Crystallization from solution specifications

Usually, synthetic polymers crystallize11 j15 from a melt or a solution in form of folded lamellae. Under specific circumstances it is sometimes also possible to obtain extended chain crystals which is the preferred arrangement in the crystallites of many natural polymers (cellulose, silk). Recently it has been found33 31 that in some cases another crystalline modification can be obtained, the so-called shish-kebabs, which are a sort of hybrid between folded lamellae and extended chain crystals. These shish-kebabs are obtained by shear-induced crystallization, a process in which the polymer crystallizes from solution under the influence of an elongated flow. 

Crystallization is the most commonly cited mechanism for junction-zone formation in polymer gel networks. A distinction must be made between crystallization from solution resulting directly in network formation and crystallization which occurs subsequently to gelation. This distinction is not always apparent in the reports of crystallization and gelation. For example, crystallization may occur in an already phase-separated network or it may involve polymer molecules not included in the network structure. Another possibility is that a specific interaction or phase separation results in the formation of a weak network but that this network is consolidated by subsequent crystallization. Here the relative importance of the various mechanisms becomes unclear. These form the grounds for the current debate on the relative importance of crystallization and phase separation in gelation. In the examples which follow crystallization has been favoured as the primary mechanism. 

Recentiy, an irmovative chiral resolution using enantio-specific co-crystallization from solution was reported by Springuel and Leyssens." In this new chiral resolution... 

Both mono- and disodium phosphates are prepared commercially by neutralization of phosphoric acid using sodium carbonate or hydroxide. Crystals of a specific hydrate can then be obtained by evaporation of the resultant solution within the temperature range over which the hydrate is stable. For the preparation of trisodium phosphate, sodium hydroxide must be used to reach the high end pH because CO2 cannot be stripped readily from the solution above a pH of near 8. 

Citric acid, anhydrous, crystallizes from hot aqueous solutions as colorless translucent crystals or white crystalline powder. Its crystal form is monoclinic holohedra. Citric acid is dehquescent in moist air. Some physical properties are given in Table 1 (1 3). The solubiUty of citric acid in water and some organic solvents is given in Table 2. The pH and specific gravity of aqueous solutions of citric acid are shown in Table 3. 

Sodium chloride and sodium cyanide are isomorphous and form an unintermpted series of mixed crystals. The ferrocyanide ion has a marked effect on the habit of sodium cyanide crystallized from aqueous solution (50). Sodium cyanide and sodium carbonate form a molten eutectic at approximately 53 wt % sodium carbonate and 465°C. The specific conductivity of molten 98% sodium cyanide is 1.17 S /cm (51). 

Working carefully with tweezers, Pasteur was able to separate the crystal into two piles, one of "right-handed" crystals and one of "left-handed" crys tals like those shown in Figure 9.6. Although the original sample, a 50 50 mix lure of right and left, was optically inactive, solutions of the crystals from eacl of the sorted piles were optically active, and their specific rotations were equa in amount but opposite in sign. 

Both DDT and chlordan are used in colorless, odorless, deobase-type solvent DDT in 5% solution and chlordan in 2% solution. The oil solvent is used because it is a nonconductor and because experience has shown that the crystals from a film of oil solvent solution adhere more firmly to the surface sprayed. A pressure-type spray tank, either a hand pump or mechanical source of air pressure, is used, with a special nozzle which gives a fan-shaped nonmisting spray. A special dripless valve is used. Several valves and nozzles which meet these requirements are on the market. The appropriate areas are sprayed with this fan-shaped painting spray stream, so that the surface glistens with the wet film but there is not sufficient quantity to run down. Experience has shown that this will leave approximately 200 mg. of DDT per square foot or an equivalent amount of chlordan. This procedure is recommended by the U. S. Public Health Service in its spraying technique for residences and food establishments treated in its program of spraying DDT only for malaria control. It has not as yet recommended chlordan for this use. Specifications for the sprayer nozzles procedure can be obtained from this source. 

Another cultured cell line of Catharanthus roseus (EU4A), which does not produce detectable amounts of vinblastine and other bisindole alkaloids, was also examined for its ability to transform 78 (183). Cell-free extracts of the culture line were prepared, and the 35,000 X g supernatant solution was used. Incubations with 2r-tritioanhydiovinblastine yielded a mixture from which radioactive vinblastine (52) was isolated. The labeled vinblastine was reisolated after unlabeled carrier was added and rigorously purified by successive thin-layer chromatography, reversed-phase HPLC, and crystallization to constant specific activity. Boiled extracts could not produce labeled 52, thus supporting the involvement of enzymes in the conversion process. 

Ciystallization from solution is an important separation and purification process in a wide variety of industries. These range from basic materials such as sucrose, sodium chloride and fertilizer chemicals to pharmaceuticals, catalysts and specialty chemicals. The major purpose of crystallization processes is the production of a pure product. In practice however, a number of additional product specifications are often made. They may include such properties as the ciystd size distribution (or average size), bulk density, filterability, slurry viscosity, and dry solids flow properties. These properties depend on the crystal size distribution and crystal shape. The goal of crystallization research therefore, is to develop theories and techniques to allow control of purity, size distribution and shape of crystals. 

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