Solutions, ideal crystallization curve

This is the equation for the crystallization curve of a solution provided that the solution is ideal, that no mixed crystals are formed, and that the difference between the heat capacities in the liquid and solid states is small enough to justify neglecting the second term in (22.4). This important equation is due to Schroder and van Laar.j ... 

The change in free energy upon phase separation only involves a mixing term, since a transition in the physical state (crystallization or evaporation) does not occur. In other words, AtrG = -AGmix. For an ideal solution (see Section 2.2), phase separation cannot occur, since it implies that AHmix equals zero, and A5m x is always positive. For nonideal solutions, it depends on the shape of the free energy curve whether phase separation can occur. 

Block (96) reported an analytical method for chloride-bromide mixtures utilizing DSC. The fact that the heat of fusion of an ideal solid solution of the type AWX — BmX or AmX — AWY is directly proportional to the concentration of solute ion was used to determine chloride-bromide mixtures in the concentration range 0-100%. Solutions containing both chloride and bromide are precipitated with silver nitrate, forming solid solutions of silver chloride-bromide. The heat of fusion of the mixed crystal is then determined, and the percent chloride or bromide obtained from a previously prepared standard curve. 

In practice, the presence of other aromatics significantly affects the solid/liquid diagram of the mixture, but, as a first approximation, only the shape of the ME liqiridus is modified. Indeed, as shown by the solubility curves or the different isomers in the aromatic C( solution, considered to be ideal (Fig. 4.11), p-xylene is the compound that crystallizes first as the temperature is lowered. 

Initially the curve W-X-X -Z follows a similar pattern to the ideal curve shown on Fig. 21. The precipitation at point A -A is due to cholic acid crystals (confirmed by polarizing microscopy and determination of melting points). Point Z probably represents the completion of the titration of NaC since, in practice, the molar amount of acid added from W to Z, closely corresponds with the theoretical calculation of the number of moles of acid required to titrate the known amount of NaC. However, at point Q, further precipitation occurs, with a corresponding jump in pW (Q-Q ). This precipitate has an appearance grossly different from the milky opalescence of the cholic acid crystals in solution. Microscopy and melting point... 

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