Demonstration Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Demonstration of Composition Changes in a Distillation Column by Use of Bromophenol Blue Indicator Sigvart Evjen,*,† Coralie Petit,‡ Mikael Hammer,§ Arne Lindbråthen,§ Gøril Flatberg,§ Sigve Karolius,§ Heinz Presig,§ and Anne Fiksdahl† †
Department of Chemistry, Norwegian University of Science and Technology, Høgskoleringen 5, 7034 Trondheim, Norway INSA Rouen-Normandie, 685 Avenue de I’Universitè, 76801 Saint-È tienne-du-Rouvray, France § Department of Chemical Engineering, Norwegian University of Science and Technology, Sem Sælands vei 4, 7034 Trondheim, Norway Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on January 23, 2019 at 18:56:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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S Supporting Information *
ABSTRACT: In demonstrations of column distillation for the general public or in other demonstration settings, the audience often lacks the equipment or knowledge to access the distillation process. Distillation of solvents usually gives colorless solution. By adding bromophenol blue (BPB) to the distillation of water and ethanol, the authors have developed a setup sensitive to different ethanol concentrations, allowing direct visualization of the distillation process. The color in the distillation column changes from blue to yellow in the distillation column as ethanol concentration changes. Bromophenol blue indicator can also be used to quickly and semiquantitatively determine the ethanol concentration in binary ethanol−water mixtures in laboratory settings. KEYWORDS: General Public, Demonstrations, Public Understanding/Outreach, Equilibrium, Separation Science, Testing/Assessment
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yellow to blue at pH 3.0−4.6 in aqueous solution.4 Out of the pH indicators, BPB exhibits one of the most visible color changes, from yellow to blue, improving color change visualization. In the following paper, the authors describe a demonstration setup where BPB indicator is used to visually display the concentration changes in a distillation column. Our setup has been used on multiple occasions for both undergraduate students and for the general public, and it has been a very popular attraction.
INTRODUCTION Distillation of liquid mixtures is an important industrial separation process and a natural part of the laboratory curriculum for chemical engineers. While simple distillations can be demonstrated by distilling a liquid out of a colored solution, fractional distillation cannot as readily be visualized. In undergraduate courses, the progression of the process and the influence of different process conditions on the resulting concentration of the distillate can be examined in great detail by means of advanced scientific methods such as gas chromatography (GC), specific gravity, refractive indices, or chemical titration.1−3 However, when performing public demonstrations or in class, such methods are not readily available and direct representation is required. To address the issue, the authors have developed a setup applying a colored indicator that is sensitive to differences in ethanol concentration. Indicators are used to observe the state or changes in the properties of a solution. Different indicators typically are dichromatic or trichromatic, and the indicator color can be used to determine a variety of properties, such as presence of functional groups in a molecule, pH, or metal content in a solution. pH indicators are widely used both in scientific laboratories and in schools as an integral part of chemical education, primarily for acid−base chemistry. Bromophenol blue (BPB) is a common pH indicator, changing color from © XXXX American Chemical Society and Division of Chemical Education, Inc.
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MATERIALS AND DEMONSTRATION The following materials are required for this demonstration: • Deionized water • Ethanol • Bromophenol blue (BPB, CAS no.: 76-59-5) For the distillation setup, the following items are required: • Multiple tray distillation column, preferable Oldershaw or other types of flat-tray column, • Boiler • Reflux condenser Received: September 11, 2018 Revised: December 16, 2018
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DOI: 10.1021/acs.jchemed.8b00736 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
• Hot plate/heating elements • Septum This demonstration is best suited for events where it is possible to start up the demonstration before the event. Providing sufficient heat to produce distillate can take between 10 and 30 min depending on the amount of liquid, effect of heating elements, and number of column trays. A schematic of the demonstration equipment is shown in Figure 1. For this setup, a boiler with an injection port on top
below the distillate outlet and distillate was transferred to the top of the column with a pump, Figure 2. The system operates
Figure 2. Distillation column setup with BPB indicator injected: (1) boiler; (2) distillation column; (3) condenser; (4) pump; (5) distillate inlet, from pump; and (6) syringe with indicator solution.
