Part 25 (1/2)
We shall first consider the solubility relations of the single salts. The complete equilibrium curve for magnesium chloride and water is represented in Fig. 113 by the series of curves ABF_{1} G_{1} H_{1} J_{1} L_{1} N_{1}.
AB is the freezing-point curve of ice in contact with solutions containing magnesium chloride, and B is the cryohydric point at which the solid phases ice and MgCl_{2},12H_{2}O can co-exist with solution. BFG is the solubility curve of magnesium chloride dodecahydrate. This curve shows a point of maximum temperature at F_{1}, and a retroflex portion F_{1}G_{1}. The curve is therefore of the form exhibited by calcium chloride hexahydrate, or the hydrates of ferric chloride (Chapter VIII.). G_{1} is a transition point at which the solid phase changes from dodecahydrate to octahydrate, the solubility of which is represented by the curve G_{1}H_{1}. At H_{1} the octahydrate gives place to the hexahydrate, which is the solid phase in equilibrium with the solutions represented by the curve H_{1}J_{1}. J_{1} and L_{1} are also transition points at which the solid phase undergoes change, in the former case from hexahydrate to tetrahydrate; and in the latter case, {286} from tetrahydrate to dihydrate. The complete curve of equilibrium for magnesium chloride and water is, therefore, somewhat complicated, and is a good example of the solubility curves obtained with salts capable of forming several hydrates.
The solubility curve of pota.s.sium chloride is of the simplest form, consisting only of the two branches AC, the freezing-point curve of ice, and CO, the solubility curve of the salt. C is the cryohydric point. This point and the two curves lie in the YT-plane.
On pa.s.sing to the ternary systems, the composition of the solutions must be represented by points or curves situated _between_ the two planes. We shall now turn to the consideration of these. BD and CD are ternary eutectic curves (p. 284). They give the composition of solutions in equilibrium with ice and magnesium chloride dodecahydrate (BD), and with ice and pota.s.sium chloride (CD). D is a _ternary cryohydric point_. If the temperature is raised and the ice allowed to disappear, we shall pa.s.s to the solubility curve for MgCl_{2},12H_{2}O + KCl (curve DE). At E carnallite is formed and the pota.s.sium chloride disappears; EFG is then the solubility curve for MgCl_{2},12H_{2}O + carnallite (KMgCl_{3},6H_{2}O). This curve also shows a point of maximum temperature (F) and a retroflex portion. GH and HJ represent the solubility curves of carnallite + MgCl_{2},8H_{2}O and carnallite + MgCl_{2},6H_{2}O, G and H being transition points. JK is the solubility curve for carnallite + MgCl_{2},4H_{2}O. At the point K we have the _highest temperature at which carnallite can exist with magnesium chloride in contact with solution_. Above this temperature decomposition takes place and pota.s.sium chloride separates out.
If at the point E, at which the two single salts and the double salt are present, excess of pota.s.sium chloride is added, the magnesium chloride will all disappear owing to the formation of carnallite, and there will be left carnallite and pota.s.sium chloride. The solubility curve for a mixture of these two salts is represented by EMK; a simple curve exhibiting, however, a temperature maximum at M. This maximum point corresponds with the fact that dry carnallite melts at this temperature with separation of pota.s.sium chloride. _At all temperatures {287} above this point, the formation of double salt is impossible_. The retroflex portion of the curve represents solutions in equilibrium with carnallite and pota.s.sium chloride, but in which the ratio MgCl_{2} : KCl is greater than in the double salt.
Throughout its whole course, _the curve EMK represents solutions in which the ratio of MgCl_{2} : KCl is greater than in the double salt_. As this is a point of some importance, it will be well, perhaps, to make it clearer by giving one of the isothermal curves, _e.g._ the curve for 10, which is represented diagrammatically in Fig. 114. E and F here represent solutions saturated for carnallite plus magnesium chloride hydrate, and for carnallite plus pota.s.sium chloride. As is evident, the point F lies above the line representing equimolecular proportions of the salts (OD).
