Crystallization from solution heat effects

Kinetics in crystallization means nucleation and crystal growth. Both effects are important in melt crystallization but not as important as in crystallization from solutions. In melt crystallization, the process is mainly controlled by the rate of heat transfer and not by mass transfer as it is in solution crystallization. 

CrystalUzation of Polymers Crystallization from solution will not be considered here because it is slow and the heat effect per unit mass of the solution is usually too small for DSC experiments. More detailed description... 

Anhydrous NaC102 crystallizes from aqueous solutions above 37.4° but below this temperature the trihydrate is obtained. The commercial product contains about 80% NaC102. The anhydrous salt forms colourless deliquescent crystals which decompose when heated to 175-200° the reaction is predominantly a disproportionation to C103 and Cl but about 5% of molecular O2 is also released (based on the C102 consumed). Neutral and alkaline aqueous solutions of NaC102 are stable at room temperature (despite their thermodynamic instability towards disproportionation as evidenced by the reduction potentials on p. 854). This is a kinetic activation-energy effect and, when the solutions are heated near to boiling, slow disproportionation occurs ... 

The fact that the initial setting process for magnesium oxychloride cements takes place without observable formation of either the 5 1 8 or the 3 1 8 phase is important. It indicates that formation of an amorphous gel structure occurs as the first step, and that crystallization is a secondary event which takes place from what is effectively a supersaturated solution (Urwongse Sorrell, 1980a). This implies that crystallization is likely to be extremely dependent upon the precise conditions of cementition, including temperature, MgO reactivity, heat build-up during reaction and purity of the components in the original cement mixture. 

Batch crystallization. Crystallization is extremely common in the production of fine and specialty chemicals. Many chemical products are in the form of solid crystals. Also, crystallization has the advantage that it can produce a product with a high purity and can be more effective than distillation from the separation of heat-sensitive materials. Crystallization has already been discussed in Chapter 10 and has two main steps. Firstly the solute to be crystallized is dissolved in a suitable solvent, unless it is already dissolved, for example, solute dissolved in a solvent from a previous a reaction step. Secondly, the solid is then deposited in the form of crystals from the solution by cooling, evaporation and so on. 

A method of preparation of the neutral salt (I) has been given by Herz [45]. He claims that the anhydrous salt crystallizes from an aqueous solution. In the light of other authors works (e.g. Zingaro), it is doubtful whether in such conditions, an anhydrous salt can really be formed. The dehydration of neutral lead styphnate (I) was investigated by Zingaro who found that complete dehydration may be effected by heating the substance at 115°C for 16 hr. At higher temperatures (135-145°C) dehydration takes place more quickly (Fig. 53). Stettbacher [46] reported that in a moist atmosphere anhydrous lead styphnate absorbs water to reform... 

The heat of transition from form III to form II is 310 keal/mol and that from form II to form I is 979 kcal/mol. Whetstone (Ref 113) studied the initiation of transition between forms III and IV. The effect of foreign substances on the transition IV III was studied by Campbell and Campbell (Ref 81) who found that in the case of a solid solution of 8. to 10% of KNOj in AN the temperature of transition of form III into form IV is depressed by about 20°. Such solid solutions can be prepared either by fusion or by co-crystallization from aqueous solutions. Hendricks et al (Ref 40) found form III to be ortho rhombic and form V to exist up to -18° and not to -16°. Bowen (Ref 30) showed that there is also a metastable inversion occurring at about 50° as follows orthorhombic form (j3)(32. l°to -16°) tetragonal form (5)(125-2° to 84.2°). 

Phosphotungstic acid crystallizes from water (in which it is extremely soluble) in very heavy white octahedra. The water solution is not stable toward light but slowly turns blue as a result of reduction. Re-oxidation is easily effected by heating with chlorine water. In spite of its great solubility in water, the acid may be completely extracted from water solution by ether. It forms with ether a dense liquid layer of a complex compound which is insoluble both in ether and in water, so that three liquid layers are formed when the water solution is extracted with an excess of ether. In addition to being very soluble in water and ether, phosphotungstic acid is readily soluble in the lower alcohols and esters. 

The 3-chloro-l-(4-indolyloxy)-2-propanol is dissolved in 50 ml of toluene and 50 ml of isopropylamine and heated to the boil for 45 h. Evaporation to dryness is effected in a vacuum, the residue is shaken out thrice between ethyl acetate and a 1 N tartaric acid solution and a 5 N sodium hydroxide solution is then added to the combined tartaric acid phases until an alkaline reaction is obtained. The alkaline solution is shaken out thrice with 50 ml of methylene chloride, the extracts are dried over magnesium sulfate and the solvent evaporated in vacuum. The residue is crystallized from ethyl acetate/ether to give the 4-(2-hydroxy-3-isopropylaminopropoxy)indole. 

In 3 L round-bottomed flask there were placed methyl 3,4-dihydro-2-methyl-4-oxo-2H-l,2-benzothiazine-3-carboxylate 1,1-dioxide, 2-aminopyridin and dry xylene. Nitrogen gas was then bubbled into the suspension for 5 min, then the reaction mixture was heated to begin a period of slow distillation, with complete solution effected during the first 10 min of heating. After 5.5 h, the period of slow distillation was discontinued and reaction mixture was allowed to heat at reflux for approximately 16 h. After that the reaction mixture was cooled to room temperature and filtered. The solid material was crystallized from chloroform with methanol and againe from methanol and then there were obtained piroxicam, melting point 197°-200°C, dec. 

Addition of 1.2 equiv of 1 to Ni(PEt3)4 in pentane at 25 °C gave a dark red solution, concomitant with the evolution of gas. Standard workup and crystallization from toluene-pentane gave 9 as a spectroscopically pure, dark red crystalline solid sensitive to air and stable during brief heating to 100-110 °C. The unusual thermal stability of the nickel bis(silyl) compound is attributed to the advantageous properties of the carboranyl unit, including electronic and steric effect. 

Heat Effect Accompanying the Cooling of a Solution of MgS04 A 30% solution of MgS04 is cooled from 150F to 50F. Data of the initial and final conditions are taken off the equilibrium diagram, Fig. 16.3(b). At the lower temperature, 27% of the mixture crystallizes out as the heptahydrate. 

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