at atmospheric pressure. Additional equipment which can be used includes a temperature control unit for power supply, liquid level, and temperature detector for the distillate. The distillation setup used in our demonstrations consists of a glass boiler of 4.8 L with four encapsulated 1 kW heating elements. Power to the heating elements was controlled by a Normag USG Leistungsteller 3 temperature control unit based on a Siemens LOGO 25 temperature controller and a PT100 temperature sensor immersed in the boiler liquid. The distillate reservoir could contain 100 mL. A complete description of the setup applied in our demonstration is given in Supporting Information. Running the Experiment
The reboiler is filled with a 9 vol % ethanol solution and 2 mL of BPB solution (1 vol % BPB in 96 vol % ethanol) per L of total solution and boiled until condensation is observed at the cooler on top of the column. The BPB solution (1 mL, 1 vol % in 96 vol % ethanol) is added to the top of the column through the septum. As BPB slowly runs down the column, the color of the indicator will change from yellow to blue according to the ethanol concentration at each level, see Figure 2. After a few minutes, much of the BPB will have leached down to the lower levels of the column. To maintain the color at the top of the column, more BPB in ethanol can be added. Because the BPB is added in solution with ethanol, the total amount of ethanol will increase as time passes, giving greater ethanol concentration throughout the column. As the ethanol concentration increases, the color shift from blue to yellow will occur at lower trays than at the start of the demonstration. Eventually, all
Figure 1. Schematic of a small scale distillation setup: (1) boiler, (2) distillation column, (3) condenser, (4) distillate, and (5) inlet for indicator injection.
of a heater or connected to heating elements is fitted with a column containing at least five trays. More trays will give better separation, until an azeotropic mixture is obtained.5 A column containing horizontal trays, such as an Oldershaw column, is strongly recommended, as this will give slower leaching of the indicator during the demonstration. At the top of the column, there is a reservoir for collecting distillate. The reservoir must allow the distillate to run back down the column when filled. A reflux condenser is fitted over the distillate, together with a septum. Due to the apparatus size, our reflux condenser was B
DOI: 10.1021/acs.jchemed.8b00736 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
levels of the column will be yellow, after roughly 100 min, using an Oldershaw column with six trays. By removing distillate (20 mL/L in boiler) and adding the same amount of deionized water to the boiler, the color gradient was restored.
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HAZARDS The main risks associated with this demonstration are the following: (i) ethanol is a flammable liquid, even in solutions with water,6 and (ii) the apparatus and heating elements will be hot during the demonstration. The demonstration setup should always be kept away from any open flames or ignition sources. The setup and heating elements should be placed on a nonflammable surface. Ethanol should be stored separately in a closed container, away from any heat or ignition sources. Appropriate firefighting equipment (suitable for ethanol or liquid hydrocarbon fuels) should be available, and if performing the demonstration at a public venue, the fire risks and precautions necessary should be discussed in advance with the venue manager. During the demonstration, proper protective equipment including safety glasses and chemically or heat resistant gloves should be worn. The equipment should be checked for leakages before demonstration to minimize risks. If the equipment is not properly fitted with a pressure release valve or opening to release pressure during experimental startup, the glass may break or shatter. Bromophenol blue is an irritant by exposure to skin, eyes, inhalation, or ingestion, but it is present in low concentrations in the apparatus during the demonstration. The demonstrator should be familiar with the MSDS of ethanol and bromophenol blue.
Figure 4. Color scale with BPB in ethanol diluted (0−96 vol %) in deionized water.
