbenes, lamella-structured silver(i) carbene complexes 27a and 27b (Figure 14) were isolated.74 It is interesting to note that the synthetic procedures for the two complexes are the same except for the use of different solvents of crystallization. The dinuclear 27a was obtained from recrystallization in dichloromethane- -hexane while the tetranuclear 27b was obtained from acetone. The structure of 27a could be interpreted as the dimeric form of [Ag(carbene)Br] bridged by intermolecular Ag-Br interactions. The Ag-G bond has a distance of 2.094(5) A. The tetranuclear 27b, on the other hand, could be regarded as two monocationic bis(carbene)silver(i) bridged by an [Ag2Br4]2 anion, with the presence of short Ag(cationic)-Ag(anionic) contact (3.0038(18) A) and comparable Ag-G bond distances (2.0945(5), 2.138(13) A). A related... 

The compounds are, in general, stable and fairly resistant to hydrolysis in spite of the >POF grouping. We showed, for example, that diphenylphosphorodiamidic fluoride could be recrystallized from aqueous alcohol. Tetramethylphosphoro-diamidic fluoride was not affected to any extent by contact with water at 18° for 6 hr. The reaction between n/2 aqueous sodium hydroxide solution and the compound was studied. The extent of hydrolysis (removal of fluorine) after 30 min. was about 8-9 per cent, and even after 500 hr. it was only 29-9 per cent. The compound is also affected by acid. 

Preparation of the intermediate for Allobarbitone (diethyl diallyl malonate). Diethyl malonate is dissolved in anhydrous alcohol and treated with one mole of clean sodium meted per every one mole of the ester. To this solution add one mole of allyl chloride and reflux for about 4 hours. Another equimolar ratio (1 mole of sodium per mole of ester) of sodium is added, followed by the same ratio of allyl chloride (1 mole per 1 mole), and this mixture is boiled for 2 hours. The alcohol is removed by distillation and the ester is extracted with benzene and distilled or evaporated in vacuo, recrystallized with a suitable "dry" solvent, and filtered. Evaporate again to remove traces of solvent. Keep this product, and any other substances that require dry reagents or solvents, stored away from contact with the atmosphere. When evaporating, filter the air coming into the evaporating vessel with a suitable drying agent. Use a little common sense. 

Caution All 1 -aryl triazenes are toxic and direct hand contact should be avoided. The checkers used hexane for the recrystallization. 

In regional metamorphic rocks and contact metamorphic (metasomatic) rocks, new mineral crystals grow in solid rocks in which there was a change in conditions. However, the process is not the same as straightforward solid state growth or recrystallization. Since volatile components such as H O and CO, which were originally present in the rock, are involved, it is better to assume a... 

Cautionl Because hydrogen cyanide is formed in this reaction, all operations up to the recrystallization of the product should he carried out in a good hood. Contact of tetracyanoethylene with the skin should he avoided. 

Caution. Ammonium acetate is an irritant. Both potassium tetrachloro-platinate and cisplatin are known sensitizing agents and cisplatin is a poison. The HCl solution used for recrystallization of cisplatin is highly toxic and corrosive. Contact with the liquid and vapor should be avoided. Appropriate precautions must be taken, and an efficient hood must be used. 

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