Part 21 (2/2)

54.93

4.21

40.86 --------------------------------------------------

The numbers in the same horizontal row give the composition of the conjugate alloys, and it is evident that the upper layer consists almost entirely of silver and zinc. On allowing the mixture to cool slightly, the upper layer solidifies first, and can be separated from the still molten lead layer. It is on this behaviour of silver towards a mixture of molten lead and zinc that the Parkes's method for the desilverization of lead depends.[325] If aluminium is also added, a still larger proportion of silver pa.s.ses into the lighter layer, and the desilverization of the lead is more complete.[326]

[Ill.u.s.tration: FIG. 86.]

[Ill.u.s.tration: FIG. 87.]

The Influence of Temperature.--As has already been said, a ternary system existing in three phases possesses two degrees of freedom; and the state of the system is therefore dependent not only on the relative concentration of the components, but also on the temperature. As the temperature changes, therefore, the boundary curve of the heterogeneous system will also alter; and in order to represent this alteration we shall make use of the right prism, in which the temperature is measured upwards. In this way the boundary curve pa.s.ses into a boundary surface (called a dineric surface), as shown in Fig. 86. In this figure the curve _akb_ is the isothermal for the ternary system; the curve _a_K_b_ shows the change in the _binary_ system AB with the temperature, with {248} a critical point at K. This curve has the same meaning as those given in Chapter VI. The curve _k_K is a critical curve joining together the critical points of the different isothermals. In such a case as is shown in Fig. 86, there does not exist any real critical temperature for the ternary system, for as the temperature is raised, the amount of C in the ”critical” solution becomes less and less, and at K only two components, A and B, are present. In the case, however, represented in Fig. 87, a real ternary critical point is found. In this figure _ak'b_ is an isothermal, _ak”_ is the curve for the binary system, and K is the ternary critical point. All points outside the helmet-shaped boundary surface represent h.o.m.ogeneous ternary solutions, while all points within the surface belong to heterogeneous systems. Above the temperature of the point K, the three components are miscible in all proportions. An example of a ternary system yielding such a boundary surface is that consisting of phenol, water, and acetone.[327] In this case the critical temperature K is 92, and the composition at this ternary critical point is--

Water 59 per cent.

Acetone 12 ”

Phenol 29 ”

[Ill.u.s.tration: FIG. 88.]

The difference between the two cla.s.ses of systems just mentioned, is seen very clearly by a glance at the Figs. 88 and 89, which show the projection of the isothermals on the base of the prism. In Fig. 88, the projections yield paraboloid curves, the two branches of which are cut by one side of the triangle; and the critical point is represented by a point on {249} this side. In the second case (Fig. 89), however, the projections of the isothermals form ellipsoidal curves surrounding the supreme critical point, which now lies _inside the triangle_. At lower temperatures, these isothermal boundary curves are cut by a side of the triangle; at the critical temperature, _k”_, of the binary system AB, the boundary curve _touches_ the side AB, while at still higher temperatures the boundary curve comes to lie entirely within the triangle. At any given temperature, therefore, between the critical point of the binary system (_k”_), and the supreme critical point of the ternary system (K), each pair of the three components are miscible with one another in all proportions; for the region of heterogeneous systems is now bounded by a closed curve lying entirely within the triangle. Outside this curve only h.o.m.ogeneous systems are found.

Binary mixtures, therefore, represented by any point on one of the sides of the triangle must be h.o.m.ogeneous, for they all lie outside the boundary curve for heterogeneous states.

[Ill.u.s.tration: FIG. 89.]

2. _The three components can form two pairs of partially miscible liquids._

In the case of the three components water, alcohol, and succinic nitrile, water and alcohol are miscible in all proportions, but not so water and succinic nitrile, or alcohol and succinic nitrile.

[Ill.u.s.tration: FIG. 90.]

[Ill.u.s.tration: FIG. 91.]

As we have already seen (p. 122), water and succinic nitrile can form two liquid layers between the temperatures 18.5 and 55.5; while alcohol and nitrile can form two liquid layers between 13 and 31. If, then, between these two temperature limits, alcohol is added to a heterogeneous mixture of water and nitrile, or water is added to a mixture of alcohol and nitrile, two heterogeneous ternary systems will be formed, {250} and two boundary curves will be obtained in the triangular diagram, as shown in Fig. 90.[328] On changing the temperature, the boundary curves will also undergo alteration, in a manner similar to that just discussed. As the temperature falls, the two curves will spread out more and more into the centre of the triangle, and might at last meet one another; while at still lower temperatures we may imagine the curves still further expanding so that the two heterogeneous regions flow into one another and form a _band_ on the triangular diagram (Fig. 91). This, certainly, has not been realized in the case of the three components mentioned, because at a temperature higher than that at which the two heterogeneous regions could fuse together, solid separates out.

[Ill.u.s.tration: FIG. 92.]

The gradual expansion of a paraboloid into a band-like area of heterogeneous ternary systems, has, however, been observed in the case of water, phenol, and aniline.[329] In Fig. 92 are shown three isothermals, viz. those for 148, 95, and 50. At 148, water and aniline form two layers having the composition--

Water, 83.5 per cent. } { water, 20 per cent.

} and { Aniline, 16.5 ” } { aniline, 80 ”

{251}

and the critical point _k'_ has the composition--

Water, 65; phenol, 13.2; aniline, 21.8 per cent.

At 95, the composition of the two binary solutions is--

Water, 93 per cent. } { water 8 per cent.

} and { Aniline, 7 ” } { aniline, 92 ”

while the point _k”_ has the composition

Water, 69.9; phenol, 26.6; aniline, 3.5 per cent.

At 50, the region of heterogeneous states now forms a band, and the two layers formed by water and aniline have the composition--

Water, 96.5 per cent. } { water, 5.5 per cent.

} and { Aniline, 3.5 ” } { aniline, 94.5 ”

while the two layers formed by water and phenol have the composition--

Water, 89 per cent.} { water, 38 per cent.

} and { Phenol, 11 ” } { phenol, 62 ”

All mixtures of water, phenol, and aniline, therefore, the composition of which is represented by any point within the band _abcd_, will form two ternary solutions; while if the composition is represented by a point outside the band, only one h.o.m.ogeneous solution will be produced.

3. _The three components form three pairs of partially miscible liquids._

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