Crystallization, solvent choice

Solvent choice is also governed by another important factor, the ease of solvent removal. Solvents with low boiling points are preferred because their removal is easy. A third consideration in selecting a solvent is the temperature at which the solvent solidifies. Benzene was once widely used in recrystallization, but when placed in an ice bath, crystals of benzene would also precipitate (benzene crystallizes at 6 Celsius). A final consideration in choosing a solvent is reactivity. Obviously a solvent that reacts with a solid cannot be used for recrystallization. 

In EPR spectroscopy, it is possible to measure spectra of paramagnetic samples in a variety of forms, including fluid solution, frozen solution, powdered solid or single crystal. Glearly, for heterogeneous polycrystalline systems, such as oxides, the problems of solvent choice, lossy samples, poor quality glass conditions when... 

Clearly the priorities for solvent choices change with each reaction. Increasing the reaction rate may be less important than the ease of work-up and isolation operations. The best solvent for a reaction may be the one from which the product can be crystallized directly [23],... 

A third choice is also possible for use as a crystallization solvent. This is mineral spirits available from hardware stores in the paint department. Mineral spirits are roughly equivalent to the petroleum ether or ligroin commonly seen in chem labs. Those brands which boast of low odor are the best choice. Before using this material it is best to fractionally distill it, and collect the lowest boiling point half of the product. This speeds crystal drying. Since the choice of mineral spirits eliminates ether from the supply loop, the clandestine operator may well go this route. Toluene is also an acceptable solvent. 

Table 2 Summary of the Effects of Vessel Temperature, Vessel Pressure, and Solvent Choice on Crystal Form Produced...

An understanding of the solubility of polysulfones is important for applications in which the polymer must be dissolved, such as in coatings and membrane applications. Solubility of the three commercial polysulfones follows the order PSF > PES > PPSE All three polysulfones can be dissolved in a small number of highly polar solvents to form stable solutions at room temperature. NMP, DMAc, pyridine, and aniline are suitable solvents for polysulfones. Also 1,1,2-trichlorethane and 1,1,2,2-tetrachloroethane are suitable but are unattractive for health reasons. Because of the lower solubility parameter of PSF, it can also be dissolved in several less polar solvents such as tetrahydrofuran (THF), 1,4-dioxane, chloroform, dichloromethane, and chlorobenzene. Solvent choices for PES and PPSF are fewer because these polymers have a propensity to undergo solvent-induced crystallization in many solvents. 

Finally, as the depletion behavior is molecular recognition by the crystal lattice and more specifically by the grovfing surface, and as the solvent can influence the solid-state form crystallizing (being it either different polymorphs or solvates) as well as the habit of the crystals, the choice of the solvent is of importance. 

Chromium Removal System. Chlorate manufacturers must remove chromium from the chlorate solution as a result of environmental regulations. During crystallization of sodium chlorate, essentially all of the sodium dichromate is recycled back to the electrolyzer. Alternatively, hexavalent chromium, Cr, can be reduced and coprecipitated in an agitated reactor using a choice of reducing agents, eg, sodium sulfide, sulfite, thiosulfate, hydrosulfite, hydrazine, etc. The product is chromium(III) oxide [1333-82-0] (98—106). Ion exchange and solvent extraction techniques have also... 

Polyarylether Ketones. The aromatic polyether ketones are tme thermoplastics. Although several are commercially available, two resins in particular, poly ether ether ketone [31694-16-3] (PEEK) from ICI and poly ether ketone ketone (PEKK) from Du Pont, have received most of the attention. PEEK was first synthesized in 1981 (20) and has been well studied it is the subject of numerous papers because of its potential use in high performance aircraft. Tough, semicrystalline PEEK is prepared by the condensation of bis(4-fiuorophenyl) ketone with the potassium salt of bis(4-hydroxyphenyl) ketone in a diaryl sulfone solvent, such as diphenyl sulfone. The choice of solvent is critical other solvents, such as Hquid HE, promote the reaction but lead to premature low molecular-weight crystals, which do not exhibit sufficient toughness (21). 

---