Ternary System

Exercise No. 2 TERNARY SYSTEMS Liezel Angelique L. Sabaupan, Bethel Faith Rezaga, and Bea Eder Date of Performance: July 1, 2013; Date Submitted: July 11, 2013; Lab Section: 1L; Instructor: Prof. Annabelle T. Abrera Abstract Ternary systems are represented in a Gibbs-Roozeboom diagram. Since a three-component system offers four degrees of freedom, temperature and pressure are held constant. A mixture of acetic acid, water and chloroform was studied in this experiment. A binodal curve was constructed by means of applying turbidimetric titration. Acetic acid-water mixture was titrated with chloroform and acetic acid-chloroform was titrated with water. The acetic acid composition was pre-assigned and the composition of each of the component in the mixture was determined and plotted in the diagram. To locate the tie lines, delta point and plait point, certain amount of three components that will fall within the binodal curve was prepared and the conjugate phases of the resulting mixtures were titrated with standardized NaOH. The composition of acetic acid was plotted in the diagram and the three parameters were experimentally determined.

The composition in the experimentally determined plait point was 40% acetic acid, 45% chloroform and 15% water. I. Introduction Aside from a binary system, systems can also have three components which are then called a ternary system. By applying Gibbs’ phase rule, a ternary system predicts four degrees of freedom which means it requires 4 dimensions in order to be plotted in a paper. To be able to come up with a two-dimensional plot, two factors should be held constant (Levine, 2002). In this case, the temperature and pressure is to be held constant. By doing this, the resulting plot would be a composition-composition phase diagram. The mole fraction of each of the component are then plotted, two of these are independent mole fractions whereas the other one is a dependent mole fraction (Mortimer, 2008). Shown below is a sample composition-composition diagram of a ternary system.

Figure 1. A phase diagram of a ternary system. In a phase diagram of a ternary system, there is a one-phase region of complete miscibility. From the term itself, it means that the three components in this specific region are in solution. There is also a region of immiscibility which contains tie lines. The tie lines represent the coexistence of the components in equilibrium (Mortimer, 2008). A diagram called the GibbsRoozeboom diagram is used to construct the phase diagram of a ternary system. This diagram is an equilateral triangle where each of the apexes of the triangle represents a component of the system. By determining the weight fraction of the two

components, the weight fraction of the third component then becomes be fixed. The triangle is divided into a hundred parts and hence, for convenience in plotting, the %w/w composition of each of the component is the parameter plotted. Shown below is the Gibbs-Roozeboom diagram.

The concentrations used were 20, 30, 40, 60, and 70% v/v. Each of the solution was titrated with chloroform until the first sign of turbidity was observed. Turbidity was associated to the appearance of a second phase. The same procedure was used for the preparation of solutions containing acetic acid and chloroform. The titrant used was water. After titration, the %w/w composition of each of the mixtures titrated was calculated. The obtained %w/w composition was then plotted in the Gibbs-Roozeboom diagram, thus resulting to the binodal curve. Four 50 g mixtures which contained all three liquids were then prepared in such a way that the composition was found within the two-phase region of the diagram. Volumes of each of the liquid to be mixed were given by the professor.

Each mixture was then equilibrated at 25˚C in a 250-mL separatory funnel. The conjugate phases were then separated and were transferred to 125-mL Erlenmeyer flasks. The conjugate phases were identified to be upper layer (Brich) and lower layer (C-rich). The density of each of the conjugate phase was determined using a pycnometer. After determining the density, a 5.00-mL aliquot of each of the conjugate phases were transferred in a 250-mL Erlenmeyer flask and was titrated with standardized NaOH up to the phenolphthalein endpoint. The composition (%w/w) of acetic acid was then determined in each of the conjugate phases. III. Results and Discussion In this exercise, a phase diagram for a ternary system consisting of acetic acid, water and chloroform was constructed. The first thing that was done was the preparation of mixture containing acetic acid and water. The volume of acetic acid was computed in such a way that the mixture contains a specified composition of acetic acid. The compositions used were 20, 30, 40, 60 and 70 %v/v. Each of the mixtures was titrated

Figure 2. Gibbs-Roozeboom diagram In a system of three components where two liquid pairs are miscible and the other pair is partially miscible, the immiscible pairs can be separated. Data from each of the separated layer can then be used to construct the bimodal curve and within this curve is the region of immiscibility. At the end of this exercise, it is expected that the students will be able to construct a Gibbs-Roozeboom diagram representing a three-component system and be able to explain the behaviour of the system in terms of the various molecular interactions present. II. Methodology Turbidimetric titration at constant temperature and pressure was employed in order to determine data points for the bimodal curve or the region of immiscibility. Three liquids were used namely: acetic acid (A), water (B) and chloroform (C). Five 25.0-mL of solution containing acetic acid and water were prepared. The concentrations of acetic acid were varied.

with the third component – chloroform – and the volume of chloroform used until the mixture was turbid was noted. Two trials were done for each of the mixture. The volume of each component in each of the mixture was noted and was converted to its mass contribution in the mixture using the density of the component. The density of acetic acid is 1.005 g/mL, for water, it’s 0.99704 g/mL, and the density of chloroform is 1.474 g/mL. After getting the mass of each component in each of the mixtures, its percent by weight was calculated dividing each of the mass by the total mass of the mixture multiplied by a hundred. The summary of the data on the calculated mass and percent by weight for the mixture containing acetic acid and water and then titrated with chloroform is shown below.

