Recrystallisation process

A brief account of aromatic substitution may be usefully given here as it will assist the student in predicting the orientation of disubstituted benzene derivatives produced in the different substitution reactions. For the nitration of nitrobenzene the substance must be heated with a mixture of fuming nitric acid and concentrated sulphuric acid the product is largely ni-dinitrobenzene (about 90 per cent.), accompanied by a little o-dinitrobenzene (about 5 per cent.) which is eliminated in the recrystallisation process. On the other hand phenol can be easily nitrated with dilute nitric acid to yield a mixture of ortho and para nitrophenols. It may be said, therefore, that orientation is meta with the... 

When the levels of impurities, including moisture, are unacceptable for particular reactions, and when large volumes of such solvents are likely to be required, it is frequently more economic to purify the commercial grades than to purchase the more expensive AnalaR grades. Solvents of the appropriate grade of purity should also be used in isolation (extraction) and purification (recrystallisation) processes, particularly in the latter stages immediately prior to spectroscopic and/or elemental analysis. 

Investigate by thin-layer chromatography the effectiveness of the recrystallisation process in the following way. Load a 20 x 5 cm thin-layer plate (Silica Gel G) with approximately 3 mm diameter spots of concentrated solutions (in acetone) of the crude and the recrystallised product. Concentrate a portion of the ethanolic mother-liquor and similarly apply to the plate. Develop the chromatogram with a toluene-ethyl acetate mixture (4 1) and dry the plate. Mark the positions of the visible spots and leave the plate in a tank of iodine vapour to reveal the rest of the components. The recrystallised p-nitroacetanilide (RF 0.07) should be free from the pale yellow o-isomer (RF 0.36) these compounds are revealed by the iodine treatment. 

Figure 5.8 Schematic representation of the dissolution-dissociation-recrystallisation process of a cyclodextrin complex with a poorly soluble guest. The complex rapidly dissolves, and a metastable oversaturated solution is obtained. The anomalously high level of dissolved guest drops bock but remains higher than the level that can be obtained with noncomplexed drug. Solid curve = complexed drug broken curve = noncomplexed drug.

The composition of a SSAS system at primary saturation or at stoichiometric saturation will be generally independent of the initial solid to aqueous-solution ratio, but will depend on the initial aqueous-solution composition existing prior to the dissolution of the solid. In contrast, the final thermodynamic equilibrium state of a SSAS system attained after a dissolution or recrystallisation process will generally depend not only on the initial composition of the system but also on the initial solid to aqueous-solution ratio (Glynn et al, submitted to Gcochim, Cosmochim. Acta). 

The timescale of the NR recrystallisation process strongly increases due to blending the NR with IR 305 and carbon black. This sensitivity of the NR recrystallisation for compounding agents offers the possibility to use this technique for mixing efficiency studies of NR based rubber compounds. 

This fusion/recrystallisation process determines the degree of perfection of the fi-crystalline phase formed and this (as we saw above) partly determines the efficiency of an annealing procedure ment to promote or-crystallinity in such an article. The extent of such an effect was first investigated by a series of DSC experiments, see Table 9.4. 

These results clearly show that the recrystallisation speed of this DPM mixture reaches a maximum value at about 60°C. The Hfl-value of 124 J/g measured for the reference sample indicates that a Hfl-value of 48 J/g for this raw DPM sample containing 31 %wt. p,p-DPM might be possible. The results presented in Figure 10.8 show that after two hours storage at 60°C a Hf-value of 49 J/g was measured. This indicates that the recrystallisation process of the p,p-DPM phase is nearly completed is after these two hours of storage at 60°C. Thus, a solid raw DPM product can be obtained within a reasonable amount of time (about two hours) if the recrystallisation process occurs at 60°C instead of 20°C. 

It is supposed that due to the short time of laser sintering undesirable recrystallisation processes are prevented or limited. Besides, optimum conditions for formation of nanoporous layer during laser sintering are provided due to specific ability of nanoparticles to consolidation. As a result, the filter materials possess increased filtration fineness and throughput in comparison with the analogical... 

Conversely, in recrystallisation processes, only the crystalline phase is purified by further treatment, not the mother liquor. 

The desire of people to colour their clothes resulted in the development of some chemical processes. It was known in ancient times that to render a dye fast the fabric must first be treated with a mordant to bind the dye to the cloth, and that the alums provided excellent mordants. Alums occur naturally quite widely, but they often contain iron salts, which would introduce an unwanted colour in the dyeing process. Although direct evidence is lacking, it would seem likely that even in ancient times alum was purified by a recrystallisation process. 

L. M. Colyer, G. N. Greaves, S. W. Carr, andK. K. Fox, Collapse and recrystallisation processes in zinc-exchanged zeolite-A a combined X ray diffraction, XAFS and NMR study, J. Phys. Chem. 5111, 10105-1114 (1997). 

As mentioned earlier, the control of properties in a supercritical fluid can allow separation to be incorporated into a recrystallisation process. This uses the so-called crossover effect, which arises when the isotherms of solubility versus pressure for two compounds at the same temperature can cross, i.e. the solubility of one compound is greater at a lower temperature, whereas that of the other compound is higher at a higher temperature, and they are equal at an intermediate temperature. Chimowitz et al. [103], by expansion of the solution from an extraction vessel down to 1 bar, measured solubility of 1 10 w/w decanediol-benzoic acid mixture in supercritical carbon dioxide. They experimentally defined the crossover behaviour for the two compounds, and subsequently achieved the separation of almost pure benzoic acid [104] from the 1 10 w/w decanediol-benzoic acid mixture. In a subsequent study, Sako et al separated phenanthrene and naphthalene to 97 wt% purity from a 1 1 w/w mixture using a similar technique [105]. 

A method was described for measuring the heat and rate of crystallisation following partial melting. The method uses a specific sequence of temperatures with a differential scanning calorimeter, and the melting and recrystallisation processes were confirmed by optical observations. The method was applied to poly(butylene terephthalate). The melting temperatures obtained from recrystallisation were used in the Hoffman-Weeks equation to deduce 236 C as the equilibrium melting temperature for PBTP. 22 refs. 

Thermoplastic polymers are also prone to recrystallisation processes, particularly after rapid cooling employed in many manufacturing processes. Sometimes these are very evident as with PET (Figure 2.5), though sometimes the overall annealing effect does not show an overall exothermic event. The kinetics of recrystallisation processes are usually described by the Avrami equation discussed below. 

Sometimes there is a requirement to observe the changes that occur in a sample when it is heated slowly, for example when measiuing cure or recrystallisation processes. On other occasions, these changes are imdesirable and prevent measurement of sample properties at higher temperatiues. Yet at the slow scan rates normally employed changes can easily occur. 

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