Spectroscopic Determination of Ternary Phase Diagrams

Ternary phase diagrams describe phase equilibria among three components as a function of composition at constant temperature and pressure. The system of n-propanol, n-heptane, and water exhibits a simple ternary diagram with two well-demarcated regions. One region represents complete liquid miscibility at high proportions of n-propanol. The second region delineates the compositions where two immiscible liquid phases coexist--a water-rich layer and a heptane-rich layer with distinct amounts of solubilized n-propanol. The ternary phase diagram for such partially miscible liquid systems is generally determined in an undergraduate laboratory experiment by both visual methods and classical techniques. Indeed, the demarcation between the one- and two-phase liquid regions is visually discernible through titrations of n-propanol-n-heptane mixtures with water until a second liquid phase appears. However, the compositions of the immiscible liquid phases within the two-phase region can not be determined using classical pH titrations or refractive index measurements. Acidic or basic species are absent, and n-heptane and n-propanol exhibit refractive indices that differ by only 0.002 at room temperature. We have designed a modern experiment using conventional absorbance or fluorescence spectroscopy to rapidly, accurately, ad conveniently construct the n-propanol-n-heptane-water ternary phase diagram.