Double salt interval formation

From what has been said, it is evident that when the formation of a double salt can occur, three temperature intervals can be distinguished, viz. the single-salt interval, the transition interval, and the double-salt interval. When the temperature lies in the first interval, evaporation leads first of all to the crystallisation of one of the single salts, and then to the separation of both the single salts together. In the second temperature interval, evaporation again leads, in the first place, to the deposition of one of the single salts, and afterwards to the crystallisation of the double salt. In the third temperature interval, only the double salt crystallises out. This will become clearer from what follows. 

Summary.— With regard to double salts we have learned that their formation from and their decomposition into the single salts is connected with a definite temperature, the transition temperature. At this transition temperature two vapour pressure curves cut, viz. a curve of dehydration of a mixture of the single salts and the solubility curve of the double salt or the dehydration curve of the double salt and the solubility curve of the mixed single salts. The solubility curves, also, of these two systems intersect at the transition point, but although the formation of the double salt commences at the transition point, complete stability in contact with water may not be attained till some temperature above (or below) that point. Only when ike temperature is beyond the transition interval will a double salt dissolve in water without decomposition (e.g. the alums). 

Formation of Double Salt— We have already learned in the preceding chapter that if the temperature is outside the transition interval, it is possible to prepare a pure saturated solution of the double salt. If we suppose the double salt to contain the two constituent salts in equimolccular proportions, its saturated solution must be represented by a point lying on the line which bisects the angle AOB e.g. 

If the formation of double salt is possible, but if the temperature lies within the transition interval, the relations will be represented by a diagram like Fig. 126. Isothermal evaporation of the solution X will lead to the deposition of the component A, and the composition of the solution will alter in the direction DE at the point E, the double salt will be formed, and the composition of the solution will remain... 

As has already been pointed out, the formation of racemic compounds from the two enantiomorphous isomerides, is analogous to the formation of double salts. The isothermal solubility curves also have a similar form. In the case of the latter, indeed, the relationships are simplified by the fact that the two enantiomorphous forms have identical solubility, and the solubility cur 7-es are therefore symmetrical to the line bisecting the angle of the co-ordinates. Further, with the exception of the partially racemic compounds to be mentioned later, there is no transition interval. 

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See also in sourсe #XX -- [ , , ]

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