Investigation of the Ternary Phase Diagram of Water–Propan-2-ol

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Investigation of the Ternary Phase Diagram of Water−Propan-2-ol− Sodium Chloride: A Laboratory Experiment Cory C. Pye,* M. Angelique Imperial, Coltin Elson, Megan L. Himmelman, Jacquelyn A. White, and Fuhao Lin Department of Chemistry, Saint Mary’s University, Halifax, NS Canada B3H 3C3 S Supporting Information *

ABSTRACT: A new laboratory experiment investigating the ternary phase equilibrium of water−propan-2-ol−sodium chloride with one-, two-, and three-phase regions has been developed. Observations of the type and number of phases of a series of mixtures of the three components were made, and the results are then used in a systematic way to more finely choose the next set of compositions to use. No specialized equipment is required.

KEYWORDS: Upper-Division Undergraduate, Physical Chemistry, Hands-on Learning/Manipulatives, Communication/Writing, Phases/Phase Transitions/Diagrams

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INTRODUCTION Ternary phase diagrams convey information about the thermodynamically stable phases of a three-component system under a given set of conditions. Typically, the pressure is taken as 1 atm, and the temperature is specified (usually 298 K). The phase regions are usually plotted on the inside of an equilateral triangle, where the three corners represent the pure compounds, and the edges correspond to the binary mixtures. The axes are labeled usually by mass fraction or mass percent, although the corresponding molar quantities may also be used. Any phase region boundary which intersects the sides gives the maximum solubility of one component in the other. Ternary phase diagrams are of great utility in industry. For example, the importance of the sodium oxide−calcium oxide− silicon dioxide ternary phase diagram to the manufacture of common glass has been emphasized by Morey.1 Most glasses have an approximate composition corresponding to Na2O· 3CaO·6SiO2. In addition, the ternary phase diagram of calcium oxide−aluminum oxide−silicon dioxide is important in the manufacture of Portland cement, used in making concrete.2,3 Most Portland cement has an approximate composition of 8CaO·Al2O3·2SiO2. Several pedagogical articles elaborate on the interpretation of phase diagrams. Davis advocates for careful analysis and application of the Gibbs phase rule when investigating solid− solid−liquid systems such as the water−boric acid−tartaric acid and water−succinic acid−tartaric acid examples discussed.4 One has to be alert to the possibility that the two solids could form a new compound, a solid solution, or remain as the separate compounds. Smith discusses the use of ternary phase diagrams to explain the common-ion effect, salting out of an organic solute, and two-temperature recrystallization.5 By the © XXXX American Chemical Society and Division of Chemical Education, Inc.

use of tie lines, Heric advocates for the determination of the plait point, where the two liquid phases in equilibrium become indistinguishable,6 and demonstrates its application to the water−acetic acid−benzene system. The wide variety of ternary liquid systems was demonstrated by Francis and Smith.7 Some examples of ternary diagram experiments proposed in this Journal include the lead nitrate−sodium nitrate−water system (gravimetric analysis),8 the water−acetic acid−1,2dichloroethane system (analysis by refractive index),9,10 the water−acetic acid−chloroform system (analysis by NMR),11 and the water−n-heptane−n-propanol system (analysis by gas chromatography,12 UV or fluorescence spectroscopy,13 or NMR14). These describe, at ambient temperatures, either two solid + liquid or three liquid component systems. The authors have developed a laboratory experiment describing the two liquid + solid water−propan-2-ol−sodium chloride system, described by Gomis and co-workers.15 The experiment consists of observations of the number and types of phases observed upon mixing various ratios of the three components at a fixed temperature of 298 K.

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MATERIALS Materials and chemicals required for this experiment include the following: top-loading balance, 24−36 culture tubes, disposable pipettes, spatula, water bath at constant temperature, test tube racks, sodium chloride (ACP, 99.0%), propan-2-ol (Fisher, 99.9%), deionized water, and 0.020 g/L solution of toluidine blue (Aldrich, 84%) in propan-2-ol. Received: April 3, 2018 Revised: May 24, 2018

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DOI: 10.1021/acs.jchemed.8b00242 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. Three representations of the ternary phase diagram of sodium chloride−water−propan-2-ol, after Part 1 of the experiment is complete. A = water, B = propan-2-ol, and C = sodium chloride. For the leftmost diagram, a yellow dot represents liquid and a black dot represents solid.

Figure 2. Three representations of the ternary phase diagram of sodium chloride−water−propan-2-ol, after Part 2 of the experiment is complete. A = water, B = propan-2-ol, and C = sodium chloride. For the leftmost diagram, a yellow dot represents liquid, a black dot represents solid, and a halfyellow, half-black dot represents both solid and liquid are present.

