Part 23 (2/2)
COMPOSITION OF SOLUTIONS SATURATED WITH RESPECT TO Na_{2}Mg(SO_{4})_{2},4H_{2}O AND Na_{2}SO_{4},10H_{2}O.
---------------------------------------- Temperature.
Na_{2}SO_{4}.
MgSO_{4}.
---------------------------------------- 22
2.95
4.70 24.5
3.45
3.62 ----------------------------------------
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From the above figures, therefore, it will be seen that at a temperature just above the transition point a solution in contact with the two solid phases, astracanite and Glauber's salt, contains a relatively smaller amount of sodium sulphate than a pure solution of astracanite would; for in this case there would be equal molecular amounts of Na_{2}SO_{4} and MgSO_{4}. A solution which is saturated with respect to astracanite alone, will contain more sodium sulphate than the solution saturated with respect to astracanite plus Glauber's salt, and the latter will therefore be deposited. From this, therefore, it is clear that if astracanite is brought in contact with water at about the transition point, it will undergo decomposition with separation of Glauber's salt (supersaturation being excluded).
[Ill.u.s.tration: FIG. 101.]
This will perhaps be made clearer by considering Fig. 101. In this diagram the ordinates represent the ratio of sodium sulphate to magnesium sulphate in the solutions, and the abscissae represent the temperatures. The line AB represents solutions saturated with respect to a mixture of the single salts (p. 268); BC refers to solutions in equilibrium with astracanite and magnesium sulphate; while BX represents the composition of solutions in contact with the solid phases astracanite and Glauber's salt. The values of the solubility are contained in the following table, and in that on p. 268, and are, as before, expressed in gm.-molecules of salt in 100 gm.-molecules of water.[352]
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-------------------------------------------------------------------------
Astracanite
Astracanite Temperature.
+ sodium sulphate.
+ magnesium sulphate.
----------------------------
------------------------------
Na_{2}SO_{4}.
MgSO_{4}.
Na_{2}SO_{4}.
MgSO_{4}.
------------------------------------------------------------------------- 18.5
--
--
3.41
4.27 22
2.95
4.70
2.85
4.63 24.5
3.45
3.62
2.68
4.76 30
4.58
2.91
2.30
5.31 35
4.30
2.76
1.73
5.88 -------------------------------------------------------------------------
At the transition point the ratio of sodium sulphate to magnesium sulphate is approximately 1 : 1.6. In the case of solutions saturated with respect to both astracanite and Glauber's salt, the relative amount of sodium sulphate increases as the temperature rises, while in the solutions saturated for astracanite and magnesium sulphate, the ratio of sodium sulphate to magnesium sulphate decreases.
If, now, we consider only the temperatures above the transition point, we see from the figure that solutions represented by points above the line BX contain relatively more sodium sulphate than solutions in contact with astracanite and Glauber's salt; and solutions lying below the line BC contain relatively more magnesium sulphate than solutions saturated with this salt and astracanite. These solutions will therefore not be stable, but will deposit in the one case, astracanite and Glauber's salt, and in the other case, astracanite and magnesium sulphate, until a point on BX or BC is reached. All solutions, however, lying to the right of CBX, will be _unsaturated_ with respect to these two pairs of salts, and only the solutions represented by the line XY (and which contain equimolecular amounts of sodium and magnesium sulphates) will be saturated with respect to the pure double salt.
Transition Interval.--Fig. 101 will also render intelligible a point of great importance in connection with astracanite, and of double salts generally. At temperatures between those represented by the points B and X, the double salt when brought in contact with water will be decomposed with separation of sodium sulphate. Above the temperature of the point {271} X, however, the solution of the pure double salt is stable, because it can still take up a little of either of the components. At temperatures, then, above that at which the solution in contact with the double salt and the less soluble single salt, contains the single salts in the ratio in which they are present in the double salt, solution of the latter will take place without decomposition. _The range of temperature between that at which double salt can begin to be formed (the transition point) and that at which it ceases to be decomposed by water is called the transition interval._[353] If the two single salts have identical solubility at the transition point, the transition interval diminishes to nought.
In those cases where the double salt is the stable form below the transition point, the transition interval will extend downwards to a lower temperature. Fig. 101 will then have the reverse form.
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 the temperature is beyond the transition interval, will a double salt dissolve in water without decomposition (_e.g._ the alums)._
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CHAPTER XVI
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