[Ill.u.s.tration: FIG. 114.]
Summary and Numerical Data.--We may now sum up the different systems which can be formed, and give the numerical data from which the model is constructed.[366]
I. _Bivariant Systems._
-------------------------------------- Solid phase.
Area of existence.
-------------------------------------- Ice
ABDC KCl
CDEMKLNO Carnallite
EFGHJKM MgCl_{2},12H_{2}O
BF_{1}G_{1}GFED MgCl_{2},8H_{2}O
G_{1}H_{1}HG MgCl_{2},6H_{2}O
H_{1}I_{1}IH MgCl_{2},4H_{2}O
I_{1}L_{1}LKI MgCl_{2},2H_{2}O
L_{1}N_{1}NL --------------------------------------
II. _Univariant Systems._--The different univariant systems have already been described. The course of the curves will be sufficiently indicated if the temperature and composition of the solutions for the different invariant systems are given.
{288}
III.--_Invariant Systems--Binary and Ternary._
-------------------------------------------------------------------------
Composition of solution.
Point.
Solid Phases.
Temper-
Gram-molecules of salt
ature.
per 1000 gram-mol. water.
------------------------------------------------------------------------- A
Ice
0
--
B
Ice; MgCl_{2},12H_{2}O
-33.6
49.2 MgCl_{2}
C
Ice; KCl
-11.1
59.4 KCl
D
{ Ice; MgCl_{2},12H_{2}O; }
-34.3
43 MgCl_{2}; 3 KCl
{ KCl }
E
{ MgCl_{2},12H_{2}O; KCl; }
-21
66.1 MgCl_{2}; 4.9 KCl
{ carnallite }
F_{1}
MgCl_{2},12H_{2}O
-16.4
83.33 MgCl_{2}
F
{ MgCl_{2},12H_{2}O; }
-16.6
{ Almost same as F_{1};
{ carnallite }
{ contains small amount
{ of KCl
G_{1}
{ MgCl_{2},12H_{2}O; }
-16.8
87.5 MgCl_{2}
{ MgCl_{2},8H_{2}O }
G
{ MgCl_{2},12H_{2}O; }
-16.9
{ Almost same as G_{1},
{ MgCl_{2},8H_{2}O; }
{ but contains small
{ carnallite }
{ quant.i.ty of KCl
H_{1}
{ MgCl_{2},8H_{2}O; }
-3.4
99 MgCl_{2}
{ MgCl_{2},6H_{2}O }
H
{ MgCl_{2},8H_{2}O; }
ca. -3.4
{ Almost same as H_{1},
{ MgCl_{2},6H_{2}O; }
{ but contains small
{ carnallite }
{ amount of KCl
J_{1}
{ MgCl_{2},6H_{2}O; }
116.67
161.8 MgCl_{2}
{ MgCl_{2},4H_{2}O }
J
{ MgCl_{2},6H_{2}O; }
115.7
162 MgCl_{2}; 4 KCl
{ MgCl_{2},4H_{2}O; }
{ carnallite }
K
{ MgCl_{2},4H_{2}O; KCl; }
152.5
200 MgCl_{2}; 24 KCl
{ carnallite }
L_{1}
{ MgCl_{2},4H_{2}O; }
181
238.1 MgCl_{2}
{ MgCl_{2},2H_{2}O }
L
{ MgCl_{2},4H_{2}O; }
176
240 MgCl_{2}; 41 KCl
{ MgCl_{2},2H_{2}O; KCl }
M
Carnallite; KCl
167.5
166.7 MgCl_{2}; 41.7 KCl
[N_{1}
MgCl_{2},2H_{2}O
186
ca. 241 MgCl_{2}]
N
MgCl_{2},2H_{2}O; KCl
186
240 MgCl_{2}; 63 KCl
[O
KCl
186
195.6 KCl]