demonstration as bromothymol blue (BTB) was found to be too unreliable. Even small concentrations of ethanol gave yellow solutions. Other indicators, such as thymol blue (pKa = 1.65) and eriochrome black (pKa = 6.3), did not give a sufficiently visible color change for ethanol−water mixtures. Other solvent mixtures, such as methanol−water, could also be used. However, all such solvent mixtures are more hazardous than ethanol−water and were discarded. For safe running of the setup, monitoring of the operations is required, e.g., on stands. Our demonstration experiment has been presented for the past two years at our university at a series of stands, aimed both at the general public and at first year and undergraduate students. The demonstration is very popular, and it often gets many visitors, both of whom are interested in the principles or just the colors. The visual cue of the colors makes it easier for students to understand the principles of how distillation works. For younger students, we suggest simplifying the part concerning the indicators and focus instead on the concentration change in the column. For more advanced practices, addition of temperature sensors at each tray would allow for a second visual aid. Lower temperatures would be observed upward in the column because of higher ethanol concentration. As shown in Figure 4, BPB allows for quick, semiquantitative assessment of the ethanol concentration in aqueous solution in the range from 20 to 90 vol % ethanol. In laboratory experiments or demonstrations, the ethanol content in solution can quickly be determined by simple addition of 1 drop of concentrated BPB in 96 vol % ethanol. BPB analysis of binary ethanol−water mixtures allows both for a visual cue in demonstration setups, but also for laboratory exercises. Hence, BPB analysis can readily be applied as a part of all ethanol−water distillation exercises.
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RESULTS AND DISCUSSION The color change is induced by changes in the degree of selfdissociation of the BPB indicator. BPB is a dichromatic compound, as deprotonation, or dissociation, of BPB gives a highly conjugated system. Dissociation of BPB is shown in Figure 3. In pure water, the pKa of BPB is 3.85, and BPB is
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CONCLUSION The described demonstration allows for an easy visual observation of a distillation process by the use of bromophenol blue (BPB) indicator. Changes in the equilibrium for the selfdissociation of BPB in solutions of water and ethanol give a clear color change from blue to yellow in a distillation column. The demonstration is easy to perform and can be run for several hours without interruption. The equipment can be reused multiple times, and cheap, nontoxic solvents are used. The demonstration has been very popular among students of all ages as it is eye-catching, and the audience more readily understands the principles of distillation. BPB addition to ethanol−water mixtures allows for quick, semiquantitative determination of the ethanol concentration.
Figure 3. Dissociation of BPB, explaining the color change of BPB solutions with ethanol/water composition.
practically completely dissociated, as the solution is blue.4 However, in ethanol, which has a weaker dipole and does not stabilize charges as well as water, BPB does not dissociate, giving a yellow solution.7 The color of BPB solutions will change with variation in the concentration of ethanol in pure water, as shown in Figure 4. The demonstration utilizes the change in dissociation with ethanol concentration to display how ethanol and water are separated through distillation. The lower boiling point compound, ethanol, is distilled off first, and the indicator color becomes increasingly yellow toward the top of the column. BPB was chosen as an indicator for this C
DOI: 10.1021/acs.jchemed.8b00736 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Demonstration
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00736. Notes for instructors (PDF, DOCX) Video of running experiment (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Sigvart Evjen: 0000-0003-2662-5861 Anne Fiksdahl: 0000-0003-2577-2421 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support of the Research Council of Norway, through CLIMIT grant 233776.
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REFERENCES
(1) Scott, T. A. Refractive Index of Ethanol−Water Mixtures and Density and Refractive Index of Ethanol−Water−Ethyl Ether Mixtures. J. Phys. Chem. 1946, 50 (5), 406−412. (2) Evans, G. M. The laboratory bubbling column in the teaching of fractional distillation. J. Chem. Educ. 1934, 11 (7), 424. (3) Edelstein, S. M. Distillation experiment. J. Chem. Educ. 1936, 13 (6), 272. (4) Meites, L. Handbook of Analytical Chemistry, 1st ed.; McGrawHill: New York, 1963; pp 3-35 and 3-36. (5) Moore, W. J. Physical Chemistry, 3rd ed.; Prentice-Hall: London, 1962; pp 140−142. (6) Ha, D.-M.; Park, S. H.; Lee, S. The Measurement of Flash Point of Water-Methanol and Water-Ethanol Systems Using Seta Flash Closed Cup Tester. Fire Science and Engineering 2015, 29 (2), 39−43. (7) Dangui, A. Z.; Santos, V. M. S.; Gomes, B. S.; de Castilho, T. S.; Nicolini, K. P.; Nicolini, J. Preferential solvation bromophenol blue in water-alcohol binary mixture. Spectrochim. Acta, Part A 2018, 203, 333−341.
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DOI: 10.1021/acs.jchemed.8b00736 J. Chem. Educ. XXXX, XXX, XXX−XXX
