Concise Physical Chemistry

c09 JWBS043-Rogers September 13, 2010 11:26 Printer Name: Yet to Come

138 THE PHASE RULE

XB

T, K

A B

.

FIGURE 9.11 A compound phase diagram with a low boiling azeotrope.

phase diagram with anazeotrope. A phase diagram like that in Fig. 9.11 is, quite

evidently, a combination of two Type I phase diagrams, one extending slightly beyond

the midpoint of the composition axis and one somewhat shorter curve. Drawing

isotherms as in Fig. 9.11 soon convinces us that the mixture cannot be separated by

conventional fractional distillation. Even the best still, starting with a mixture to the

right of the azeotropic point (•), will produce component B and azeotrope, whereas

starting to the left of the point will produce pure A and azeotrope. One does not get

pure A and pure B by either of these methods as one did by distilling mixtures of

components having a simple Type I phase diagram. If the curve in Fig. 9.11 is turned

upside down, the azeotrope is said to behigh boiling. Ethanol and water form a high

boiling azeotrope at about 95% ethanol, which is why the common laboratory ethanol

is only 95% pure while most reagent grade solvents are 99+%.

9.8 TERNARY PHASE DIAGRAMS

Any mixture of three components can be represented within an equilateral triangle.

All points within the triangle represent one and only one of the infinitely many possi-

ble mixtures. Because there are now three composition variablesf=C−P+ 2 =

3 −P+2, bothTandpare held constant for the three-component triangular rep-

resentation on a two-dimensional surface. If only one phase is present, all solutions

are permitted and the composition can take any point in the triangular 2-space. When

two phases are present, the composition is restricted to the locus of points on the

coexistence curve. Like the Type II phase diagram, many three-component phase

diagrams are known having a dome-shaped coexistence curve. They are widely used

for characterizing and selecting solvents used in industrial processes. Intersections of

tie lines with the coexistence curve indicate the composition of phases in two-phase

systems formed when the total composition point is below the coexistence curve.

In Fig. 9.12, solvents B and C are nearly completely immiscible, but addition of sol-

vent A brings the composition of the two coexisting phases, the termini of the tie lines,

closer together until finally there are ternary solutions of A, B, and C that are homoge-

neous. These solutions correspond to ternary mixtures above the coexistence dome.