Supersaturated solutions of the

The entire amount of the ammonium chloride-mandelic acid mixture may be boiled with the benzene but this gives a supersaturated solution of the acid in the benzene and much difficulty is met in the filtration. The solubility of mandelic acid in hot benzene is approximately i g. in 50 cc. 

Tubes A and B. which are connected, are filled with a supersaturated solution of the racemate and a single crystal of one enantiomer is placed in tube B. A slow circulation is maintained and A is kept at a slighlly higher temperalurc than B. The crystal growth in B is discontinued after about 10% of the enantiomer has been deposited on the crystal. It is then replaced by a single crystal of the other enantiomer and the process repeated. 

An important group within the ammonium nitrate explosives are emulsion explosives, which consist essentially of a water-in-oil emulsion of an aqueous solution, supersaturated at room temperature, in an oil matrix which is a fuel. The oil phase is the continuous phase and includes small droplets of the supersaturated solution of the oxidizing agent. 

Silver nitrate in ammoniacal solution may be completely reduced to silver by aqueous arsenious oxide. The reduction is hindered by the presence of ammonium sulphate, owing to the decrease in concentration of the hydroxyl ions 5 neutral salts such as sodium sulphate or sodium nitrate have no effect. Similarly, auric chloride may be reduced to gold.6 At 20° C. an aqueous solution of vitreous arsenious oxide reacts 4 to 5 times as rapidly as an aqueous solution of the octahedral form 7 the greater rate of dissolution in water of the former variety has been mentioned (p. 137), but from supersaturated solutions of the two forms there is no appreciable difference in the rates of deposition. The explanation of the inferior reducing power of the crystalline variety may be that there exist anisotropic molecules which only slowly lose their anisotropic properties. An ammoniacal solution of arsenious oxide heated with cupric sulphate in a sealed tube at 100° C. causes reduction... 

TYPICAL PROCESSES OF PREFERENTIAL CRYSTALLIZATION If a conglomerate derivative is found, a moderate supersaturated solution of the racemic modification is prepared. Then, appropriate quantity of the (+) or (-) seed crystal of the optically active compound is inoculated into the solution, and it is left standing or stirred gently to crystallize. If a certain amount of the optically active compound (e.g. 1 g) was crystallized out by the first inoculation, then we can obtain about 2 g each of the optically active compounds after adding 2 g each of the racemic modification and repeating the operation. 

Alexander McPherson and Paul J. Shlichta have suggested using insoluble minerals as heterogeneous nuclei for the crystallization of macromolecules. They obtained excellent protein crystals, which could be cleaved from the mineral nucleus and used for X-ray diffraction studies. The mineral is introduced into a supersaturated solution of the material to be crystallized. As supersaturation increases, nucleation occurs on a specific face of the mineral nucleus, and a crystal begins to grow. The orientation and periodicity of the molecules on the nucleus surface promote an oriented overgrowth that has a similar periodicity. 

In principle, the crystallization of a protein, nucleic acid, or virus (as exemplified in Figure 2.2) is little different than the crystallization of conventional small molecules. Crystallization requires the gradual creation of a supersaturated solution of the macromolecule followed by spontaneous formation of crystal growth centers or nuclei. Once growth has commenced, emphasis shifts to maintenance of virtually invariant conditions so as to sustain continued ordered addition of single molecules, or perhaps ordered aggregates, to surfaces of the developing crystal. 

Color plate 5 Crystallization of sodium acetate from a supersaturated solution (Section 12A-2). A tiny seed crystal is dropped into the center of a petri dish containing a supersaturated solution of the compound. The time sequence of photos taken about once per second shows the growth of the beautiful crystals of sodium acetate. 

Figure 5.1 shows the procedure for preferential crystallization. Mutual seedings of small amounts of crystals of both enantiomers of a racemate to a supersaturated solution of the racemate alternatively give large amounts of the corresponding enantiomers. Thus, the preferential crystalhzation is very simple and therefore a fascinating way to obtain both enantiomers of a racemate. However, preferential crystalhzation is not always apphed to any racemate. 