The average percent by weight of the acetic acid-water mixture and acetic acid-chloroform mixture in each prescribed composition of acetic acid was then determined. The calculated average percent by weight was then plotted in the GibbsRoozeboom diagram. A curve was drawn using the dots plotted and this curve corresponds to the bimodal curve which encloses the region of immiscibility. The area outside the binodal curve is known to be the one-phase region. In this region, all the three components, acetic acid, water and chloroform are in solution. Though acetic acid and chloroform are organic compounds, it is still in solution with water because the amount of water in the solution is much greater than the amount of either acetic acid or chloroform. Aside from this, acetic acid and chloroform molecules are relatively small making it easier to solvate the molecules.

Also, chloroform and acetic acid has dipole-dipole moment due to the presence of an electronegative atom in the molecule. The chlorine atom in the chloroform dives electrons towards it giving the molecule a dipole moment and for the acetic acid, the dipole moment is due to the oxygen atom. The presence of dipole-dipole interaction permits the interaction between the organic molecules and the water molecules. After the composition of each component was determined, four 50 g mixture that contained the three components were prepared in such a way that the composition will lie within the twophase region of the diagram. Each of the mixture was equilibrated at 25˚C in a 250mL separatory funnel. The conjugate phases were separated and transferred in 125-mL Erlenmeyer flasks. The upper layer was mostly acetic acid, the basis being the density of the components. Acetic acid has a lower density compared to chloroform and so the lower layer is consist mostly of chloroform. The calculated density of each of the conjugate phases is summarized below.

Using a pycnometer, the density of each of the conjugate phases was determined. After the determination of the density, each of the conjugate phases was titrated with NaOH. Before titration, the titrant was first standardized using potassium acid phthalate as the standard. A 5.00-mL aliquot of each of the conjugate phases was then obtained and was titrated with the standardized NaOH up to the phenolphthalein. The purpose of this was to determine the amount of acetic acid present in each of the conjugate phases. By a simple neutralization reaction and stoichiometry, the amount of used base for titration is equal to the amount of acetic acid present in the mixture. By calculating the mass of the aliquot using the calculated density of each of the conjugate phases and by the determination of the mass of acetic acid in the mixture using the data on the amount of titrant used, the %w/w of acetic acid in each of the conjugate phases was determined. The data on the acetic acid composition in each of the conjugate phases is summarized in the table below.

In the experimentally determined plait point, the component of the mixture is about 40% acetic acid, 45% chloroform and 15% water. The components in equilibrium are acetic acid and chloroform and they have almost the same composition. The phase diagram of a ternary system is useful in industrial processes. The phase diagram is used to select and characterized solvents in different processes (Rogers, 2011).

IV. Conclusion

The average composition of acetic acid in each of the conjugate phases was then plotted in the Gibbs-Roozeboom diagram. For each mixture, there were two points – one point corresponds to the composition of acetic acid in the upper layer and the other point corresponds to the composition of acetic acid in the lower layer. The points plotted were along the bimodal curve and each pair of points was connected and extended. The line connecting the paired points is called the tie line which represents the composition of the components in equilibrium. The tie lines were extended in such a way that they would have a point of intersection. This point of intersection is called the delta point. From the delta point, a line was drawn tangent to the curve. The point of intersection of the tangent line drawn and the bimodal curve corresponds to the plait point. The plait point is the point at which

The design of the experiment was simple yet gave good results. The students were able to construct a Gibbs-Roozeboom diagram which represented the acetic acidwater-chloroform mixture and the binodal curve, tie lines, delta point and plait point were also located and interpreted in the diagram using the data gathered in the experiment. The behaviour of the system in terms of the molecular interactions present was also explained. For further learning, experimenters could use different components of a mixture. They can also study the effect of temperature on the binodal curve constructed and even on the plait point of the system.

V. References

Levine, I.R. 2002. Physical Chemistry, 6th ed. New York: McGraw-Hill. 385-386. Mortimer, R.G. 2008. Physical Chemistry. 3rd ed. San Diego, California: Elsevier Academic Press. 290. Rogers, D.W. 2011. Concise Physical Chemistry. Hoboken, New Jersey: John Wiley and Sons Inc. 138-139.