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EXPERIMENT The students normally work in groups of three during a single 3 h laboratory period on a rotating schedule of laboratories. The approach taken is simply to mix accurately weighed masses of the three components summing to approximately 10 g per sample, as close to a prescribed ratio as practicable, in a culture tube with a cap. The capped culture tubes are then vigorously shaken, placed in a 25.0 ± 0.1 °C water bath until equilibrated, shaken again, returned to the water bath, and then visually observed to determine the number and nature of the phases. The design is such that all regions of the phase diagram are observed, and areas where the number of phases change (the phase boundaries) are investigated in more detail. The students plot their data approximately as they go. The 1:2:5 grid (by mass) is chosen for selecting the compositions. For example, initially, the three pure components are examined (100%, the 1-grid). The error in any phase boundary can be 50% by mass at this point. This gives students a feel for what 10 g of a substance looks like. Next, the three 50:50 mixtures are examined (100%/2, the 2-grid), which gives the students an idea of the solubility of the components in each other. The error in any phase boundary is reduced to 25%. Finally, the 12 binary and six ternary compositions 20, 40, 60, and 80% (multiples of 20%, 100%/5, the 5-grid) are examined. Toluidine blue is used to contrast the liquid phases. At this point, the error is reduced to 10% by mass. If time permits, additional points on the 10% grid chosen near phase boundaries can be obtained. For groups of two students, this last step is often skipped due to time constraints.

high vapor pressure of propan-2-ol can occur when opening culture tubes.

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RESULTS

Student Experiment

As the laboratory progresses, the phase boundaries become more precisely located. When the first part (Stage 1) of the lab is complete (visual observations of the pure components), students observe that, at 298 K, sodium chloride is solid and that both water and propan-2-ol are liquid. At this point, the phase diagram can be represented as shown in Figure 1. The left representation simply plots the phase descriptions at the compositions investigated, the center representation corresponds to the traditional representation of the phases, and the right representation is an attempt to include the uncertainty in the boundary position without bias. Visual observations of the samples indicate that 10 g of propan-2-ol takes more volume than 10 g of water (propan-2-ol is less dense). Because two of the phases are liquid and one is a solid, there must be a phase boundary separating at least two regions, a liquid region and a solid + liquid region. To locate this boundary more precisely, we proceed to Stage 2. When the second part of the lab is complete (Stage 2, visual observations of the 50:50 mixtures), students observe that, at 298 K, the water−propan-2-ol mixture is a single phase, whereas both of the other mixtures consist of two phases. There is noticeably less solid sodium chloride present in the water−sodium chloride mixture than the propan-2-ol−sodium chloride mixture, which means that sodium chloride is quite soluble in water at 298 K. At this point, the phase diagram can be represented as shown in Figure 2. At this point, students can hypothesize that water and propan-2-ol are completely miscible at 298 K and that all binary mixtures with more than 50% salt are biphasic.

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HAZARDS Standard personal protective equipment normally available in the laboratory (safety glasses, lab coat, and gloves) should be used when handling propan-2-ol. Minor splashing due to the B

DOI: 10.1021/acs.jchemed.8b00242 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 3. Three representations of the ternary phase diagram of sodium chloride−water−propan-2-ol, after Part 3 of the experiment is complete. A = water, B = propan-2-ol, and C = sodium chloride. For the leftmost diagram, a yellow dot represents liquid, a black dot represents solid, and a halfyellow, half-black dot represents both solid and liquid are present.

Figure 4. Representations of the ternary phase diagram of sodium chloride−water−propan-2-ol, after Part 4 of the experiment is complete. A = water, B = propan-2-ol, and C = sodium chloride. For the leftmost diagram, a yellow dot represents liquid, a black dot represents solid, a half-yellow, half-black dot represents both solid and liquid are present, and a third-yellow, third-blue, third-black dot represents a solid and two liquid phases are present.

When the third part of the lab is complete (Stage 3a, visual observations of the 20:80, 40:60, 60:40, and 80:20 binary mixtures), students confirm that, at 298 K, water and propan-2ol are totally miscible in the absence of sodium chloride and all of the other mixtures except one are biphasic. A dilute solution of toluidine blue in propan-2-ol is used instead of propan-2-ol. Being very dilute, the toluidine blue does not appear to affect the results. The 20:80 sodium chloride−water mixture is a single phase, confirming that sodium chloride is very soluble in water at 298 K. The mixtures with 60% sodium chloride content or greater are difficult to mix well, especially those with water, and observing the liquid phase can be difficult for the 80% sodium chloride mixtures. At this point, the phase diagram can be represented as shown in Figure 3. At this point, students would hypothesize (falsely) that mixtures of water, sodium chloride, and propan-2-ol form either one or two phases, without having done any true ternary mixtures. When the fourth part of the lab is complete (Stage 3b, visual observations of the 20:20:60, 20:40:40, 20:60:20, 40:20:40, 40:40:20, and 60:20:20 ternary mixtures), students refute their hypothesis that all ternary mixtures are biphasic. All of these mixtures are triphasic, consisting of one solid and two distinct liquid layers. The liquid layers are distinguishable because toluidine blue is much more soluble in the propan-2-ol-rich liquid phase (top) than in the water-rich liquid phase (middle). At this point, the phase diagram becomes much more complicated to draw (Figure 4). However, we know that because of the Gibbs phase rule, the three-phase region must appear as a triangle on the phase diagram, and if we assume that one of the phases is pure sodium chloride, then it must be a vertex of that triangle. Two sides of the triangle are the boundary between the threephase and the two different two-phase solid + liquid regions (sodium chloride + water-rich liquid, sodium chloride +

