SCHONITE 87
2. Cryohydric temperature for potassium sulphate, —1-2°,
marked a in the figure.
3. Cryohydric temperature for the mixed sulphates,— 4*5°.
The composition of the solution at the latter temperature
may be calculated by an application of Meyerhoffer's rule,
according to which, at a transition point, the solubility
curve for the salt remaining untransformed suffers no
special modification. The content of potassium sulphate of
the solution saturated with both sulphates at —4-5° is
therefore given by producing the line for the K2S04
content on simultaneous saturation with E2SO4 and
K2Mg(S04)2.6H20 (H2L2 in the horizontal projection). In
the same way the content of MgSO4 is given by producing
HILI in the vertical projection, so that M represents the
composition of the solution at the cryohydric point. The
ice area is therefore an almost vertical plane through B, TYT, G,
and A (freezing point of water), whose bounding lines and
points have the following meaning:—
A, freezing point of pure water (o°).
AB, freezing point of magnesium sulphate solution.
B, cryohydric point of magnesium sulphate solution (— 6°).
AQ-, freezing point of potassium sulphate solution.
o, cryohydric point of potassium sulphate solution (—1-2°).
BM, freezing point of solutions saturated with magnesium
sulphate, with gradual increase of potassium sulphate
content.
M, cryohydric point for magnesium and potassium sul-
phates (—4-5°).
GM, freezing point of solutions saturated with potassium
sulphate, with gradual increase of magnesium sulphate
content, up to saturation with the latter also at M.
Finally, there is the steam area.
In the first place, we have to consider the boiling point
of saturated magnesium sulphate on the vertical projection
plane at D (108°), and that of saturated potassium sulphate
on the horizontal plane at F (ioa ).