Supersaturated solution crystallisation

It is a well-known fact that substances like water and acetic acid can be cooled below the freezing point in this condition they are said to be supercooled (compare supersaturated solution). Such supercooled substances have vapour pressures which change in a normal manner with temperature the vapour pressure curve is represented by the dotted line ML —a continuation of ML. The curve ML lies above the vapour pressure curve of the solid and it is apparent that the vapour pressure of the supersaturated liquid is greater than that of the solid. The supercooled liquid is in a condition of metastabUity. As soon as crystallisation sets in, the temperature rises to the true freezing or melting point. It will be observed that no dotted continuation of the vapour pressure curve of the solid is shown this would mean a suspended transformation in the change from the solid to the liquid state. Such a change has not been observed nor is it theoretically possible. 

Occasionally substances form supersaturated solutions from which the first crystals separate with difficulty this is sometimes caused by the presence of a little tar or viscous substance acting as a protective colloid. The following methods should be tried in order to induce crystallisation —... 

It appears as if an axiom of stereochemistry, the absolute identity of the most important chemical and physical properties of chiral isomers, is no longer valid. Experiments using the amino acid tyrosine (Tyr) showed unexpected differences in the solubility of D-and L-Tyr in water a supersaturated solution of 10 mM L-Tyr crystallised much more slowly than that of D-Tyr under the same conditions. The saturated solution of L-Tyr was more concentrated than that of D-Tyr. Supersaturated solutions of DL-Tyr in water formed precipitates containing mainly D-Tyr and DL-Tyr, so that there was an excess of L-Tyr in the saturated solution. The experiments were carried out with extremely great care in order to exclude the possibility of contamination. Further experiments will show whether this is a particular property of tyrosine, or whether other amino acids will show similar behaviour. Possible... 

General Considerations.—Solid crystallisable substances are usually obtained at the end of a reaction in the form of a crude product which separates in more or less pure form from the solvent on cooling, either directly or after concentration. The rate at which organic substances crystallise varies within very wide limits, and their tendency to form supersaturated solutions is extraordinarily great. But even when supersaturation is counteracted by dropping a crystal into the solution—by seeding —the attainment of equilibrium in the cold saturated solution is often exceedingly slow. The cause is indeed the slow rate of crystallisation. Hence the full yield of crude product is often obtained only after the solution has been left for many hours. 

Solution crystallisers are usually classified according to the method by which supersaturation is achieved, that is by cooling, evaporation, vacuum, reaction and salting out. The term controlled denotes supersaturation control whilst classifying refers to classification of product size. 

In the rate of crystallisation of a substance from a supersaturated solution two independent factors have to he considered, firstly the rate of nucleus formation from which crystallisation may proceed and secondly the rate of growth of a nucleus once it is formed. 

Increase in temperature causes a slight decrease in refractive index. The heat of dissolution 4 of the octahedral form is - 7530 calories at 18° C. The velocity of crystallisation from supersaturated solutions corresponds with 5 - dcjdt = fre4, where c is the concentration the temperature coefficient for the interval 0 to 25° C. is zero. 

Dihvdroxyselenazole crystallises in centimetre-long prisms, M.pt. 1475 C., readily soluble in alcohol, moderately soluble in water forming supersaturated solutions. It sublimes at 100° C., and is feebly acid towards litmus, showing some phenolic properties. It does not form salts with acids. 

Supersaturated Solutions.—When a solution of a solid in water, already saturated at a given temperature, is heated up with more of the solid until the whole of the latter has passed into solution, crystallisation or precipitation of the excess of solute does not always take place upon cooling the system to the original temperature. It is clear that the solution must now hold a greater quantity of substance than corresponds to the ordinary solubility and is said to be supersaturated. Such solutions can readily be prepared by heating up sodium thiosulphate, sodium acetate, or sodium sulphate with water, and allowing to cool without agitation. 

The growth affinity is the difference between the chemical potential of one mole of growth units of the crystallising substance in its supersaturated solution and in its crystalline form divided by RT. The growth unit is defined according to the stoichiometry of the crystal. 

Because of the difficulties associated with the characterisation of heteronuclei in solution, few studies have attempted to explain experimental results in a quantitative way. If it is assumed that, once nucleation occurs, the particles grow without recrystallisation, then it is possible to get information about the particle density from a consideration of the geometry of the particles and the growth kinetics. One approach is to add heteronuclei to supersaturated solutions and measure the crystallisation kinetics and, from the data obtained, estimate the surface area of the growing crystals. In this way, it is feasible to obtain information about the nucleation capability of different heteronuclei and the effects of pretreatments on the nucleation capability. An example of such an application will be discussed in Sect. 5.4. 

With supersaturated solutions he found oscillations of density prior to crystallisation. 

The requirements for homochirally pure a-amino acids have not ruled out any of these general synthetic methods (which all give racemic products), since resolution of DL-a-amino acids and their derivatives is a simple, albeit time-consuming, solution to this need. Classical methods for resolution include physical separation of the DL-amino acids themselves (by chromatography on a chiral phase e.g. resolution of DL-tryptophan over cellulose, see Section 4.15), fractional crystallisation of certain racemates or supersaturated solutions (through seeding with crystals of one enan-... 

Precipitation is carried out by a controlled mixing of the reactants in order to obtain a supersaturated solution from which nucleation takes place. Amorphous primary particles are formed that later crystallise into desired phases and in parallel agglomerate to larger secondary particles. Precipitation processes need in line pH meters and possibilities for automatic particle size distribution analysis coupled to the ageing vessel. 

Buchanan7 determined the densities of many salt solutions with an emergent stem hydrometer and without thermostatic control. With supersaturated solutions he found oscillations of density prior to crystallisation. 

With regard to the limits of supersaturation and the spontaneous crystallisation of the solute from supersaturated solutions, see Jaffe, Z. physikaL Chem., 1903, 43, 565 j Miers and Isaac,y. Chem, Soc, 1906, 89, 413 1908, 93, 384 Hartley, Jones, and Hutchinson, Jones, 1739 Fouquet, Compt rena.j 1910,... 

Rapid crystallisation from super-cooled melts and supersaturated solutions often results in a "fluffy" tree-like crystal formation called dendrites. The main crystal... 

These two mechanisms are usually responsible for what is called primary nucleation. Once crystals appear in the supersaturated solution further crystallites are formed in the vicinity and this establishes the macro-crystallisation process, generally called secondary nucleation. The relationship between the various forms of nucleation are shown on Fig. 8.10. 

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