propan-2-ol-rich). The two mixed-component vertices are connected by curves to the solubility limits on the edges, separating the one-phase liquid region from the two-phase solid + liquid regions. One can now carry out a thought experiment. If pure sodium chloride is successively removed from a composition in the three-phase region, corresponding to a line connecting pure sodium chloride to the original composition, then this line must intersect the phase boundary between the three-phase region not containing pure sodium chloride. At this point we have two liquids saturated in sodium chloride. This corresponds to a new region, not yet observed, consisting of only two liquids. Further removal of salt eventually results in the two liquid phases becoming one. If the (optional) fifth part of the lab is completed (Stage 4, visual observations of the remaining binary and ternary mixtures), the compositions are chosen so that mass percentages are multiples of 10, and to hone in on the regions where phase boundaries exist. To demonstrate the logic, 20% sodium chloride−water is one phase, whereas a 40% sodium chloride−water mixture is biphasic. The point selected would be 30% sodium chloride, 70% water. Similarly, 10:90 sodium chloride−propan-2-ol would also be selected. Other compositions (sodium chloride−propan-2-ol−water) determined in this way would be 20:10:70, 20:70:10, 30:10:60, and 10:x:90 − x, x = 10, 20, ..., 80). For points close to phase boundaries, slight errors in weighing from the ideal compositions can give results different from here because the composition may appear on the other side of the phase boundary. Actual mass ratios need to be used instead of the ideal mass ratios. This results in a slight tweaking of the boundaries of the above phase diagrams. With an error of 5% mass, the invariant points of the phase diagram are 25% NaCl in H2O; 5% NaCl in iPrOH; 15% NaCl + 10% i PrOH in H2O; and 5% NaCl + 25% H2O in iPrOH. C

DOI: 10.1021/acs.jchemed.8b00242 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Possible Extensions of the Experiment

for her efforts towards developing a method for analysis of propan-2-ol. We thank Darlene Goucher (Chemistry, SMU) for technical assistance. C.C.P. thanks Christa Brosseau, Department of Chemistry, SMU, for a critical evaluation of a previous version of this manuscript.

Several possible variations of the experiment were investigated for possible use as a 2 week lab, term project, or demonstration. These include the use of finer grids, replacing propan-2-ol with propan-1-ol, titrating NaCl(aq) with propan-2-ol, analysis of chloride content,16 and analysis of propan-2-ol content. These are described in the Supporting Information.

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PEDAGOGICAL GOALS This experiment introduces and reinforces how to read ternary diagrams, as this is required in order to be able to choose points correctly. In addition, the experiment demonstrates how an unknown ternary diagram can be constructed from scratch with no a priori knowledge. The experiment as described in the accompanying lab manual introduces “miniature” hypotheses that are tested as the lab progresses, including some false hypotheses. The hypotheses are generated by the students after each stage is completed, with some Socratic questioning by the instructor if needed. The assessment is by a “semi-formal” laboratory report, in which results and discussion are submitted, but could be expanded to a full formal laboratory report, or condensed to the generated phase diagram. The 37 students generally did well in the 2 years that the experiment has been conducted (average lab grade 70−75%). The majority of laboratory experiments investigating ternary phase equilibra have two liquid phases in equilibrium, with accurate determination of the compositions. This experiment can have up to three phases in equilibrium, and the observations consist simply of the examination of the number and type of phases for a given mass ratio, the trade-off being that the composition is only known to half of the fineness of the grid.

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CONCLUSIONS A new laboratory experiment investigating the ternary phase equilibrium of water−propan-2-ol−sodium chloride with one-, two-, and three-phase regions has been developed for physical chemistry students. The method developed approximately locates the major regions of the ternary phase diagram.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00242. Additional Material for Instructors (PDF, DOCX) Laboratory manual for students (PDF, DOCX)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Cory C. Pye: 0000-0002-3253-6913 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.C.P. thanks the Saint Mary’s University (SMU) Chemistry 2312 Laboratory Students of the Fall 2016 and Fall 2017 Semesters for working through this new laboratory experiment. C.C.P., C.E., M.L.H., and J.A.W. thank Patricia Granados (Center for Environmental Analysis and Remediation, SMU) D

DOI: 10.1021/acs.jchemed.8b00242 J. Chem. Educ. XXXX, XXX, XXX−XXX