Three Component System
The solids and liquids of a three component system can be shown on a triangular diagram, it is necessary to consider both the pressure and the temperature as fixed. The phases of the system as a function of the composition can then be shown. The relative amounts of the three components, usually presented centages by weight can be shown on a triangular plot.
The corners of the triangle labeled A, B, C correspond to the pure components A, B, and C respectively. The side of the triangle opposite the corner labeled A, for example, implies the absence of A. thus the horizontal lines across the triangle show increasing percentages of A from zero at the base to 100 percent at the apex. In a similar way the percentage of B and C are given by the distances from the other two sides to the remaining two apices. From the three composition scales of the diagram the composition corresponding to any point can be read off. This procedure for handling the composition of three component systems is possible, and the total composition is always 100 percent, because of the geometric result that the sum of the three perpendicular distances from any point to the three sides of the triangle is equal to the height of the triangle.
As with two-component systems, the simplest three component systems are those in which ha liquid system breaks down into two phases. Over a certain range of temperature the system acetic acid chloroform water is such a system. A two phase region occurs in system at 18˚C is necessarily part of the diagram is the tie lines through the two phase region joining the compositions of the two phases that are in equilibrium. (in all previous two phase diagrams, such lines could have been drawn but since they would have been horizontal constant temperature lines, it was unnecessary to exhibit them.) thus a total composition corresponding to point a in the two phase region gives two phases, one of composition b and the other of composition c. a unique point on the two phase boundary is indicated by d. this point, called the isothermal critical point or the plate point, is similar to the previously encountered critical solution temperatures, or consulate points, in that the compositions of the two phases in equilibrium become equal at this point.
Application of the phase rule to a system corresponding to a point in the two phase region gives
Ø = C – P + 2 = 3 – 2 + 2 = 3
The 3 degrees of freedom can be accounted for the pressure, the temperature, and one composition variable. Thus the composition of both phases cannot be arbitrarily fixed. If one is fixed, the tie from that composition fixes the composition of the second phase.
Three-component systems involving solids and liquids can be introduced by considering systems of two salts are somewhat soluble and the diagram gives the curves for the saturated solution compositions. Such diagrams are perhaps more easily understood if tie lines are in equilibrium with the solid salts B and C, solutions along DF and EFare in equilibrium with the solid salts B and C, respectively. Point F corresponds to a system in which the solution is in equilibrium with both salts. Removal of water from point F moves the total composition toward the base of the triangle. The effect is to form more solid salts, which remain in equilibrium with the decreasing amount but constant concentration of saturated solution.
Finally, three component systems in which the three components taken in pairs form simple eutectics can be illustrated by the system bismuth-tin-lead. A three-dimensional representation shows descriptively the phase behavior as a function of composition and temperature at the fixed pressure of 1 bar. For quantitative work it is more suitable to express the data at various constant temperatures on triangular plots. Such diagrams for a few temperatures are therefore included. Tie lines are shown to indicate more clearly the solution composition that are in equilibrium with the solid components.
If a solution containing 32 percent lead, 15 percent tin, and 53 percent bismuth is cooled, it is found that it remains liquid until, 96˚C, all three solid components start separating out. The phase rule indicates that at such a point, called a ternary eutectic, there is
C – P + 2 = 3 – 4 + 2 = 1 degree of freedom
Since this degree has been used by the fixed pressure, the system has no remaining variables. It is characteristic of such ternary eutectics that the eutectic point is at a low temperature compared with the melting points of the pure components; e.g. the ternary eutectic of the metal system of the present example will melt in boiling water.
Services:- Three Component System Homework | Three Component System Homework Help | Three Component System Homework Help Services | Live Three Component System Homework Help | Three Component System Homework Tutors | Online Three Component System Homework Help | Three Component System Tutors | Online Three Component System Tutors | Three Component System Homework Services | Three Component System