A ternary phase diagram is used in fields like chemistry, materials science, and engineering to visualize the behavior of three-component systems. This diagram illustrates how different combinations of three substances interact and exist in equilibrium at a specific, fixed temperature and pressure. By plotting every possible mixture, the diagram allows researchers to quickly identify the stable physical state—or phase—of any given composition. Understanding how to read this diagram is foundational to predicting the outcomes of mixing three chemicals, whether they form a single uniform solution or separate into multiple distinct layers.
The Geometric Foundation of the Diagram
The foundation of a ternary phase diagram is typically an equilateral triangle, which provides a two-dimensional space to represent the ratios of the three components. Each of the three corners, or vertices, represents one of the pure components at 100% concentration. Moving away from a vertex signifies a decrease in the concentration of that specific component.
The three sides connecting the vertices represent binary mixtures, meaning every point on an edge contains a mixture of only two components. The entire area inside the triangle represents all possible ternary mixtures. This geometric structure is effective because the concentration of the third component is automatically determined once the concentrations of the first two are known, as the sum must always equal 100%.
Reading the Composition of a Single Point
To determine the exact composition of any point within the triangle, a reading method must be applied for each of the three components. The concentration of a component is read by using the set of lines drawn parallel to the side opposite that component’s vertex. For example, to read the percentage of component A, one must follow the lines that are parallel to the side connecting the B and C vertices.
Concentration increases linearly from 0% at the base (the side opposite the vertex) to 100% at the vertex itself. If the composition lines are marked in 10% increments, a point resting on the line halfway between the base and the vertex indicates a 50% concentration of that component. This process is repeated independently for the other two components.
Interpreting Phase Regions and Boundaries
The lines and curves inside the diagram divide the triangle into distinct regions that indicate the physical state of the mixture. A large, often dome-shaped area is known as the two-phase region, where the mixture separates into two distinct liquid phases that do not fully mix. Any composition falling outside of this two-phase area is considered a single-phase region, meaning the components are completely miscible and form a homogeneous solution.
The boundary line separating the one-phase and two-phase regions is called the binodal curve. This curve represents the saturation limit, where the mixture is just on the verge of separating or fully dissolving. At the very top of the binodal curve, the two liquid phases in equilibrium become identical, a unique composition known as the plait point or critical point.
Utilizing Tie Lines and the Lever Rule
Inside the two-phase region, a key feature is the presence of straight lines known as tie lines, or conodes, which connect the compositions of the two immiscible liquid phases that exist in equilibrium. For any overall composition that falls on a tie line, the ends of that line indicate the exact compositions of the two separate phases into which the mixture has split. All mixtures that lie along the same tie line will separate into the same two phases, regardless of their overall proportion.
The Lever Rule is a mass balance technique used to determine the relative amounts, or proportions, of the two equilibrium phases for a specific overall composition on a tie line. Conceptually, the overall composition point acts as the fulcrum of a lever, with the two phase compositions at the ends. The amount of one phase is proportional to the length of the tie line segment on the opposite side of the overall composition point.