Moreover, for droplets of diameter d Q = 10 pm and for T = 353.15 K, P = 8 MPa (so for mass transfer coefficient K = 5 x 10 m/s) results in tvap = 0.125 s, which is much larger than the saturation time td- Because the equilibrium concentration of CO2 in the droplet is too small to create a supersaturated solution of the solute, the supersaturation of the nonvolatile solute is created by its concentration due to solvent evaporation from the droplet... 

In a few cases it is possible to crystallize the desired optical isomer by seeding a supersaturated solution of the race-mate with the desired isomer. This has been used with a sulfide ester alcohol intermediate in the synthesis of the drug diltiazem (10.11).2d... 

Supersaturated solutions of the surfactant can be prepared in the usual way by heating a biphasic suspension to a temperature at which all of the surfactant dissolves, and then cooling to the original temperature. They are also generated by mixing saturated or nearly saturated surfactant solution in water with aqueous NaCl solution so that the surfactant solubility is reduced — and the solubility is very sensitive indeed to salinity. In both cases the resulting solution is at first visually clear and transparent and at room temperature remains so for hours or days or, in the most extreme cases, weeks. It may be that nucleation of the partially ordered, liquid crystalline, surfactant-rich phase is slow. 

When a seed crystal of sodium acetate is added to a supersaturated solution of the compound (A), solute begins to crystallize out of solution (B) and continues to do so until the remaining solution is saturated (C). 

A useful technique for the precipitation of relatively insoluble electrolytes is the stopped-flow method (Sohnel and Mullin, 1978b). If two stable solutions, which react to form a supersaturated solution of the reactant, are mixed together instantaneously, no detectable changes occur for some time. However, as soon as the reactant starts to precipitate the concentration of the electrically conductive species begins to decrease and this causes the solution conductivity to diminish. The period of conductivity steadiness is inversely proportional to the supersaturation, and for highly supersaturated solutions it can be less than a millisecond. 

Unfortunately, less than 10% of organic compounds crystallize as a conglomerate (the remainder form racemic crystals) largely denying the possibility of separating enantiomers by simple crystallization techniques - such as by seeding a supersaturated solution of the racemate with crystals of one pure enantiomer. 

Liquid-phase deposition is a method for the non-electrochemical production of polycrystalline ceramic films at low temperatures, along with other aqueous solution methods [chemical bath deposition (CBD), successive ion layer adsorption and reaction (SILAR), and electroless deposition (ED) with catalyst] has been developed as a potential substitute for vapor-phase and chemical-precursor systems. The method involves immersion of a substrate in an aqueous solution containing a precursor species (commonly a fluoro-anion) which hydrolyzes slowly to produce a supersaturated solution of the desired oxide, which then precipitates preferentially on the substrate surface, producing a conformal coating... 

Note that attached, part spherical bubbles are subject to the same Ostwald ripening process as free bubbles, and will therefore disappear in a saturated or even a mildly supersaturated solution of the gas phase, or when the temperature is sufficiently lowered. It is a matter of experimental observation that bubbles that detach from surfaces are generally of a size such that Pe > 1, and bubbles released from surfaces generally rise rapidly through the solution and burst at the free liquid surface. 

Reaction (1) describes the electrochemical oxidation of the arene (Ar) at the anode to give a short lived intermediate cation radical which will be stabilized by complexation to a dimer cation radical as described by equ. 2. The conducting crystals finally grow from the supersaturated solution of the dimer-complex on the electrode surface (equ. 3). Following this principle a large number of different structures has been synthesized. A complicating factor arises when the structure allows for the inclusion of solvent. In these cases solvent will replace some of the counterions X and - in order to keep electro-neutrality-a deviation from the stoichiometray (Ar) is observed in other words, the stack of tne arenes contains more neutral than positive centers. 

Water-splitting electrodialysis can be improved by using a supersaturated solution of the fi-ee acid. Sodium acetate inhibits crystallization of succinic acid, but free acetic acid, which is produced by water-splitting electrodialysis, promotes crystallization of succinic acid (Berglund et al. 1991). 

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